GENERAL INTRODUCTION TO OCTAHEDRAL COBALT(III) SUBSTITUTION REACTIONS

The chemistry of metal complexes has been studied quite

extensively both in solid state and in solution. However cer­

tain aspects of solution chemistry of metal complexes will be

reviewed in some detailsforthe.~ake.of bravity and resemblence

of the present wor~. A scan through chemical literature reveals

lot of theoretical and experimental discussions on the reactions

S . 1-21 of co-ordination complexes in solution. everal recent rev1ew

are the indications of the yet unfulfilled aim which is to

bridge the gap between experimental findings and theory. With

the advancement of y~ars, as systematisation of ideas proceed

in the theoretical understanding of the processes, more and

more experimental data accumulate, a part of which strengthens

the theoretical frame work while other parts may demand its

overhaul.

Reaction in co-ordination comple~es in solution may be

broadly classified as follows:

i) Substitution reactions involving substitution of metal

or ligand, ii) Reactions of co-ordinated ligands, iii) Redox

reactions.and iv) Isomerisation and racemisation reactions.

Present works belong to the first category of reaction namely

ligand substitution reaction.

- 2 -

To find the most probable path ways through which the

reaction proceeds, considerable amount ~f work has been done

with octahedral complexes of Co(III), Cr(III), Rh(III), Ru(III),

Ir(III), Mo(III) and the square planer complexes of Pt(II),

Pd(II) and Au(III). Such complexes are relatively inert and

hence the slow ligand substitution reactions can be followed

with sufficient accuracy by using conventional titrimetric,

spectrophotometric, electrometric and polarimetric methods.

Fast reaction techniques like i) flow methods, ii) polaro­

graphy, iii) nuclear magnetic resonance, iv) flash photolysis,

and v) relaxation methods are available to study the very fast

reaction like formation and dissociation of Ni(II), Co(II),

and Fe(III) complexes22- 25 • Detection of transient interme-

·diates26 formed in the oxidation of ligand by Co(III) and

V(V), ligand exchang~27 · of Mn(II), Fe(II), Co(II), Ni(II) and

Cu(II) have been studied by adopting fast reaction techniques.

These methods have also been applied to study the fast reaction

of complexes of alkaline earth metals28 and lanthanides 29 •

In order to know the most probable pathway through which

the reaction is taking place, kinetists use to study the

following :

i) Number of elementary steps which constitute the overall

reaction e.g. stