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SOLUTION CHEMISTRY OF SOME
GROUP Va ORGANOMETALLICS
by
GREGORY LEWIS KUYKENDALL, B.S. in Chem,
A DISSERTATION
IN
CHEMISTRY
Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for
the Degree of
DOCTOR OF PHILOSOPHY
Approved
AfeiA-37
A ^1
r .
ACKNOWLEDGP^IENTS
Tlie author wishes to express his sincere gratitude to Dr.
Jerry L. Mills for his direction of this dissertation. Also
acknowledged are the Texas Tech Graduate School and the Robert
A. Welch Foundation for their generous financial support.
11
CONTENTS
ACKNOWLEDGEMENTS, ii
LIST OF TABLES, iv
LIST OF FIGURES
I. INTRODUCTION.
II. EXPERIMENTAL.
Instrumental.
Special Apparatus 10
Kinetic Measurements 10
Decomposition Analysis 13
Preparations
III. RESULTS AND DISCUSSION. 23
Solution Structure of Penta-para-tolylantimony,
Solvolysis Reactions, 34
NMR Spectra 41
Exchange Reactions 50
Sb(V)-Sb(V) Exchange Reactions 50
Sb(III)-Sb(V) Exchange Reactions 57
As-Sb and As-As Exchange Reactions 64
Kinetics and Mechanism. 66
Eauilibrium Studies 73
REFERENCES, 80
lii
LIST OF TABLES
1. TI NMR Measured Widths at Half-Height for Methyl Hydrogens in (£;-tol)cSb as a Function of Temperature 29
13 2. C NMR Measured Widths at Half-Height for the 02(07)
and 03(05) Carbons in (£-tol)5Sb as a Function of Temperature 33
3. Kinetic Data for Solvolysis of Penta-p^tolylantimony 37
4. H NMR Data for £-tolyl Compounds 42
5. H NMR Data for Ligand Exchange Systems 43 13
6. C NMR Data for £-tolyl Compounds 44
7. Kinetic Data for the (£-tol)5Sb-MecSb Exchange Reactions 72
8. Equilibrium Concentrations for all Sb(V) Species at Various Temperatures 76
9. Equilibrium Constants for M^Sb + T5Sb Exchange Reactions at Various Temperatures 78
10. Thermodynamic Parameters for M^Sb + T Sb Exchange Reactions 79
IV
LIST OF FIGURES
1. Low Temperature H NMR Spectra of £-tolyl Methyl Hydrogens in Penta-£-tolylantimony 27
2. Log k-j vs. 1/T for Penta-£-tolylantimony from H NMR Spectra 28
13 3. Low Temperature C NMR Spectra of Penta-^-tolylantimony. . . 30
13 4. Log k vs . 1/T for Penta-£-tolylantimony from C NMR
Spectra 31
5. H NMR Spectra of (£-tol)3Sb Before and After Solvolysis in CDCI3 36
6. H NMR Chemical Shift vs . Electronegativity for the Me3SbX2 and (£-tol)2SbX2 Series 45
7. H NMR Chemical Shift V£. Electronegativity for Mixed Aryl-Alkyl Antimony Compounds 47
13 8. C Chemical Shift vs_. Electronegativity for the
Triphenyl and Tri-£-tolyl Group V 49
9. Rate of Equilibration for M^Sb + T Sb at 75* 53
10. H NMR Chemical Shift for Methyl Groups vs . Number of
£-Tolyl Groups for K^_ Sb and M T Sb Compounds 54 11. Rate of Equilibration for M^Sb + T Sb at 25 o 58
12. Second Order Plot for M5Sb + T5Sb Exchange Reaction at 60*, A = B 70
' 0 0
13. Second Order Plot for M -Sb + T^Sb Exchange Reaction at 75% A = B 71
' 0 0
CHAPTER I
INTRODUCTION
The chemistry of persubstituted pentacoordinate organometallic
compounds of Group Va began in 1952 with the preparation of pentaaryl-
1 2 and pentaalkylantimony compounds by Wittig ' and co-workers. A
review of the structures of pentacoordinate compounds of Group Va
3 elements has been recently published as well as studies of stereo-
4 chemical non-rigidity. Since the initial synthesis of these
compounds, several reactions involving both heterolytic and homolytic
cleavage of antimony-carbon bonds have been observed. The former
includes exchange or redistribution reactions and the latter
solvolysis or free radical decomposition reactions. As in any general
area of research, there is a constant need for updating and collating
new results, as well as a general compilation of trends in physical
and/or spectroscopic properties.
Structural Determination
As stated, one of the major areas of investigation in this field
is the determination of the stereochemical configuration of penta
coordinate compounds of Group Va. The geometry of a molecule, to a
first approximation, is determined by its sigma bond structure. By
virtue of having five valence electrons in their outer shell, the
3 3 elements of Group Va below the second row may have d 2sp or d 2 2^P
hybridization which by VESPR theory leads to either trigonal bi-
pyramidal or square planar geometry. These geometries may be assigned
to the point group D-, or C. , respectively, from the geometrical loca
tion of the ligands. Most pentacoordinate R-Sb compounds adopt the
4-11 expected trigonal bipyramidal structure with local D., symmetry.
Considerable interest has been generated by reports that pentaphcnyl-
antimony, Ph^Sb, has approximately square pyramidal symmetry in the
12 13 solid state. ' It was thought that perhaps "crystal packing
forces" might explain the apparently anomalous structure, and there
fore, a solution vibrational analysis was performed on pentaphenyl-
14 15 antimony in halocarbon solvents. ' The conclusion from the
vibrational study was that even in solution, pentaphenylantimony is
of local C, symmetry. An IR-Raman spectroscopic study on pentacyclo-
propylantimony indicated a soltuion structure of C, symmtery for
that compound also.
Structural determinations of complex molecules such as pentaphenyl
antimony by vibrational spectroscopy are at best fraught with con
siderable possibility for error or misinterpretation. H NMR is
virtually useless as a method for the determination of the solution
stereochemistry of pentaphenylantimony. The phenyl ring protons
comprise an AA'BB^C spin system since all of the hydrogen atoms are
magnetically non-equivalent, resulting in a complex second-order
spectrum. The second order spectrum arises when the coupling constant
between two nuclei is of the same order of magnitude as the chemical
shift between them, lifting the degeneracy of the energy levels. In
the case of the AA*BB*C spin system, with at least 120 resonance fre
quencies, it would be virtually impossible to derive chemical shifts
and coupling constants for the phenyl protons in two types of phenyl
rings with overlapping resonances, even with modem computer
capabilities. Penta-^-tolylantimony, which has been found to be
9
approximately trigonal bipyramidal in the solid state, was chosen
as a model compound for pentaphenylantimony, so that the £-tolyl-
methyl protons could be used as a "tag" for h NMR. The ambient
temperature spectrum showed only a single £-tolyl-methyl absorption,
attributable to a rapid intramolecular exchange process which
renders all five positions geometrically equivalent, at least on the -3
NMR time scale of 10 seconds. This fluxional behaviour is caused
by rearrangement through vibrational modes resulting in conversion
from D-, to C. symmetry and vice versa.
It was decided to undertake a low temperature NMR study of penta-
£-tolylantimony to determine if the fluxional behavior would cease,
allowing a determination of the solution structure. As the tempera
ture was lowered, the width at half-height of the resonance fre
quencies increased as resonance lifetime on the two distinct positions
decreased. Below the coalescence temperature, the exchange process
will cease and the resonances of the two positions will become 1 13 apparent. The H and C results indicate there is no static solution 13
structure at temperatures as low as -130*. Low temperature C NMR
experiments to -130* on pentaphenylantimony produced analogous
results for this compound, as have investigations by other workers
using a NMR on penta-£-tolylphosphorus at temperatures to -60*.
From the relationship that the lifetime is proportional to the first
order exchange rate constant, the low temperature experiments pro
vided an energy barrier to exchange of 1.60 kcal/mole.
Exchange Reactions
Another area of interest that had previously only been partially
explained is the ability of R^Sb, R^SbX2, and R^Sb compounds to under
go ligand exchange reactions in solution. One of the earliest studies
18 1
by Long, et al., showed using H NMR that trimethylantimony di-
halides underwent halogen exchange to produce mixed halide species
of the type R.,SbXY. However, no mention was made of a proposed ex
change mechanism or of any equilibrium constants for the exchange 19 process. Van Wazer measured the reaction rate for the second order
exchange reaction between trimethylantimony and antimony trichloride,
20
and proposed a four-center transition state. In 1968, Long re
ported equilibrium constants for trimethylantimony dihalide scrambling
reactions and suggested that the equilibrium constants might approach
the value expected for random distribution of the exchanging ligands.
21 Van Wazer concurred with this latter hypothesis in 1973 in a study
22 of redistribution reactions on phosphorus centers, as did Lappert in
1975 in an investigation of halogen exchange in phenylboron dihalide-
23 boron trihalide systems. Barker and Kahn have also found evidence
24 25 for a four-center reaction intermediate first postulated in 1955. '
The work presented here is the first example of an aryl-alkyl or
ayrl-aryl ligand exchange process producing mixed ligand species. The
ligand exchange process has been verified for Sb(V)-Sb(V) mixtures
of Ph^Sb - (£-tol)^Sb and Me^Sb - (£-tol)^Sb, as well as for three
Sb(III)-Sb(V) mixtures. The order of the appearance of products in
the redistribution process provides strong evidence for an associative
mechanism proceeding through the four-center transition state pre
viously discussed.
For the (£-tol)cSb - (Me)cSb system, the rate constant for the
exchange reaction
(£-tol)^Sb + Me^Sb -> (£-tol) (Me) Sb + (£-tol) (Me)^Sb
was determined using the method of initial rates. The value of k_ was
found as a function of temperature and the thermodynamic parameters
4 4 4 AH', AS' and AG' were calculated. Analysis of the individual reactions
in the overall process indicates that the rate of exchange between
two pentacoordinate species is about five hundred times faster than
that for Sb(III) - Sb(V) exchange, and Sb(III) - Sb(III) exchange
does not occur for R.,Sb species, even after 60 days at room tempera
ture.
Solvolysis Reactions
Several types of solvolysis reactions of pentaarylantimony
26 compounds have been well characterized. Wittig and Helwinkel have
reported that phenylbis-biphenyleneantimony is converted to ethoxy-
phenyl-2-biphenylyl-biphenyleneantimony by the action of refluxing
27 ethanol. Briles and McEwen have carried out a similar reaction
between pentaphenylantimony and methanol, producing methoxytetra-
28 phenylantimony plus benzene. Razuvaev and Osanova have prepared
tetraphenylphenoxyantimony by reaction of pentaphenylantimony with
29
phenol. McEwen and Lin measured the rate of alcoholysis of penta
phenylantimony and tetraphenyl-£-tolylantimony and showed that the
process does not undergo a free radical mechanism. Under more
vigorous conditions, i.e. - in a sealed tube at 100* or under strong
30 31
ultraviolet light -, Razuvaev, et al., ' showed that pentaphenyl
antimony reacts with chloroform and carbon tetrachloride solvents via
a free-radical mechanism to form tetraphenylstibonium halide salts.
A similar reaction occurs with methyl iodide or in the presence of
mercury.
32 In 1973 we first reported the reaction of penta-£-tolylantimony
with halogenated hydrocarbon solvents under ambient conditions where
pentaarylantimony compounds were previously thought to be stable.
Again, the utility of the £-tolyl-methyl "tag" was demonstrated when
samples of penta-£-tolylantimony showed the emergence of a new singlet
absorption after sitting overnight. The analogous reaction would be
undetectable in the complex spectrum of pentaphenylantimony. The
Identity of the products, tetra-£-tolylstibonium chloride and toluene,
was confirmed by matching the "TI NMR spectra with the synthesized
stibonium salt and a toluene sample. A solution IR study in halo-
carbon solvents proved that the same reaction occurs for pentaphenyl-
33
antimony as well. McEwen and Lin, in a very detailed study, pre
sented a proposed reaction mechanism and carefully characterized all
products of the photolytic solvolysis of pentaphenyl- and tetraphenyl-
£-tolylantimony in carbon tetrachloride, confirming our results. The
importance of these findings is that no previous structural determina
tion studies have considered the possibility of the reaction in solution
of the pentaaryl compound of interest. Indeed, some of the vibrational
14 assignments by Beattie for the pentaphenylantimony closely match
those found for tetraphenylstibonium chloride. Though the reaction
is not particularly fast, with approximately 10% reaction after
several hours, the very nature of vibrational studies requires samples
of high purity. Future workers in this area might do well to consider
the possibility of sample decomposition.
The kinetic data for the pseudo-first order reactions with halo
genated hydrocarbon solvents are presented and shown to be dependent
upon the solvent used and amount of ultraviolet light received by
the sample. The products indicate a free radical mechanism and the
reaction mixture was analyzed to identify the major products of the
decomposition. Gas chromatography showed that benzene was the only
volatile component produced, and no chlorobenzene was present.
In contrast to the results for the pentaarylantimony compounds,
the analogous arsenic compounds are much less reactive. Penta-£-
tolylarsenic did not undergo solvolysis even after 2 months at 70* in
the presence of light. A proposed mechanism is presented that is
consistent with the kinetic data and the products of the reaction.
NMR Studies
In addition to the structural studies and reactions previously
1 13 mentioned, a comprehensive compilation of H and C data is presented
for the compounds involved in the research, as well as trends ob-
13 tained from the spectral data. The C chemical shift data for some
8
triaryl compounds of Group Va show a direct relationship between the
chemical shift of the a-carbon and the electronegativity of the
central metal atom. Further, there are concurrent changes due to
substituent effects on the ligand. a NMR results on some trialkyl-
and triarylantimony dihalides show deshielding effects due to the
electronegativity of the halogen substituent.
CHAPTER II
EXPERIMENTAL
Instrumental
Proton nuclear magnetic resonance (NMR) spectra were obtained
using either a Varian Associates XL-100-15 NMR spectrometer operating
13 at 100.1 MHz or a Varian Associates A-60 NMR spectrometer. All C
NMR spectra were obtained on a Varian Associates XL-100-15 NMR spectro
meter operating at 25.2298 MHz incorporating a Fourier Transform pro
cedure by interfacing with a Varian Associates computer model 620-L.
All reported chemical shifts were obtained on a Varian Associates
XL-100-15 NMR spectrometer. All air sensitive NMR samples were pre
pared by condensing the desired amount of compound into a NMR tube
containing degassed solvent. Following addition of tetramethylsilane
as an internal standard, the tubes were flame sealed and removed from
1 13 the high vacuum system. All H and C chemical shifts are reported
with respect to internal TMS.
The infrared (IR) spectra were recorded from 3000 to 270 cm
using a Perkin-Elmer model 457 grating infrared spectrophotometer.
The IR spectra of the solid compounds were recorded in KBr pellets or
in thin layer liquid cells equipped with Csl windows. IR spectra of
the liquid samples were recorded in a 10-cm gas phase IR cell equipped
with Csl windows.
The mass spectra were obtained using a Varian MAT-311 mass
spectrometer operating at 70 eV.
10
Special Apparatus
All manipulations of air-sensitive compounds were performed on
a high-vacuum system of the type generally described by Shriver.
The high-vacuum system was equipped with ground glass stopcocks
greased with Apiezon M. Pressures were maintained between 10~ and
10 Torr through the use of a mercury diffusion pump coupled with
a Welch Scientific Duo Seal vacuum pump. Pressures were monitored
via either a mercury manometer or a Scientific Associates McLeod Gauge.
All areas of the vacuum line were calibrated assuming ideal gas
behavior with manometer volume taken into account. Trap-to-trap
distillations were performed at temperatures maintained by liquid/solid
slush baths of various organic solvents.
Kinetic Measurements
The samples for the exchange reactions were prepared by weighing
approximately 0.1 mmole of solid compound and transferring the compound
to an NMR tube fitted with a high-vacuum standard taper joint, adding
approximately 0.20 ml solvent and evacuating after freezing with liquid
nitrogen. After degassing the solvent, 0.10 mmole of liquid compound
was vapor transferred into the NMR tube from the previously calibrated
high vacuum line. The NMR tubes were also previously calibrated by
adding water, measuring the height of the liquid, and weighing the NMR
tube before and after addition of the water. This gave a value of
.00896 ml/mm which was used to calculate the volume and consequently
the molarity of the solutions. The kinetic data for the initial rates
11
was obtained by monitoring the decrease in concentration of (CH^)cSb vs
time using H NMR until no change in the concentration of any species
was observed for six intervals of five minutes each. All spectra
were obtained on a Varian Associates XL-100-15 NMR spectrometer
operating at 100.1 MHz interfaced with a Varian Associates Model 620-L
computer. The probe was heated to the reaction temperature and a
constant temperature maintained throughout the experiment by a Varian
Associates variable temperature controller. Data were taken every
five minutes throughout the course of each experiment. The spectra
were obtained in the FT mode using five accumulations of 0.40 seconds
prior to Fourier Transformation and subsequent data display, making the
measurement time essentially instantaneous on the experimental time
scale. With a 5000 Hz spectral width, the peak intensity, which is an
output parameter of the FT program, is essentially directly proportional
to peak area since the widths at half height are virtually equal and,
from a correlation of intensity to concentration, the decrease in
intensity is a sensitive measure of decrease in concentration of
(CHo)cSb. This method also permits convenient determination of the
increase of product concentrations during the exchange process. The
kinetic rate data were resolved using a computerized linear least
squares fit to a second order rate equation.
The premise that peak height is directly related to peak area
may be rigorously shown as follows.
If the usual assumption that NMR peaks are Lorentzian in shape is
made, then
12
2 2 a) f(v) = a/b + (V-VQ) .
For a NMR peak shape the peak height, h, can be represented as
I 2 b) h = f(v) = a/b = f(v) max.
'v=vo
Let V* = V where the height is 1/2 of h max; then
c) f(v') = f(v) max/2 = a/2b^ = a/b^ + ( V ' - V Q * ) ^
resulting in the expression for the width at half height, w- .„,
d) b = (v'-v'o) - ^1/2/2.
Since the area under the peak may be defined as follows:
N T 1 + iT/ J "h? adv a-rr e) Peak area = Jf (v)dv = J —^ TT- = v—
-<» b + (V-VQ)
substituting expressions for height, h, and width at half-height, 1/2*
w- . ' h f) Peak area = — ^ .
Therefore the relative areas of two peaks depend only upon the height
or intensity of the peak for equal widths at half-height.
Insomuch as the £-tolyl methyl peaks are not resolvable, the
equilibrium concentration of the persubstituted £-tolylantimony (III)
and (V) compounds had to be calculated rather than directly measured.
Furthermore, in order to compensate for changes in magnetic field
homogeniety, the methyl peak areas were converted to mole fraction of
methyl concentration. By knowing the original (Me)^Sb concentration,
and therefore the original methyl concentration, the equilibrium con-
13
centrations of all methyl containing species are directly determined
by dividing the methyl concentration of the (Me).(£-tol) _.Sb species
(' = 3, i = 1-3; n = 5, i = 1-5) by i to arrive at the antimony con
centration at equilibrium. Since the original (£-tol) Sb (n = 3,5)
concentration is also known, the equilibrium value of (£-tol)^Sb or
(£-tol)^Sb is arrived at by the difference in equilibrium antimony
concentration and the total initial antimony concentration.
For the solvolysis reactions, the rate of disappearance of (£-tol)t.-
Sb was monitored on a Varian A-60 NMR spectrometer. The NMR spectra
were taken at a 100 Hz sweepwidth with 500 sec sweep rate for maximum
resolution. All spectra were integrated three times with both forward
and reverse scans and the average values for the areas used to compute
f , the fraction of total methyl resonances attributable to (£-tol)t.Sb.
The use of the fraction of reactant remaining was used to internally
standardize the spectra in case of changes in field homogeniety during
the course of the experiment. The reaction was monitored through at
least 3 half-lives in all cases and three experiments were performed for
each set of experimental conditions. All kinetic results were obtained
from a linear least squares analysis of the data obtained as described
above.
Decomposition Analysis
A Beckman GC-2A gas chromatograph was used to determine the
percent benzene in the Phj-Sb decomposition mixture in chloroform by
measuring the area under the benzene retention peak. The system was
previously calibrated using standard solutions from 0.5 - 25% by
14
volume solutions of benzene in chloroform, with a vaporization tempera
ture of 70* and a carrier flow rate of 20 ml per minutes. After
benzene analysis the solution was filtered through a sintered glass
frit, the solvent was evaporated, and the residue extracted with
boiling water to remove the remaining Ph.SbCl. The solution was then
treated with an excess of sodium perchlorate in 0.05 M NaCl solution
and the Ph/Sb cation precipitated quantitatively as Ph.SbClO^.
Preparations
36 a) Tri-p-tolylstibine , {^T^i^i^2^^* ^^^ prepared by the method
of Talalaeva" ^ by the dropwise addition of 2.28 g (0.010 mol) of
antimony trichloride in 3 00 ml of ether to 3.23 g (0.033 mol) of
p-tolyllithium in ether with stirring under an inert atmosphere.
After addition was complete, the mixture was refluxed for 2 hours and
cooled. Following hydrolysis with 100 ml water, the two layers were
separated and the aqueous layer was washed three times with 20 ml
portions of ether. The ether portions were combined, dried over
magnesium sulfate and the solvent removed under vacuum. The yellowish
product was dissolved in acetone, and precipitated by cooling,
yielding 2.5 g of white crystals melting at 126°, representing a
63% yield of the purified product. Purity was confirmed by melting
point and tl NMR.
b) Tri-p-tolylantimony difluoride, (£-C^H^)^SbF2, was prepared by
slowly adding 48% aqueous hydrofluoric acid to 0.51g (0.58 mmol) of
oxybistri-£-tolylantimony dichloride ({(£-tol)^SbCl}20) in 20 ml of
15
acetone until the solid dissolved. Addition continued until turbidity
persisted and water was added until formation of a white flocculent
precipitate ceased. Tne solution was then stirred for 15 minutes in
an ice bath. Filtration and washing yielded 0.44 g of white crystals
(88% yield) melting at 92*.
36 c) Tri-p-tolylantimony dichloride, (£-0 H ) SbCl2, was prepared
as in part (b) using concentrated aqueous HCl. The yield was 86%.
38 In a second method of preparation, 4.6 g of (£-tol)^Sb was dissolved
in petroleum ether (35-60*) at 0* and chlorine gas was bubbled
through the solution while stirring, with immediate formation of a
white precipitate. After stirring for one hour at 0*, filtration of
the cold solution yielded 2.9 g of product (54% yield). The white
crystals from both methods of preparation melted at 154-6*.
d) Tri-p-tolylantimony dibromide, ( £-C^H^)^SbBr^, was prepared
by treating 1.0 g of [(p-tol)«SbCl]20 with 48% aqueous HBr as in part
38 (b), resulting in an 87% yield. In a second method of preparation, a
solution of bromine in chloroform was added to 3.0 g (12.6 mmol) of
(£-tol)oSb in chloroform until a slight yellow color persisted. The
solvent was reduced by 70% and the solution was cooled. The product
was filtered and recrystallized from ether, with 5.4 g being obtained,
representing a 75% yield. The product from both methods melted at 233*.
36
e) Tri-p-tolylantimony diiodide, (£-C^H^)^Sbl2, was produced as
in part (b) from 1.0 g of [(£-tol)^SbCl]20 and 48% aqueous HI, yielding
1.28 g of products (85% yield), melting at 198-200°. 39
f) Oxybistri-p-tolylantimony dichloride. [(£-tol)»SbCl] 0, was
16
prepared by two different methods. In one method, first reported here,
50 mmoles of £-tolyllithium was added dropwise to 11.4 g (50 mmol)
of antimony trichloride in 200 ml ether in an inert atmosphere with
stirring. During addition the solution turned dark brown and was
stirred for 12 hours with gentle reflux. After hydrolysis, workup
was done as in part (a). The solid residue was placed in acetonitrile
and the solution was filtered, resulting in 0.91 g of insoluble product,
melting at 261*, representing a 70% yield.
39 The method of Kolditz was used to prepare [(£-tol)^SbCll^O
to verify the product described above by comparison. The products had
identical melting points (261*), infrared spectra, and TI NMR spectra.
g) Penta-p-tolylantimony, (£-C H-,) Sb, was prepared by the
dropwise addition of 2.9 g (6.2 mmol) of (£-tol) < SbCl2 in 75 ml
benzene to 13.6 mmoles of tolyllithium in ether in an inert atmosphere.
The reaction mixture was refluxed for 3.5 hours cooled and hydrolyzed.
The workup was as in (a). After drying, the volume of benzene was
reduced to 20 ml and petroleum ether was added, with precipitation of
the product upon cooling to 0*. The yield was 3.0 g (84%) melting at
196-7°. The product was dissolved in boiling acetonitrile, filtered,
and precipitated by adding methanol and cooling. The purified product
9 1 17 melted at 186* and purity verified by melting point, H NMR, and
mass spectral data.
h) Tetra-p-tolylstibonium bromide, (£-C H^),SbBr, was prepared
by the dropwise addition of 5.5 g (9.9 mmol) of (£-tol)^SbBr2 in 40 ml
benzene to an ether solution containing 13 mmol of £-tolylmagnesium
17
bromide in an inert atmosphere. The addition took 2.5 hours and the
solution was stirred for 4 hours more. The mixture was hydrolyzed
with 10 g of ice and 6 ml of 48% HBr. The organic and aqueous layers
were separated and the aqueous layer was extracted once with ether.
All organic portions were combined and the solvent removed under
vacuum at ambient temperature. The resulting yellow oily liquid was
extracted five times with 50 ml portions of hot water. The water
portions were combined and the volume decreased under vacuum. The
resulting (£-tol)^SbBr melted at 223-225*. The purity was checked
by "SI NMR.
40 i) Tetra-p-tolylstibonium chloride, (£-C^H^),SbCl. An anion
exchange column was prepared with Amberlite IR-4-B resin and charged
with chloride ion by passing 1 M HCl through the column. The (£-tol),-
SbBr was dissolved in boiling water, cooled, and passed through the
column using water as the eluent. The water was collected, evaporated
under vacuum and the product, tetra-£-tolylstibonium chloride, had a
melting point of 220-222*. The conversion was essentially quc.ntitative
using very slow elution times, i.e., 2 ml/minute.
41 j) Triphenylstibine dibromide, (C H )- SbBr2, was prepared by the
dropwise addition of a solution of 9.19 g (57.5 mmol) of bromine in 20
ml chloroform to 19.3 g (54.7 mmol) of triphenylstibine in 50 ml
chloroform with stirring. The solution was stirred for 3 hours, con
centrated to 20 ml and cooled overnight at 0°. The solution was
filtered and recrystalized from ether, yielding 24.8 g of purified
38 product (88%), melting at 216°.
18
k) Pentaphenylantimony, (C,Hc)cSb, was prepared by the dropwise
addition of 0.041 mol of phenyllithium in ether to 9.5 g (0.0185 mol)
of Ph2SbBr2 in 50 ml benzene with stirring under a dry nitrogen
atmosphere. Following addition, the solution was refluxed for 6 hours,
cooled and hydrolyzed. The workup as in part (a) yielded 5.1 g (55%)
of Pb^Sb after recrystallization in acetonitrile, which melted at
13 168-169°. Purity was checked by melting point.
1) Pentaphenylarsenic, (C,Hc)cAs, was prepared by the dropwise
addition of 0.012 mol of phenyllithium in ether to 4.19 g (0.010 mol)
of Ph.AsCl in ether under a dry nitrogen atmosphere, was stirred for
2 hours and hydrolyzed. Workup as in part (a) yielded 0.30 g of
product melting at 126-127* following recrystallization from aceto
nitrile.
m) Tri-p-tolylarsine, (£-C^H_)„As, was prepared by the dropwise
addition of a 3.8 g (.021 mol) of arsenic trichloride to an ether
solution containing .065 mol of £-tolylmagnesium bromide under a
nitrogen atmosphere. The reaction was very exothermic and required
external cooling. Following hydrolysis, workup as in part (a) yielded
5.9 g of product (81%) melting at 149-151°. Purity was confirmed
1 43 by melting point and H NMR.
44 n) Tri-p-tolylarsine dibromide, (£-C^H^) .3AsBr2» was prepared by
the dropwise addition of a solution of 1.6 g (0.010 mol) of bromine
in 20 ml chloroform to 3.48 g (0.010 mol) of (£-tol)^As in 50 ml
chloroform. The mixture was treated as in part (a), yielding 4.3 g
(£-tol)-AsBr2 representing an 85% yield of purified product.
19
o) Penta-p-tolylarsenic, (£-CyH ) As, was prepared by the drop-
wise addition of £-tolyllithium (0.020 mol) in ether to 5.02 g
(0.010 mol) of (£-tol)2AsBr2 in 50 ml benzene under a dry nitrogen
atmosphere. The solution was refluxed for 3 hours, cooled and
hydrolyzed. The workup as in part (a) yielded 2.72 g (51%) of
product melting at 139-140°. Purity was confirmed by melting point
1 43 and H NI-IR.
p) TrimethyIstibine, (CH„)^Sb, was produced by the reaction of
0.63 moles of methylmagnesium iodide in ether with 47.7 g (0.21 mol)
of antimony trichloride in 125 ml of ether. The SbCl^ was added drop-
wise with stirring, and all reactions were performed under an inert
atmosphere. The reaction mixture was stirred for 12 hours, and
distilled to dryness into a second 3-neck flask. The ether-stibinc
mixture was very difficult to separate by distillation, and the follow
ing isolation method was employed. The trimethyl stibine was converted
46 to trimethylantimony dibromide by adding Br2 in ether until a slight
yellow color persisted. The solution was filtered, the solvent reduced
and the solution filtered again, yielding white crystals that de
composed at 178-80°, representing a 32% yield based on the SbCl-
used. The (CH )^SbBr2 was reduced with zinc to (CH^)2Sb in the
following manner. The (CH.^)^SbBr2 was placed in a 25 ml round-bottom
flask fitted to a Y-tube, one side of which was connected to a high
vacuum line and the other to a rotating side arm containing an excess
of granular zinc metal. Water (0.5 ml) was added to the (CH-)^SbBr2,
the system was frozen with liquid nitrogen and evacuated. The zinc
20
was then added to the (CH^)2SbBr2-H20 mixture and a vigorous reaction
proceeded as the water melted. The mixture was stirred during the
reduction, and the cessation of bubbling indicated the end of the
reaction after about 10 min. The products were condensed into a
high-vacuum system during the reaction while the pressure was
monitored via a mercury monometer, and held to £a.. 30-40 Torr. The
product was purified by trap-to-trap distillation, with water con
densing in a chlorobenzene slush bath (-45°) and trimethylantimony in
an ethyl acetate slush bath (-83°). The purity was checked by vapor
47 48 1 49 pressure, IR, and H NMR.
2 q) Pentamethy1antimony, (CH^)^Sb, was prepared by the reaction of
3.26 g (10 mmol) of (CH2)2SbBr2 with 22 mmoles methyllithium in ether
under an inert atmosphere in a three-necked flask with suitable stop
cocks to allow direct transfer to a high-vacuum system. The (CH-)^-
SbBr- forms a suspension in ether rather than a solution. The CH-,Li
was added dropwise with stirring and the reaction mixture was refluxed
for 12 hours. The major portion of the ether was distilled from the
mixture prior to separation by high vacuum trap-to-trap distillation.
The pentamethylantimony condensed in a chlorobenzene slush bath (-45°)
with the ether and other highly volatile substances being trapped at
-196°. An IR spectrum showed that no ether was present in the final
product. Purity was checked by tl NMR and IR.
r) Trimethylarsine, (CH^)-As, was prepared by the dropwise
addition of 18.15 g (0.1 mol) of arsenic trichloride in 125 ml of
xylene to a solution of methylmagnesium iodide in n-butyl ether under
21
an inert atmosphere. The temperature of the reaction was kept well
below 50°. The trimethylarsine was distilled into a three-necked
flask fitted with suitable stopcocks to allow direct transfer to a
high-vacuum system. The trimethylarsine was purified using standard
trap-to-trap distillation procedures with an ethyl acetate (-83°)
slush bath used to trap the purified product. Purity was verified
t- 47 52 1 49 by vapor pressure, gas phase IR, and H NMR measurements.
53
s) Trimethylarsine dibromide, (CH,,)-AsBr2, was prepared by
the dropwise addition of Br2 in ether to an ether solution of (CH^)^As
until a slight yellow color persisted. The solvent was reduced under
vacuum by 75% and filtration yielded white powdery crystals melting
at 156-160°. Purity was checked by H NMR.
t) TriphenyIs tibine, (CgH^)^Sb, was purchased from Alfa Inorganics
and used following recrystallization in chloroform.
u) Triphenylarsine, (C^Hc.)oAs, was purchased from PCR Incorporated
and used following recrystallization in chloroform.
v) Tetraphenylarsonium chloride, (C^Hc),AsCl, was purchased from
Eastern Chemical Corporation and used following recrystallization in chloroform.
54 w) Pentamethylarsenic, (CH„)c.As. Several attempts to prepare
and isolate pentamethylarsenic were unsuccessful, probably due to
disproportionation or decomposition. The dropwise mixing of methyl-
lithium with trimethylarsenic dibromide in ether under an inert
atmosphere indicated that a reaction was taking place, however, only
trimethylarsine could be isolated via high-vacuum techniques. All
22
previous attempts by other workers to prepare (CH_)t.As have also been
unsuccessful, and it seems likely that a method of synthesis at low
temperature must be developed to isolate the product.
x) Tristrifluoromethylstibine, (CF.,) Sb, was prepared according
to Dale. To 13.1 g (108 mmol) of antimony metal was added 9.8 g
(50 mmol) of iodotrifluoromethane in a sealed stainless steel bomb.
The mixture was heated for 7 hours at 165-170°, then all condensable
material was transferred to a high-vacuum system. The mixture was
separated by trap-to-trap distillation, with (CF^)Sbl2 and (CFo)2SbI
condensing in a benzyl alcohol slush bath (-15*) and (CF^).Sb con
densing in a chloroform slush bath (-63.5°). The unreacted CF„I was
collected at liquid nitrogen temperatures (-196°). The product was
a clear liquid with a slight yellow color. Purity was checked by
- 55 , , ^^55 vapor pressure measurements and gas phase IR.
CHAPTER III
RESULTS AND DISCUSSION
Solution Structure of Penta-para-tolylantimony
Penta-£-tolylantiraony might seem to be a structural analog of
pentaphenylantimony; however, the solid state structure of the former
indicates that it has the expected D symmetry while the latter has
12 13 the unusual C^^ symmetry. ' NMR experiments were performed to try
to determine the solution stereochemistry of (£-tol)cSb. An H NMR
spectrum of (£-tol)Sb at -60* has been previously reported to yield
a single absorption for the methyl protons, presumably indicating
rapid intramolecular ligand exchange, as is common for pentacoordinate
4
Group Va compounds, yielding all methyl protons magnetically equiva
lent on the time scale of the experiment. We have recorded the H
13 and C NMR spectra of (£-tol)cSb in dichlorofluoromethane solvent
13
at temperatures down to -130*. The concurrent use of C NMR spectro
scopy with H NMR virtually eliminates the possibility that the
observance of a single H NMR peak for the methyl protons in (£-tol)£.Sb
could be due to accidental proton magnetic equivalence of a "static"
solution structure of D , or C, , rather than to rapid intramolecular
exchange. In the case of "static" solution structure without
accidental magnetic equivalence of the methyl protons, two separate
methyl proton resonances should occur with relative areas of 3:2
(equitorial:axial) or 4:1 (basal:apical) for D., or C. symmetry,
respectively.
23
24
The width at half-height, w^^2» ^^^ the methyl proton ''"H NMR
resonance in (£-tal)^Sb varied from 1.9 Hz at 30* to 8.7 Hz at -129*
(See Figure 1). We attribute this broadening to slowing of the rapid
intramolecular exchange process, inasmuch as the w . of internal TMS
broadened very little in this temperature range. Reliable '''H NMR
data could not be obtained below -130*, at which temperatures the
coalesced peak should presumably separate into two resonances.
The NMR experiment provides access to T., the mean lifetime of
an absorbing nucleus at the site giving rise to the i th resonance
absorption. The inverse mean lifetime, l/x., is directly propor
tional to k, the first order rate constant for transfer out of the
i th site. In the region of rapid exchange between sites on the D., 3h
and C, geometries, designated A and B, the rate of transfer is faster
than the precision frequency and the resonance frequency appears at
a point between the actual absorption frequency of the two individual
geometries. This mean lifetime can be designated as " AR • By the
Heisenberg uncertainty principle, AE*At~b, and for NMR line shapes,
At~T, so the uncertainty in energy is shown by an increased line
width. Aw. ,„. Therefore w- .--AE/h-l/x and an increase in w^ ,„ produces
an increase in l/x or a decrease in x, the resonance lifetime. In
the region of intermediate exchange rates, the increased width of line 35
AB (Lorentzion) is (^1/2^AB~^^I/2^AB ^ ^^^^AB' ^^^^^ ^^1/2\B ^^
the width at half-height for line AB, and (w, /2)!T, is the width at
half-height under rapid exchange conditions. The increase in w, ,„ ^^^
concurrent decrease in x causes a similar increase in x and x .
AB A B
25
Since the lifetime at position AB results from a time average of
transition between position A and B, as the lifetime at AB decreases,
the lifetimes at positions A and B must increase at the same rate.
The value of k that is actually measured is the rate constant for
transfer out of the AB state into the A and B states, which increases
as the temperature decreases; however this rate is the same as that
for transfer into the individual states A and B. As the lifetimes
X. and Xg increase, the value of k for transfer from state A to state
B through AB decreases. A plot of log k vs . 1/T, where k is derived
from I/TTX, should be linear with a slope of -E /2.3R. Such a plot
(see Figure 2) for the tolyl-methyl proton resonance in (£-tol)t.Sb
from -36* to -129* was linear and yielded an activation energy of
* -1.60 + 0.18 kcal/mole. Experimental values are in Table 1, where
Figure 1 shows the low temperature H NMR spectra of (£-tol)cSb.
The activation energy derived above is, as previously discussed, for
transfer out of the AB state and therefore, the activation energy for
transfer between the A and B states is +1.60 +0.18 kcal/mole. The
state AB may be considered the transition state, and the rate of
transfer out of the transition state is the same as the rate of
transfer into the transition state. Further, since the process of
The uncertainty in the measurement was calculated as the standard
deviation in the slope of the graph (Figure 2) as outlined by Y.
Beers, "Theory of Error", Addison-Wesley Publishing Company, Reading,
Mass., 1953.
26
going from the transition state AB to the individual state A has a
negative activation energy, the reverse process has a positive activa
tion energy.
1 13
At ambient temperature, the H decoupled C NMR spectrum of
(£-tol)^Sb consisted of five sharp resonances due to the five non-
equivalent types of carbon atoms in the £-tolyl ligand. The single
resonance for each position on all five ligands is attributed to rapid
C r — C . / ^ \
(C3— c / r V s b ' ^ 6 — ^7
intramolecular exchange of ligand position. (See Figure 3). As the
temperature was lowered to -135*, the C resonance moved 85 Hz upfield
while there was no change in the C, and C . resonances. The 05(0^)
and C^(C^) peaks began to lose intensity as the temperature was
lowered. At 0*, the intensity ratio of 02(0^) or 0^(0^) was 3:1;
at -90* it was 2:1; and at -124*, it was less than 1:1. The C2(C^)
and Co(C^) resonances had disappeared completely in the -130* to
-135° range, which was the low temperature limit for these experiments
The peaks reappeared upon warming and the spectrum was identical to
that before cooling. A line shape analysis of the 02(0,) and C^(C,)
absorbances was performed to obtain an alternate determination of the
exchange energy barrier for geometrical interconversion. A plot of
log k, obtained from corrected widths at half-height, (w. ,„)., versus
1/T (Figure 4) gave an activation energy, E , of 1.39+0.11 kcal/mole
for the intramolecular positional exchange barrier in the (£-tol)£.Sb
27
h 25 HZ
-129
_ -102"
-75'
-36'
30*
Fig 1. — Low Temperature H N tR Spectra of £-Tolyl Methyl Hydrogens in Penta-£-tolylantimony.
28
1.6 ^
1.2 .
:^ 0.8 .
o
0.4 -
2.0
T r T r
4.0 6.0 1/1x10^
8.0
Fig. 2 -spec t ra .
Log k V£ 1/T for Penta-£-tolylantimony from H NMR
.29
TABLE 1
W . FOR METHYL HYDROGENS IN (£-tol)^Sb ' AS A FUNCTION OF TEMPERATURE^'^
-1 2 c Temperature (°C) w . (Hz) ^i/2~^l/2** " ^ k(sec x 10 )
30°
-16
-36
-46
- 6 0
- 7 5
- 9 0
-102
-124
-129
1.9
2 .5
2 .5
2 .6
3 .0
3.4
4 . 8
5.7
7 .5
8.7
0.6 1.9
0.6 1.9
0.7 2.2
1.1 3.5
1.5 4.7
2.9 9.1
3.8 11.9
5.6 17.6
6.8 21.4
^All values for w., ,„ are the average of peak widths obtained during
cooling and subsequent heating. A linear least squares treatment
of the data resulted in an activation energy of 1.60 +0.18 kcal/mole
' The value of k is calculated from the expression k Z l/x where
l/x = TTAW , , and represents the rate of transfer out of the transi
tion state intermediate between the two individual geometries.
30
v^'W'^^''
140 120 PPM
13.
IV
-124
-100'
40
' " 'KvA-"^* 30'
20
F i g . 3 — Low Temperature C NMR Spectra of Pen t a -£ - t o ly l -antimony.
2.0 n
31
O
1.5
1.0
o — o - J' .
3.0 4.0 5.0 6.0 7.0
1/T X 10
Fig. 4.—Log k v£ 1/T for Penta-£-tolylantimony from -'-•C NMR Spectra
32
system. The argument here is the same as that used for the -""H NMR
spectra to justify the positive values for the activation energy for
the stereochemical energy barrier. The (w, ,2)• values were corrected
for viscosity effects by subtracting the broadening of the solvent
peak from the broadening of the 02(0^) and 03(0^) peaks (Table 2).
13 For the C spectra, the system entered the region of moderately slow
exchange at -90°; at higher temperatures the system was undergoing
13 rapid exchange. Figure 4 shows the C NMR low temperature spectra
for (£-tol)^Sb, including the low field half of the solvent doublet
to illustrate broadening due to nonexchange effects.
Stereochemical nonrigidity is a common occurrence in penta
coordinate chemistry, especially of Group Va compounds. The Berry
56
mechanism accounts for all known polytopal rearrangements by a
bending mode in which a trigonal bipyramidal geometry passes through
a square pyramidal form to a rearranged trigonal bipyramidal shape.
For species with five identical ligands, the rearrangement barrier may A
be quite low, and Muetterties has suggested that kT may not be an
unreasonable lower limit. The rearrangement barrier is c£. 6 kcal/mole
51 58
for aminophosphoranes, R„NPF, , and for Cl2PF~ and Br^PF... With
these species having an electronegativity difference in the ligands,
the pseudorotation barrier would be expected to be larger than as in
a R-Sb molecule. Therefore, it seems quite plausible that the value of
1.50 kcal/mole (ave.) found here is quite accurate. 1 13
Both the H and C NMR results substantiate the conclusion that
the solution stereochemistry question of Group Va pentacoordinate
33
TABLE 2
13 C NMR MEASURED WIDTHS AT HALF-HEIGHT FOR THE C2(Cy) AND
0^(0^) CARBONS IN (£-TOL)^Sb AS A FUNCTION OF TEMPERATURE a,b
Temperature
30
0
-20
-40
-60
-90
-100
-110
-124
(°C) \j^.^(nz)
15
20
20
20
31
37
48
47
57
corr. w^.2 ( 2)
15
17
15
15
16
17
23
27
42
k( -l.c
sec )
47.1
53.4
47.1
47.1
50.2
53.4
72.3
84.8
131.9
All values for w ,2 ^^ ^^ average of 02(0^) and 0^(0^) widths b
obtained upon cooling and subsequent warming. A linear least
squares treatment of the data resulted in an activation energy of
1.39 + 0.11 kcal/mole. The value of k is calculated from the
exp ression k : l/x where - = TTAW- ,_, and represents the rate of trans-'1/2 fer out of the transition state intermediate between the two in
dividual geometries.
34
compounds is not readily answered. There is no static solution
structure except at extremely low temperatures, and the low energy
barrier suggests that ligand size is probably not a dominant factor
in limiting the exchange process.
Solvolysis Reactions
Both pentaphenyl- and penta-£-tolylantimony undergo solvolytic
decomposition at 25°, both in the presence and absence of light, and
the rate of reaction has been determined. The reaction proceeds by a
33 free radical mechanism in the manner outlined by McEwen, in which
the pentaarylantimony reacts with halogenated solvent to produce the
respective tetraarylstibonium halide (Ar,SbX), the protonated aryl
species, the a-linked biaryl (Ar-Ar) and other polymeric species in
lesser amounts.
A freshly prepared solution of penta-£-tolylantimony in CDCl.
had a H NMR methyl resonance at 2.29 ppm and an AB pattern (£ & m
hydrogens) at 7.01 and 7.24 ppm. After 1.5 hours at ambient tempera
ture a new methyl singlet at 2.39 ppm began to appear. After 3.0
hours there were resolvable methyl proton singlets at 2.29 ppm (penta-
£-tolylantimony), 2.39 ppm (toluene and p,p'-bitolyl, which have
overlapping methyl resonances), and 2.39 ppm (tetra-£-tolylstibonium
chloride). There were distinguishable AB patterns at 7.01 and 7.24
ppm due to (£-tol)^Sb, 7.20 and 7.46 due to (£-tol)2, 7.24 and 7.67
due to (£-tol)/SbCl, and a singlet at 7.20 due to toluene. The small
chemical shift dependence of + .02 ppm upon concentration occasionally
allowed resolution of the methyl resonances of toluene and (£-tol)2.
35
The chemical shifts of the products were matched to those of authentic
compounds prepared by independent synthesis, and a composite solution
had a spectrum identical to that of the decomposition solution. The
tt chemical shifts are given in Table 4 and the NMR spectra are shown
in Figure 5.
The rate of decomposition of (£-tol)^Sb was determined in various
solvents, with the reactions occurring both in ambient laboratory
fluorescent light (UV irradiation) and under conditions where no
light reached the samples. All reactions were pseudo-first order
over several half-lives in the disappearance of (£-tol)^Sb. In all
solvents, exposure to ambient laboratory light greatly accelerated
the decomposition process, and to a lesser degree the rate was also
affected by variation of solvent (see Table 3). The decomposition of
(£-tol)-Sb was monitored by observing the tolyl methyl resonance using
H NMR. The amount of decomposition was determined by obtaining the
fraction of the total methyl resonance attributable to (£-tol)cSb.
The reactants were found to have undergone approximately ten percent
decomposition in 1.5 hours.
First order plots of (-In f ) vs. time,where f is the fraction
of the total tolyl-methyl area attributable to (£-tol) Sb, were linear
for all solvent systems with the slope being equivalent to the observed
rate constant. (All rate constants were obtained from a linear least
squares analysis.) All kinetic data are summarized in Table 3.
The standard error of the slope, and hence of the first order rate
constant, varied up to about 12.5 for the kinetic experiments at
room temperature. The error in the samples receiving ultraviolet light
36
C/Hs &(p-tol)2-i 2.37
(p-tolLSbCI-^
(p-tol)^Sb 2.29
'V.^wAr~._^.
8 7 — n -
3 ppm
Fig. 5 — TI NMR Spectrum of (£-tol) Sb Before (A) and After (B) Solvolysis in CDCl .
37
TABLE 3
KINETIC DATA FOR THE SOLVOLYSIS OF PENTA-£-T0LYLANTIM0NY'
Solvent
CHCl.
CH2CI2
CH2Br2
CDCl^
CHCl,
CH2CI2
CH2Br2
Conditions
UV irrad.
UV irrad.
UV irrad.
UV irrad.
no UV irrad
no UV irrad
no UV irrad
.b,c. . -1. _-4 k (mm ) X 10
2.07 + .10
1.82 + .04
1.19 + .20
1.37 + .09
0.486 + .036
0.474 + .06
0.419 + .051
^"1/2^^^^^^
2.32
2.65
4.04
3.50
9.98
10.15
11.48
a b Data were obtained on a Varian A-60 spectrometer. First order rate constant for the disappearance of (£-tol)^Sb. ^Each value is the average of three runs for each set of experimental conditions, error reported is the standard deriation of the slope.
38
was about half of that for the samples kept in the dark. The difference
is due to the reception of incidental light during sample transfer
and handling, resulting in non-uniform reaction rates. The error in
integration would be the same for both samples and was 2-5% for the
six values obtained for each data point. As a consequence of the
slow measurement time, the assumption that all uncertainty is concen
trated in the y-value may not be quite valid. There will be essen
tially the same time uncertainty in all measurements, though, and
the above-mentioned treatment should be valid. The division of error
between the x- and y-values would require an a priori decision and this
procedure was not undertaken. The relatively small degree of relative
error is satisfactory, and opens up the possibility of extending NMR
as an analytical tool through development of computer assistance and
more sophisticated monitoring techniques.
To substantiate the NMR results for the decomposition reaction,
a mixture of Phj-Sb and CHCl., was quantitatively analyzed for the
relative amounts of products formed in the solution. A solution of
1.289 g (2.54 mmol) of Ph^Sb was dissolved in 50 ml of CHCl^ and
allowed to react in laboratory light for 1 month, with clear crystals
of Ph.SbCl fonning after 2 weeks. The amount of benzene formed was
measured using gas chromatography. The retention times were the same
for benzene in chloroform in the calibration samples and the major
peaks for the decomposition product (700 sec at 70° and 20 ml/min flow
rate). Analysis yielded a value of 0.6% benzene in chloroform, corres
ponding to 0.253 g (3.24 mmol) produced. The Ph.SbCl isolated by
filtration weighed 0.136 g and further complexation with perchlorate
39
yielded Ph^SbClO^ corresponding to 0.691 g of Ph^SbCl. The total
Ph^SbCl recovered (0.827 g, 1.78 mmol) represented a 70% conversion
of the Ph^Sb. There remained 0.130 g of polymeric residue that was
insoluble in both hot water and acetone. The 1.210 g of product
recovered account for 94% of the starting material.
For the reaction,
CHCl Ph^Sb > Ph^SbCl + C^H. + minor products
the excess of benzene produced and the slightly less than theoretical
amounts of Ph.SbCl formed suggest that the conversion is not quanti
tative, with the formation of some antimony polymers likely. As a
result, the possibility exists for the use of this reaction as a
synthetic route to tetraarylstibonium compounds with varying anions,
without the use of anion exchange columns. Adjustment of the reaction
parameters of temperature and UV light could conceivably improve the
present 70% conversion rate with concurrent increases in reaction rate
as well. Yields by presently used methods are in the 40-50% conversion
6,8,40 range.
An infrared study was undertaken to check the validity of the
assumption that pentaphenylantimony reacts analogously to penta-£-tolyl-
antimony. A solution of PhcSb in CHCl. was monitored by IR in a
liquid cell for two weeks and found the spectra changing, showing the
formation of Ph.SbCl. The results showed that pentaphenylantimony
does indeed react in the same manner as penta-£-tolylantimony in halo
genated hydrocarbon solvents. These results suggest that recent IR
14 15 and Raman studies ' of pentaphenylantimony in CH2CI2 and CH2Br_
40
must be viewed with caution, due to the possible decomposition of the
compound in solution. It is interesting to note the (£-tol)cAs does
not undergo any observable decomposition in CDCl., solvent even after
two months in laboratory light or after heating for two weeks at 70°
in light.
The following mechanism is proposed using Ph^Sb and CHCl., as
specific examples. The general reaction sequence may be followed for
any pentaarylantimony species in halogentated solvent with only minor
changes.
Initiation
CHCI3 ^ Cl^C- + H-
Propagation
Ph Sb + -CCl^ -> Ph^Sb-CCl^
Ph^Sb-CCl^ -> Ph^Sb-CCl^ + Ph-
Ph- + CHCI3 -> Ph-H + -CCl^
Termination
Ph- + Ph- -> Ph-Ph
Ph. + H- - Ph-H
•CCI3 + -CCl^ -> CI3C-CCI3
Subsequent Reactions
Ph^Sb-CCl3 -> Ph^SbCl + :CCl2
-.CCI2 + CHCI3 -> •CHCI2 + -0013 -> HCI2C-CCI3
There was no evidence for the presence of £-chlorotoluene in any of
the solvolytic reactions, hence the formation of Cl- radicals is
essentially ruled out and no reaction such as
41
Ph^Sb + CI- -^ Ph^SbCl + Ph-
since the reaction
Ph- + CI- -> Ph-Cl
would take place in the termination sequence. The previous reaction
sequence produces all of the products found in the reaction mixture
and no others. Furthermore, there was a small, unidentified peak in
the gas chromatographic analysis with a very short retention time that
would correspond to the low molecular weight products of radical
couplings, i.e., CI2HC-CCI3 and/or CI3C-CCI3.
NMR Spectra
The ^H NMR data for the persubstituted antimony and arsenic
compounds are listed in Table 4, the -'-H NMR data for the mixed compounds
produced in the ligand exchange experiments are in Table 5, and the
13
C NMR data are in Table 6.
There are several observations which may be made with respect to
substituent effects for the H NMR results. A plot of chemical shift
vs. Sanderson equalized electronegativity values was linear for
both the Me.3SbX2 and (£-tol)3SbX2 series (X = halogen). As shown in
Figure 6, the change in chemical shift per unit change in electro
negativity is greater for the Me3SbX2 series. This is expected since
the £-tolyl methyl group is more electronically isolated from the
effects of halogen substituents. The small deviations from linearity
are probably due to solvent effects to which proton chemical shifts
are particularly susceptible. The same type of relationship exists
42
TABLE 4
H NMR CHEMICAL SHIFT DATA^*^ FOR £-TOLYL COMPOUNDS
Compound
(£-tol) Sb
(£-tol)3SbF2
(£-tol)3SbCl2
(£-tol)3SbBr2
(£-tol)3Sbl2
(£-tol),SbCl
(£-tol),SbBr
(£-tol)cSb
(£-tol)2
toluene
(£-tol)3As
(£-tol)cAs
Methyl Resonance (ppm)
2.33
2.38
2.43
2.40
2.42
2.39
2.40
2.29
2.37
2.37
2.36
2.31
AB Resonance (ppm)
7.11
7.32
7.35
7.32
7.32
7.24
7.30
7.00
7.20
7.20^
7.20
7.09
7.33
8.02
8.11
8.04
8.02
7.67
7.66
7.23
7.46
7.46
7.23
J (Hz) om
8
8
8
8
8
8
8
8
8
—
8
8
^All spectra were obtained on a Varian Associates XL-100-15 spectrometer using CDCI3 as the solvent. Chemical shift values are reported downfield from TMS as an internal standard. ^The chemical shift values are concentration dependent to + .02 ppm. ^The ring protons in toluene show only a single absorption.
43
TABLE 5
"TI NMR CHEMICAL SHIFT DATA FOR LIGAND EXCHANGE SYSTEMS^
Compound
M^Sb
M^TSb
M3T2Sb
M2T3Sb
MT^Sb
T^Sb
M3Sb
M2TSb
MT2Sb
T3Sb
C3M2Sb
C2M3Sb
C^M, Sb 4
C2MSb
C^M2Sb
T3AS
T^As
M3AS
• — •
Methyl Resonance (ppm)
0.60
0.84
1.12
1.40
1.64
2.06^
0.58
0.78
1.01
2.07^ •
1.27
0.95
0.75
1.09
0.90
2.17^
2.07^
0.78
AB Resonance (ppm)
c
c
c
c
6.96 7.60
c
c
6.96 7.47
7.04 7.42
6.94 7.35
All spectra were obtained on a Varian Associates XL-100-15 spectrometer using benzene-d5 as the solvent. Chemical shift values are reported downfield from TMS as an internal standard. Notation for compound nomenclature: T, p-tolyl; M, methyl; C^, trifluoromethyl. AB resonances were not assigned. "Values are for the £-methyl group on the tolyl ring.
TABLE 6
13 C NMR CHEMICAL SHIFT DATA FOR £-TOLYL COMPOUNDS^
44
Compound
Ph3As
Ph3Sb
Ph3P^
Ph3Bi*
Ph^As
Ph^Sb
Ph^Sb^
(£-tol)3As
(£-tol)3Sb
(£-tol)3P^
(£-tol) cAs
(£-tol)^Sb
Toluene
Benzene
Solvent
CHCI3
CHCl 3
CHCI3
CHCl
CHCl
CHCl
CS2
CHCI3
CHCI3
CHCI3
CHCI3
CHCl
CHCl 3
CHCI3
«c,
139.3
139.0
137.2
136.9
131.3
146.3
136.2
134.6
134.2
136.3
144.6
125.5
128.7
502(0^)
133.3
135.8
133.6
130.3
134.6
134.4
134.4
129.1
135.8
133.5
133.3
135.6
128.3
128.7
603(0^)
128.1
128.1
128.4
127.5
131.5
127.2
127.3
126.4
129.3
129.1
129.0
129.1
129.1
128.7
5C, 4
128.3
128.4
128.5
130.2
132.5
127.7
127.7
138.0
137.8
138.1
137.6
138.5
137.7
128.7
5C
20.9
21.3
21.1
21.1
21.4
21.2
All spectra were obtained on a Varian Associates XL-100-15 spectrometer using CDCI3 as the solvent. Chemical shift values are reported down-field from TMS as an internal standard. ^L. F. Johnson and W. C, Jankowski (Eds.), "Carbon 13 NMR Spectra", Wiley Interscience, New York, N. Y., 1972. ^0. A. Gansow and B. Kimura, Chem. Commun., 1970, 1621. ^Ref. 14; the C, resonance was not observed in CS_ solvent.
45
E Q.
CO
CO o
• mmim
E o
O
3. 0-
2. 0
1 • Ov
o M3SbX2 A T3SbX2
3.4 T 3.9
1 4 . 4
Electronegativity Fig. 6 — H NMR Chemical Shift vs Electronegativity for the
Me3SbX2 and (£-tol) SbX2 Series.
46
for the mixed alkyl-aryl species produced in the ligand exchange
studies. Table 5 shows that as the number of £-tolyl groups increases
in the mixed species, the resonance field strength of the remaining
methyl groups decreases. This result is expected since the £-tolyl
group is electron withdrawing with respect to the methyl group, and
consequently the methyl protons are deshielded by the addition of
£-tolyl groups, leading to the observed downfield shift. A plot of
the H chemical shift of the methyl protons vs . the Sanderson
59 equalized electronegativity (Figure 7) was linear for both the
Sb(V) and Sb(III) series of mixed ligand compounds. The pentacoordinate
series showed a change of 2.26 ppm per unit change in electronegativity
while the tricoordinate series had a change of 1.17 ppm per unit
change. These results indicate that the electron density at the
methyl carbon center is more sensitive to substituent effects for the
Sb(V) compounds.
The substitution of a £-tolyl group for a methyl group should pro
duce a larger relative change in the Sb(III) compounds, where the
decrease in electron density will be shared equally by three bonding
sites, rather than by five sites as in the Sb(V) series. Since the
observed change is greater in the Sb(V) series, a plausible explana
tion is that the lone pair of electrons in the Sb(III) compounds play
an integral role in the molecular orbital scheme and provide
additional electron density to the metal center.
13 It is interesting to note some trends in C NMR data for tri- and
pentaaryl congeners in Group Va. Bearing in mind the problems often
associated with simple chemical shift arguments, it is generally found
47
1.5 .
E a
1.0,
CO
o
"E 0)
JZ O
0 . 5 i f - '"
2.8 3.0
Electronegativity
3.2
Fig. 7 — H NMR Chemical Shift v£. Electronegativity for Mixed Alkyl-Aryl Antimony Compounds.
48
13 that C shieldings decrease with increasing electronegativity of
substituent atoms directly attached to the carbon atom. The plot
of the C^ chemical shifts vs . Sanderson*s equalized electronegativity
values is shown in Figure 8, for both the triphenyl- and tri-£-tolyl-
Group Va species. The value for the phenyl group was initially used
as the electronegativity value for the £-tolyl group. The fact that
the slopes are parallel indicates that the shielding effects are similar
for both systems. An increase in metal atom electronegativity de-
shields the C-. carbon atom. Since the slopes are parallel, it is
possible to derive a value for the Sanderson electronegativity for
the £-tolyl group. An analysis of the data produced an equation for
the linear plot for the triphenyl species of
6 = 11.09 (electronegativity) + 100.50
and using the same slope and intercept for the £-tolyl data yielded
a value of 2.98 for the equalized electronegativity of the £-tolyl
group. This value is, as expected, intermediate between that for a
phenyl group, 2.32, and for a methyl group, 2.62. The presence of
a methyl group para to the C. carbon causes an increase in shielding
relative to the phenyl analog, and hence the chemical shift is de
creased. This is consistent with the inductive effect of the methyl
group which affects the position para to itself most strongly. This
is further supported by the chemical shift data for the C^ position,
where the C shift values are larger by about 10 ppm for the £-tolyl
series that the phenyl series. Indicating increased shielding, and
therefore, increased electron density at the C, position for the tri-
49
140
E Q. CL
CO
"5 o •g 13 5 <D
O
o
130 3.0 3.5
(p-toDaM
4.0
Elec t ronegat iv i t y
13 Fig. 8 — C Chemical Shift v£ Electronegativity for the Triphenyl and Tri-£-tolyl Group Va Compounds.
TEXAS TECH LIBRARY
50
phenyl species in Group Va. This model is consonant with the resonance
structures available for toluene.
There are no observable trends for the pentacoordinate species of
arsenic and antimony. The C^ shift values for both phenyl and £-
tolylantimony species indicate much less shielding than in the
corresponding arsenic(V) compounds, and either a large change in
electronegativity or a difference in orbital overlap may explain this
anomaly. Evidence from the lack of decomposition of the pentaaryl-
arsenic species indicates a stronger As-C sigma bond and concurrent
increased electron density at the C, position, resulting in increased
shielding.
Exchange Reactions
Ligand exchange reactions occurred between any two Sb(V) species
mixed in d -benzene solution as well as for Sb(III)-Sb(V) species.
The Sb(V)-Sb(V) species reached an equilibrium which is temperature
dependent. No exchange occurred when one or both of the species in
solution was either a tri- or pentacoordinate arsenic compound.
A. Sb(V)-Sb(V) Exchange Reactions
The initial H NMR spectrum of the Me-Sb - (£-tol)cSb mixture
(0.10 mmol and 0.10 mmol in d -benzene) consisted of singlet resonances
at 0.58 ppm (methyl hydrogens) and 2.06 ppm (£-tolyl-methyl hydrogens)
and an AB pattern centered at 6.91 and 7.57 ppm (£-tolyl ring hydrogens)
At 25", the onset of exchange was indicated within 15 minutes by the
appearance of new methyl resonances at 0.84 and 1.64 ppm. The reaction
51
was monitored for 19 days at room temperature or until the spectra
ceased to change for 30 minutes during the experiments at elevated
temperatures. The plot of mole fraction v£. time for all species
containing methyl groups is shown in Figure 9 for the reaction at
75 C, showing the change in methyl concentration during the course
of the reaction. All of the concentrations were measured directly
by proton NMR.
1 *
While the original H NMR spectrum of the TcSb-M^Sb mixture had
only a single methyl resonance (6 = 0.58 ppm) due to M(.Sb, there were
four new singlet resonances in the methyl region at equilibrium.
These appeared stepwise, and in pairs at 0.84 and 1.64 ppm, and at
1.12 and 1.40 ppm. The methyl resonances were identified as arising
from M^TSb (0.84 ppm), M3T2Sb (1.12), M2T3Sb (1.40) and MT^Sb (1.64).
A plot of chemical shift v^ n, the number of £-tolyl groups present in the pentacoordinate M^ T Sb compounds (n = 0-4) is linear with a
_)—n n
change of +0.27 ppm for sequential replacement of a methyl group by a
£-tolyl group as shown in Figure 10.
For an exchange reaction involving two pentacoordinate species,
ten processes are required to fully describe the equilibrium state. In
the exchanging system of McSb and T^Sb, the following ten reactions
represent the total scrambling process.
(1) T^Sb + M^Sb = MT^Sb + M^TSb
(2) MT^Sb + M^Sb = M2T3Sb + M^TSb
*For brevity and conciseness, the following abbreviations will be used
F hereafter: £-tolyl, T; phenyl, P; methyl, M; and trifluoromethyl, C .
52
(3) M2T^Sb + M Sb = M T Sb + M.TSb 5 - ^ 3 2' " 'W
(4) M3T2Sb + M^Sb = 2M^TSb
(5) M^TSb + T Sb = M^T2Sb + MT^Sb
(6) M3T2Sb + T Sb = M2T3Sb + MT^Sb
(7) M2T3Sb + T Sb = 2MT Sb
(8) M^TSb + M2T3Sb = 2M^T2Sb
(9) M^T Sb + MT^Sb = M3T2Sb + M2T3Sb
(10) M3T2Sb + MT^Sb = 2M2T Sb
where the overall stoichiometry is
4M^Sb + 4T5Sb = 2M^TSb + 2M3T2Sb + 2M2T3Sb + 2MT^Sb
The initial reaction must involve the two distinct pentacoordinate
species in solution, M^Sb and T^Sb, with the appearance of MT,Sb and
M.TSb as products of the forward reaction of eq. 1. These products
undergo further exchange by the forward reactions of eq. 2 (MT,Sb) and
eq. 5 (M^TSb), with eq. 9 also contributing. If the forward rate
constant of eqs. 2 and 5 are the same, M^T2Sb and M^T-Sb will be pro
duced without a decrease in the concentration of M/TSb and MT.Sb since
the former is produced as the latter reacts, and vice versa. By the
time the concentrations of M^T^Sb and M^TpSb have increased enough to
become involved in the reaction scheme, the concentrations of McSb,
TcSb, M,TSb and MT,Sb are relatively equal; consequently M2T3Sb under
goes exchange by the forward reaction of eqs. 3 and 8 while the M3T2Sb
exchange proceeds for the same reason via eqs. 6 and 10. As the con
centrations of the mixed species increase, the system begins to rapidly
approach equilibrium with eqs. 8, 9 and 10 being the predominant path-
53
o
O
o
/—\ CO (1) 4-1 3 C
•H S ^ . z
0) B
.H H
O i n
o
o i n r^
•U)
rt
.o to
m H
+ .o CO
m S u o
U-l
C o
-r-i • U
rt V4
^ Ti r-\ •H 3 cr w « 4 - l
o 0) 4-)
rt p^
1 1
ON
• 60
•rA ^
(uoTZiDiBj.^ 3IOH) uoTijpaiuaouoo -[^H^^W
54
E a a
CO
"co o
E CD
O
1.5_
1 0 .
0 .5
S b ( V )
S b ( i l i )
0 -r 3
umber of p-Toly! Groups, n Fig. 10 — H NMR Chemical Shift of Methyl Groups vs_. Number of
£-Tolyl Groups, n, for M T Sb and n^_ T Sb Compounds.
55
way. While there is no way to obtain specific rate constants for all
10 reactions, the maximum in the concentration of M,TSb and MT,Sb 4 4
occurs because the forward reaction of eq. 1 is faster than that of
the other equations. The rate of appearance of MT,Sb is essentially
the same as that for the disappearance of McSb, while the appearance
of M^TSb is slower. The subsequent reactions of M.TSb, then, must
occur more rapidly than those for MT,Sb, therefore k„<k^. Since the
4 ' 2 5
rate of appearance of M3T2Sb and M2T3Sb are equal, one can estimate
that V.^'^As.r, kgO-k , and k,'\>k .
Experiments were performed to establish that the exchange process
actually was an equilibrium situation. After the reaction had ceased,
the NMR sample tube was opened on a vacuum line, and all volatile
components were transferred by high-vacuum technique to another NMR
sample tube which was subsequently sealed. The "H NMR spectrum showed
the presence of McSb, M/TSb, M^Sb, M^TSb, toluene and trace amounts
of MT2Sb, MoT2Sb and M2T3Sb. The toluene and tricoordinate mixed
ligand species were generated through decomposition (vida infra).
Vapor pressure measurements indicated the presence of either ethane
or methane in the mixture. A portion of the volatile material
collected after passing through a -75° trap showed H NMR resonances
for McSb, M2TSb and toluene. The original NMR sample tube was re-
sealed after addition of more d -benzene solvent and TMS. The brown
oily residue regenerated the volatile components, albeit in small
concentrations. The M2T.,Sb and M^T^Sb very likely have a relatively -2 -1
high vapor pressure at room temperature (ca. 10 - 10 Torr) and the presence of these compounds was probably due to transfer of vapor
56
through sublimation during the heating done to assure complete trans
fer of the volatile components. These latter compounds were present
at about 3% of the M^Sb concentration.
Attempts were made to synthesize the mixed aryl-alkyl species
to verify the spectral assignments. In an attempt to prepare MT.Sb,
methyllithium was added to (£-tol)^SbCl in benzene under inert
conditions and reflexed for 30 minutes. A H NMR spectrum showed
the presence of the entire ensemble of exchange products. The only
product that could be isolated was (£-tol)^Sb, presumably formed by
2 the mechanism postulated by Wittig in the case of Ph.SbCl reacting
with CH3Li. Further reactions were attempted in benzene solvent;
(£-tol)3SbCl2 + 2CH3Li -> (£-tol)3(CH3)2Sb
and
(CH3)3SbCl2 + 2(£-tol)Li -> (CH3) (£-tol)2Sb
however in each case only an inseparable mixture of products was ob
tained. In each reaction, sample was removed directly from the
reaction mixture and the H NMR spectrum obtained. All members of
the expected ensemble of products were obtained, arising from the
exchange process occurring as the mixed ligand products were formed.
The extreme lability of the mixed species prohibits their isolation
except perhaps in a low temperature synthesis.
29 McEwen reported the synthesis and isolation of Ph,(£-tol)Sb
by the hydrolysis of [P^TSbl~Li . The air and water sensitivity of
the alkyl-aryl compounds precludes the use of this method for the
production of the compounds used in this study. McEwen's subsequent
57
use of Ph^TSb in solvolysis reaction studies neglected the fact of
possible exchange in the reaction medium. Under conditions used by
33
McEwen for these reactions, Ph^Sb and T Sb undergo an exchange
reaction to produce at least 3 new resolvable £-tolyl-methyl peaks
which were unassignable. Tliis perhaps casts some doubt on the
rationale used to explain the distribution of products in their
study. This exchange system also shows that the mechanism does not
require a methyl group for one or both of the bridging ligands to
form the activated complex in the proposed mechanism (vida infra).
B. Sb(III-Sb(V) Exchange Reactions
The initial H NMR spectrum of the (CH3) Sb-(£-tol)^Sb mixture
(0.10 mmol and 0.10 mmol in d -benzene) consisted of singlet resonances
at 0.60 ppm (methyl hydrogens) and 2.07 ppm (£-tolyl-methyl hydrogens)
and an AB pattern centered at 6.94 and 7.59 ppm (£-tolyl ring hydrogens)
The mixture was allowed to react at room temperature, with 10% of
the M-Sb having reacted after 6 days. The experiment was monitored
for 130 dajTS, and the plot of mole fraction v£. time for all species
containing methyl groups is shown in Figure 11. At the completion of
the experiment, five new methyl resonances had developed and assigned
as follows: M2TSb (0.78), MT2Sb (1.01), M3T2Sb (1.12), M2T3 (1.40),
and MT-Sb (1.64). The assignments for the pentacoordinate compounds
were taken from the Sb(V)-Sb(V) exchange experiment, while the tri
coordinate species were assigned by comparison and analogy. The
change in chemical shift is +0.21 ppm as a methyl group is replaced
by a £-tolyl group for the M3_^T Sb series (n = 0-2). The change in
58
0)
E; •H EH
T o 00
o o o o
o ID (N
4J
fd
s> in
+
en
u o
c o
• H -P fO 5-1
J3 •H rH •H :3 CT"
W
4-1
o (U
(d
0) JH
• H
59
chemical shift with n is shown in Figure 10. There are a minimum
of 12 reactions required to fully explain the exchanging system which
is a complex system of competitive, consecutive second order reactions,
with overall stoichiometry 3T^Sb + 3M3Sb = T^MSb + T3M2Sb + M TSb +
MT2Sb + T3Sb.
1) M^Sb + T^Sb = M2TSb + MT,Sb
2) M2TSb + T^Sb = MT2Sb + MT,Sb
3) MT2Sb + T^Sb = T3Sb + MT^Sb
4) MT^Sb + M3Sb = M2TSb + M2T3Sb
5) M2T3Sb + M3Sb = M2TSb + M3T2Sb
6) M2T3Sb + T^Sb = 2MT^Sb
7) M T2Sb + M3Sb = M2TSb + M^TSb
8) M^TSb + M3Sb = M2TSb + M^Sb
9) MT.Sb + M2TSb = M2T3Sb + MT2Sb
(10) M2T3Sb + M2TSb = M3T2Sb + MT2Sb
(11) M T2Sb + M2TSb = M^TSb + MT2Sb
(12) M TSb + M2TSb = M^Sb + MT2Sb
During the course of the experiment, the first products obtained were
M2TSb (0.78 ppm) and MT,Sb (1.64 ppm), arising from reaction 1, by
necessity the first reaction to occur. At this point reactions 2
and 4 are initiated, the first generation competing reactions. The
more rapid increase in M^TSb concentration than that for MT.Sb in
dicates that reaction 4 occurs faster than reaction 2, as reaction
2 depletes MT2Sb while producing MT,Sb and the converse is true for
reaction 4. Since MT.Sb and M2TSb are produced at the same rate by
reaction 1, the difference in rate of production clearly shows k,>k2.
60
The apparent inconsistency that the appearance of M T Sb is slower
than that for MT2Sb arises from reaction 6, where two five coordinate
species are exchanging. From the initial reaction times for 10%
completion, 15 min in the Sb(V)-Sb(V) system vs. 6 days in the Sb(III)-
Sb(V) system, it is apparent that the rate constants are much
larger for Sb(V)-Sb(V) exchange. The difference of several orders of
magnitude between k^ and the other Sb(III)-Sb(V) rate constants
reduces the apparent rate of formation of M T^Sb. As a consequence of
reaction 6 and the lack of ability to measure the production of T„Sb
in reaction 3, there is no way to determine the relative magnitudes
of the second generation consecutive reaction rate constants k., and
k^ or their relationship to k^, k2 and k,. The analogy may be made
to k,>k_, however, and one might expect kj.>k. . The rapid increase
in M^T concentration with subsequent decrease as the M. Sb concentra
tion approaches zero, allows reaction 2 to predominate and continue
to produce MT2Sb while the production of MT,Sb slows significantly.
In the absence of any M.,Sb, however, the production of M„T^Sb must
occur by reaction 10. Reactions 9-12 occur slowly, though they
must be the source of MT2Sb production which occurs even after the
depletion of T^Sb and cessation of reaction 2. As a result, one
sees that kg and k ^ are about equal, with k j and k^2 being even
smaller (i.e. - M^TSb and M^Sb are not produced). Further, the same
argument may be used to place k. and k^ in the range of k-. and k-2'
The relative order of the rate constants then, is k >>k.. >k,>k >kc
The McSb-T3Sb exchange system reacted in much the same way as the
61
M3Sb-T^Sb system. The original H NMR spectrum of the mixture
(0.10 mmol and 0.10 mmol in d -benzene) consisted of singlet resonances
at 0.58 ppm (methyl hydrogens) and 2.06 ppm (£-tolyl-methyl hydro
gens) and an AB pattern centered at 6.96 and 7.45 ppm (£-tolyl ring
hydrogens). The experiment was monitored for 146 days at room
temperature and for 15 days at 70°. At 70°, the first evidence of
reaction was visible 8 hours after mixing by the appearance of a new
singlet resonance at 0.84 ppm, due to M.TSb. The resonance due to
MT2Sb (1.01 ppm), the second product of the initial reaction was
evident after 21.5 hours. The difference in time for the appearance
of the two compounds produced at the same rate is due to M,TSb having
four times the methyl area that MT„Sb has, therefore the MT2Sb
resonance is not resolvable at the same concentration as that for
M.TSb. After 43 hours, the peak at 0.78 ppm arising from M2TSb is
apparent, as is that from M^Sb (0.60 ppm). After 112 hours, the
M.,T„Sb peak is apparent (1.12 ppm) and after 216 hours the M2T3Sb
peak is present (1.40 ppm).
A set of reactions similar to those for the T^Sb-M3Sb system
may be written to describe the exchange system as the reaction pro
ceeds to equilibrium, with overall stoichiometry:
3M Sb + 3T Sb = M^TSb + M3T2Sb + M2T3Sb + 1^2^^ "*" ^2"^^^ " ^3^^*
(1) M^Sb + T3Sb = M^TSb + MT2Sb
(2) M^Sb + MT2Sb = M.TSb + M2TSb
(3) M^Sb + M2TSb = M^TSb + M3Sb
(4) M^TSb + T3Sb = M3T2Sb + MT2Sb
62
(5) M3T2Sb + T3Sb = M2T3Sb + MT2Sb
(6) M2T3Sb + T3Sb = MT^Sb + MT2Sb
(7) MT^Sb + T3Sb = T3Sb + MT2Sb
(8) M3T2Sb + M^Sb = 2M,TSb
(9) M^TSb + M2TSb = M3T2Sb + M3Sb
The first products to appear were generated by reaction 1,
necessarily the initial reaction. Reactions 2 and 4 may now pro
ceed to produce M2TSb and M3T2Sb, respectively. The appearance of
M2TSb before M_T2Sb indicates that the forward rate constant for
reaction 2, k , is larger than that for reaction 4, k,. Further
more, k. must be of the same magnitude as k^ since M^Sb is produced
as rapidly as M2TSb. Reactions 2 and 4 regenerate M/TSb and MT2Sb
respectively, therefore the M.TSb concentration increases slowly
until the M_TSb concentration has built up enough to make reaction 9
an important pathway also. The apparent slow rate of appearance of
M«T«Sb is attributed to reaction 8, which is very fast compared to
the Sb(III)-Sb(V) exchange reactions. Reaction 5 is inconsequential
until the M-T. Sb concentration builds up, as is reaction 6 also.
There is no indication that M.TSb is ever produced, probably due to
depletion of T. Sb earlier in the reaction scheme. Of course, all of
the intermediate exchange reactions for the Sb(V)-Sb(V) system are
scrambling the pentacoordinate species during the Sb(III)-Sb(V)
exchange. The relative order of rate constants may be tentatively
assigned as follows:
kQ»kT >k„'\ k„'\'kj>k,'\/k 'vk 'vk_ o 1 ^ 3 9 4 5 6 7
63
The relative order of k^, k^, k^, and k^ are indeterminable from the
data presently available, however, they are all certainly slower than
k^, k2, k^, and kg.
At the end of the experiment, the concentration of all of the
tricoordinate species are larger than those for the pentacoordinate
compounds. The samples in all exchange experiments undergo some dis
coloration, with the most severe changes occurring after long periods
of time at elevated temperatures. The solutions, which are nearly
colorless at inception, become a dark yellow or brown color, with
formation of some solid material. This is attributed to decomposi
tion of the pentacoordinate compounds in a formal oxidation-reduction
reaction to form Sb(III) products as well as polymeric residue and
other unidentifiable products. Further evidence for this explana
tion comes from the appearance, after extremely long periods of time
(ca. 2 months, of methyl proton resonances for MT^Sb, M^TSb and M^Sb
in the Sb(V)-Sb(V) exchange systems. Also, storage ampules of M^Sb
slowly develop a slight yellow color and must be refractionated to
remove the M^Sb that has been produced.
A third Sb(III)-Sb(V) exchange experiment was conducted between
(CF. )«Sb and (CH3)cSb in d -benzene solvent. The original spectrum
had a singlet at 0.64 ppm arising from M^Sb. Upon the initiation of
the exchange process, new peaks appeared at 0.75 and 1.09 ppm, ten-
F F tatively assigned to M,C Sb and MC Sb respectively. These resonances
decreased in intensity as peaks at 0.97 and 1.27 ppm appeared. At this
time the sample tube had turned yellow and crystalline material was
, present as well as an immiscible darker yellow liquid layer, more
64
dense than benzene. The final spectrum had resonances at 0.66 and 0.90
ppm. Apparently, the perfluoromethylated pentacoordinate species
are either unstable at room temperature or light sensitive, causing
rapid decomposition after production. The final assignments for
the d NMR resonances in the mixture (values given as ppm) are
M^Sb (0.64), M3Sb (0.66), M^C^Sb (0.75), M2C^Sb (0.90), M3C2Sb (0.95),
F F F
MC2Sb (1.09) and M2C3Sb (1.27). The MC.Sb species was evidently never
produced in the reaction scheme. A mixture of (CF3)3Sb and (CH3)3Sb
did not react at room temperature after one week. The failure of
this reaction to proceed hampered the previous assignment of the H
NMR resonances, as the only products would have been M^C Sb and F
MC2Sb for the above mixture. The above assignments are only tentative
and further experiments might be carried out to help elucidate this
complex system. C. As-Sb and As-As Exchange Reactions
Several experiments were conducted in the area of As-Sb exchange,
however exchange products were identified in only the T As-Mj-Sb
reaction mixture. In case that the difference in metal atom radius
between As and Sb prevented good overlap for formation of the
transition state intermediate. As(III)-As(V) and As(V)-As(V) exchange
systems were also attempted. In no As-As system did any exchange
occur.
A mixture of T^As and M^Sb (0.1 mmol & 0.1 mmol in d -benzene)
had an initial H NMR spectrum with a methyl resonance (M^Sb) at 0.58
ppm, and a £-tolyl-methyl singlet at 2.06 ppm and an AB pattern
65
centered at 6.95 and 7.37 ppm (T^As). After 4 months at room tempera
ture, peaks were evident for M3Sb, M2TSb, and M^TSb. The M3Sb
concentration was 50% of the M^Sb concentration, the M^TSb was 11%
and M2TSb concentration was 13% of that for M^Sb. There were no
other products present in the mixture after 25 days at 50° sub
sequently followed by 9 more days at 70°.
A mixture of M^As and T^As would have helped discern if lack
of overlap hindered the M3Sb-T As exchange, however the synthesis of
M^As could not be effected, as described in the experimental chapter.
As a second choice, Me3AsBr2 (0.1 mmol) was mixed with T As (0.1
mmol) in d -benzene, resulting in two singlets, 2.51 ppm (M3AsBr2) and
2.06 ppm (£-tolyl-methyl) and an AB pattern centered at 6.95 and
7.39 ppm (£-tolyl ring hydrogens). No exchange had occurred after 1
day at room temperature followed by 16 hours at 50°. After 20 days
at 50°, an apparent reaction had taken place and an immiscible brown
liquid was in the bottom of the sample tube. The heating had caused
the thermal decomposition of the Me.AsBr^ to MeAsBr and CH^Br, shown
by singlets at 1.43 and 1.96 ppm respectively. The mixture was heated
for 9 days longer at 70°, with no further spectral changes, indicating
that no ligand exchange had taken place.
A mixture of M.,As and T^As (0.1 mmol and 0.1 mmol in d -benzene)
had a methyl singlet at 0.78 ppm, a £-tolyl-methyl singlet at 2.07
ppm and an AB pattern at 6.94 and 7.35 ppm. There was no spectral
change after 3 days at room temperature followed by heating at 50°
for 17 days and at 70° for 9 days. The M^As-T^As mixture under the
same conditions underwent no exchange, indicated by a lack of any
66
spectral change. The initial •'"H NMR spectrum had a methyl singlet
at 0.78 ppm, a £-tolyl-methyl singlet at 2.17 ppm and an AB pattern
at 7.05 and 7.42 ppm.
The partial exchange in the As(V)-Sb(V) system and complete
lack of exchange in any As-As system seems to indicate that the
major difference in reactivity in As and Sb for the exchange process
may arise from differences in covalent radius of the metal atom.
The Sb covalent radius is larger than that for As (1.40 A vs. 1.20 A),
and using a covalent radius of 0.77 A for carbon, the Sb-CH covalent
bond length is 2.17 A, compared to 1.97 A for the AS-CH3 bond.
62 Zahrobsky has developed a model for predicting the stereochemistry
for multi-ligand complexes, where the principal region of steric in
fluence of a bonded ligand atom is considered to lie within a "steric
angle", 0, defined by 0 = 2 arcsin (BC/AC) where BC is the Van der
Waals radius and AC is the covalent bond length. An estimate of
o
2.0 A was given as the Van der Waals radius for the methyl group.
For the arsenic-methyl bond, the Van der Waals radius (2.0 A) is o
greater than the covalent bond length (1.97 A) and 0 is indeterminant.
Physically, this means that the repulsive forces greatly hinder
the formation of an As-C bond in the activated complex in the transi
tion state, and therefore the arsenic containing systems exchange
only slightly, if at all.
D. Kinetics and Mechanism
An exchange reaction may be characterized as two consecutive S I
substitutions or as a concerted S-.2 process. All evidence from this N
67
work indicates that the mechanism involved here is an S ,2 sub-N
stitution with simultaneous ligand exchange between two metal centers.
Kinetic studies have shown that the initial reaction, at least, is
first order in each of the two reacting species, and second order
overall. The postulated exchange mechanism proceeds through a
transition state consisting of a di-bridged intermediate, with an
increase in coordination number for each metal atom. The concept of 19
a bridged dimer activated complex is not new. As stated. Van Wazer
23 and Barker each have proposed an associative process whereby bridging
ligands hold two metal centers together in the transition state.
The second order rate constant for the Me„Sb-SbCl~ exchange found by
Van Wazer"*" was 6.7 + .05 x 10~^ M'-' sec"-'" at 72°, with AH' = 18 kcal/
i 23
mole and AS' = -25 eu. The second order rate constant found by Barker
for SbCl. -SbClt. halogen exchange was 6.0 x 10 M sec at 68°, with
AHT = 15 kcal/mole and AS' = -29 eu. At 75°, the second order rate
constant for the initial exchange between T^Sb and M^Sb was 1.41 + .15
X lO"" M~''"sec~' with AH' =5.5 kcal/mole and AS r = -56.2 eu. These
data are consistent with the results from this study, in that no Me^Sb-(p-tol)«Sb exchange occurred, in the £-tolyl-methyl system,
3 3
Sb(III)-Sb(V) exchange was very slow and Sb(V)-Sb(V) exchange was
relatively rapid. This trend is expected from simple steric arguments
that the expansion in coordination is more facile for the Sb(V)
species than for the Sb(III) species. The formation of a rigid four-
center transition state from two fluxional molecules produces a
decrease in entropy. The particularly large negative entropy value
probably arises from the additional geometrical requirements placed
68
on the already constrained four-membered ring by the bulkiness of
the £-tolyl ligand, along with an almost certain change in solution
during formation of the activated complex. From the data previously
discussed, it can be seen that while the entropy becomes more negative
as the coordination number of the reactants increases, the values of
AF increases and AH' decreases, resulting in an overall increase
in the rate of ligand exchange. The value of AS' is considered to
be valid, and is comparable to that found for the scrambling of
dimethylamino vs . t-butoxyl ligands around titanium of -47 eu. From
the analysis of relative magnitude of the forward rate constants in
the Sb(III)-Sb(V) systems, it is apparent that the relative magnitude
of the individual rate constants decreases as the number of £-tolyl
groups on the tricoordinate species increases, also probably due to
steric interactions of the bulky £-tolyl in forming the activated
complex. The change in rate with coordination number of the reactants
suggests that the rate determining step in the mechanism is the forma
tion of the transition state intermediate, and not the subsequent bond
cleavage as suggested by Barker. If the limiting process is scission
of the bridged complex, then the nature of the ligand would be minimized
in determining the relative magnitude of the individual rate con
stants. All kinetic data for the initial reaction between T^Sb and
McSb are summarized in Table 7 and typical second order plots are
shown in Figures 12 and 13. The entire set of rate constants for all
10 reactions was not derived.
There are several sources of error inherent in the methods used
for this kinetic study. The measurement of small quantities of
69
reactants on the high vacuum line required pressures of 3.5 mm of
Hg for 0.10 mmoles of reactant, and there was some small uncertainty
involved in calibration of the vacuum line. Further, the method
for determination of the molarity of the solutions assumed all of
the NMR sample tubes had the same inside diameter and were pefectly
concentric as well. Another source of small amounts of error was
the assumption that all the NMR peaks had precisely the same width
at half-height, so that the ratio peak heights was the same as
the ratio of peak areas. The data at elevated temperatures was
generally within a 10% limit of confidence, while that at ambient
temperature lay within about a 20% limit. The sample left at room
temperature was susceptible to fluctuations in the ambient tempera
tures, resulting in data that was not quite as accurate. The values
4= =1= 4= of AF', AH' and AS' derived from the temperature dependence of the
reaction rates only had a 1-4 percent error, however. All error
calculations were done as previously described by the method of Beers,
In terms of the mechanism postulated here, the intermediate for
reaction 1 in the Sb(V)-Sb(V) system would be as shown below.
CH.3 CH„
Sb'
CH- CH,
' - ^ y " ; - ^
- C H 3 " "
" ^ Sb '' 1 " \
C^H^ ^ ^ 7
For a dissociative mechanism, the reaction would follow a first order
rate law, with generation of a free ligand in solution being the
rate determining step. Further evidence for the proposed mechanism
comes from the order of appearance of the products in the exchange
70
o O (D
- r
o o
c • • • M
E Ui
o in
<N
o 4J o rt 0)
c rt
X 1x3
CO i n
CO
m
u o
)-i 0)
13
O
na c o a
CO
I I
o
°o/t-o/i.
CM II r-i
O • <
to —i '^ o o
4-)
71
5 ^
*
T I M E , min.
Fig. 13 ~ Second Order Plot for M3Sb + TcSb Exchange, Ao =|= Bo
*f (x) = -z—7— In Bo-Ao Bo a
72
TABLE 7
KINETIC DATA FOR Sb(V)-Sb(V) EXCHANGE^
Concentration (m) TcSb M,Sb v^ -1 -1 3 J 5 K (l-mol -sec ) x 10
25°
0 .250 0 .250 .27 + . 0 6
0 .238 0 .260 .24 + . 0 8
0 .253 0 .240 ^29 + .12
Ave .27 + .09
60°
0 .247 0 .247 0 .93 + .09
0 .252 0.237 0.90 + .17
0 . 2 4 1 0 . 2 7 1 0.95 + .12
Ave 0 .93 + .09
75°
0 .268 0 . 1 8 1 1.43 + .33
0 .268 0 .268 1.30 + .10
0.225 0 .322 1.10 + .06
0 .203 0.406 1.80 + .07
Ave 1.41 + .15
AH"" = 5.5 + .2 kcal^ from k = (—•) exp AS'/R 'exp AH'/RT
AS"" = '56.2 + .2 eu.^ = (|) exp A F V R T
AG' = 22.2 + .2 kcal^
a b For the reaction: M3 + T5 = M^T + MT4. The reported value of k is the second order rate constant and all results are from a least squares fit to the data. ^These values are obtained from a linear least squares analysis of the rates as a function of 1/T.
73
reaction. In all cases, the first products evident resulted from a
one-ligand exchange. For the reaction involving T Sb and M^Sb, the
initial products were MT^Sb and M^TSb. Following this, M2T3Sb and
M^T2Sb appeared, with their respective concentrations increasing
at essentially the same rate. As shown in Figure 9, the products
appear stepwise, and in pairs, as expected from a series of one
ligand exchanges. For inter-molecular exchange, the mixed species
would not necessarily emerge in a stepwise manner at the same rate
for each pair as previously discussed. In all three Sb(III)-Sb(V)
systems, the order and rate of appearance was as expected for the
proposed mechanism, therefore the mechanism is the same regardless
of the oxidation state of the antimony.
In view of the preceeding kinetic and mechanistic arguments, a
complete reaction mechanism may be postulated in terms of a two metal
center di-bridged activated complex, where
RcSb + R'Sb = R.Sb^ ^ SbRl = R.R'Sb + R.'RSb 5 5 4 \j^i/ 4 4 4
is the initial reaction, and further exchange can be written generally
as
R, R» Sb + R' R Sb = R, R'Sb-^ ^SbR! R 5-n n 5-n n 4-n n ^"^ni-^ ^"^ ^
R, R' ,Sb + R! R .,Sb (n = 0-4) 4-n ni-1 4-n n+1
E. Equilibrium Studies
There has been a considerable amount of work done in the area of
IR 91 21 22 63 equilibria of exchange or redistribution reactions ' ' ' ' . For
most scrambling reactions, the ligand distribution is considered to be
74
largely statistical, and an "ideal" equilibrium constant, K^, has
been defined, corresponding to completely random distribution of
63 products. Van Wazer has proposed that the deviation from random
interchange is reflected in the free energy and may be calculated by
^^dev = '^'^ ^"^ (Keq/Ki)
where Ki may be calculated from statistical distribution values,
which are simply the binomial coefficients. The use of the above
equation produces values nearly completely attributable to non zero
values of the enthalpy and may be equated to AH° for any of the
individual reactions in the equilibrium process. The values obtained
above closely correspond to values for AH° obtained from
AH° = TAS° + AG°
where AS° = R In Ki for an individual reaction and AG° = -RT In Keq
for the same reaction. The use of this type of calculation would pro
vide another means for assessing the individual reactions present
in the overall equilibrium process.
During the course of this work, a myriad of attempts were made
to correlate equilibrium constants and temperature so as to complete
a thermodynamic analysis of the equilibrium. The deviations in Keq
at any one temperature were so large, however, that little reliable
data could be accumulated. The decomposition of the Sb(V) compounds
into Sb(III) species and polymer make the calculation of the equilibrium
concentration of T^Sb unreliable. For the overall reaction as pre
viously described for Sb(V)-Sb(V) redistribution,
Keq = [M^TSb]^[M3T2Sb]^[M2T3Sb]^[MT^Sb]^/[M3Sbl^[T3Sb]^
75
and Keq is sensitive to small variations in the equilibrium concen
tration of T^Sb. As outlined before, the equilibrium concentration
of T^Sb is calculated from the original reactant concentrations
and the equilibrium concentrations of the methyl containing species.
However, if reduction is occurring for the Sb(V) compounds, then
the relation that
[M3Sb]^ + [T3Sb]^ = [M3Sb]^ + [M^TSb]^ + [M3T2Sb]^ + [M2T3Sb]^
+ [MT^Sb]^ + [T3Sb]^
is no longer valid, leading to incorrect values for the T^Sb concen
tration at equilibrium. Other error may arise from difficulty in
measuring the original concentrations, as previously discussed.
A quantitative discussion may be made, however, with a view
toward determining the relative value of AH° and AS° for the reactions
Table 8 gives the approximate concentration of all species at various
temperatures, that is, calculated as if no decomposition occurred.
As the temperature increases, there is a general decrease in concen
tration of the M„T.^Sb and M.3T2Sb species, while all others increase,
causing an overall decrease in K . Table 9 gives the equilibrium
° eq
constants for the McSb-T^Sb exchange as a function of temperature for
the 10 individual reactions in the overall equilibrium system. Table
10 gives the thermodynamic parameters for the same individual reac
tions derived from second order plots where RT In K = TAS°-AH°.
Several values of Keq were discarded due to large deviations in the
plots, and these are denoted as such in Table 9-
Several observations may be made from this data. For the over-
TABLE 8
EQUILIBRIUM CONCENTRATIONS FOR Sb(V) SPECIES AT VARIOUS TEMPERATURES^'^»^
76
T(°C) M3Sb M^TSb M3T2Sb M2T Sb MT.Sb T3Sb keq
25
50
60
75
,010
,014
,026
036
.074
.079
.102
.148
.224
.262
.223
.206
425
332
301
186
.192
.223
.248
.299
.075
.089
.099
.124
5.8 X 10
9.7 X 10"
6.6 X 10
7.2 X 10-
^Concentration unit is mole fraction of Sb. T3Sb concentration was calculated neglecting any decomposition. ^The values in the Table are all from one sample at various temperatures. ^The value of Keq was calculated as described above.
all reaction, a plot of In K vs . 1/T yields a value of AH° = -24.4
+ 6.9 kcal/mole, AS° = 51.9 + 21 eu and AG° = -8.97 + .64 kcal/mole
at 25°C. From Table 10, the sums of the individual values yield
AH° = -32.51 kcal/mole, AS° = -73.95 eu, and AG° = -10.46 kcal/mole,
and all are within the limits given by the analysis of the overall
equilibrium constants. This indicates that the values obtained for
the 10 individual reactions are at least reasonably accurate.
The value of AG°^^ was calculated for the 10 reactions (Table
10) and all are quite small, and vary only slightly from the values
for AG° calculated at 25° from AH° and AS°. These results indicate
that the redistribution is not simply statistical. All but two of
the individual reactions are exothermic, with reactions 3 and 7,
both involving M2T3Sb, being endothermic. As a consequence, the
77
overall equilibrium system is exothermic and Keq decreases with an
increase in temperature. The change in free energy, AG°, is greatest
for the reactions involving the less substituted species, i.e. -
reactions 5, 2, 1, and 9 -, and smaller for the more mixed species,
i.e. - reactions 7, 4, 3, and 10. The conclusion that may be drawn
is that the system preferentially produces the more substituted
species M2T3Sb and M3T2Sb in an exothermic reaction from the less
substituted species M^Sb, T3Sb, M^TSb and MT^Sb. Interestingly,
reactions 2 and 5 have a larger AG° than reaction 1. This is due
to larger enthalpy change for reactions 2 and 5 than for reaction
1, but at 75°, the entropy factor reverses the order for the free
energy, so that reaction 1 is now more favorable. The equilibria,
then, depend a great deal on the entropy as well as the enthalpy.
For AH° = 0 in an isothermal equilibrium, AS° = R In K and the
experimental values of AS° calculated from the individual equilibrium
constants agree quite well from those calculated from a statistical
distribution. The overall process, then, seems to be somewhat
statistical, with deviations arising from the difference in stability
of the mixed ligand products. The increase in substitution enhances
stability of the species, and is very likely due to electronegativity
90
equalization in the products. Moreland , has suggested that the
redistribution approaches statistical behavior as the difference
in electronegativity decreases and our results suggest that the
electronegativity difference is the source of the non-zero value of
the enthalpy in the exchange reactions discussed here.
78
TABLE 9
EQUILIBRIUM CONSTANTS FOR M5Sb + TsSb EXCHANGE REACTIONS AT VARIOUS TEMPERATURES^
Reaction^ K(25°) K(50°) K(60°) K(75°) Ki^
1
2
3
4
5
6
7
8
9
10
nK^
1 8 . 9
1 6 . 4
3 .9
2 .4
7 . 8 ^
4 . 9
1 .1
1.6^
6.7
4 . 2
5 .5x10^
1 4 . 1
8.4
4 . 5
1.7
8 .3
3 .2
1.7
2 . 6 ^
4 . 9
1.9
9.9x10^
9 . 8 ^
4 . 8
2 . 9 ^
1.8
5 .5
3 .4^
2 . 1
1.6
2 .7
1.6
6.7x10^
9.9
2 .6
4 .6
3 .0^
3.4
2 .2
3 .9^
1.5
.87"^
.se"'
7.6x10^
25
10
5
2 .5
10
5
2 .5
2
4
2
6.25x10
^^alues were calculated from data in Table 8 where K^^ = n[products] / nrreactants]^. ^Reaction numbers refer to the reaction as written in the Sb(V)-Sb(V) discussion, pg. 50. ^Ki is calculated from statistical concentration values. ^These values of Keq were rejected due to large standard deviation from the other values.
79
TABLE 10
THERMODYNAMIC PARAMETERS FOR M3Sb + T3Sb EXCHANGE REACTIONS*
Reaction AH ob
AS ob
AG' AG dev
1. Mc + T^ = M.T + MT, 5 5 4 4
2. MT^ + M3 = M2T3 + M^T
3. M2T3 + M3 = M3T2 + M^T
4. M3T2 + M3 = 2M^T
5. M^T + T3 = M3T2 + MT^
6. M3T2 + T3 = M2T3 + MT^
7. M2T3 + T3 = 2MT^
8. M^T + M2T3 = 2M3T2
9. M^T + MT^ = M2T3 + M3T2
10. M T2 + MT^ = 2M2T3
-2.47
-7.07
0.74
-1.78
-7.79
-3.18
3.50
-4.53
-4.49
-5.44
-2.76
-18.08
5.22
-4.29
-19.6
•7.54
11.96
-12.20
-11.20
-1.65
-1.68
-0.82
-0.50
-1.95
-0.93
-0.06
-0.89
-1.15
-15.46 -0.83
0.165
-0.29
0.147
0.24
0.147
0.012
0.486
0.132
0.305
-0.439
^Data obtained from Keq values listed in Table 9. Values obtained by second law plots of In K vs_. 1/T. ^AG° computed at 25°.
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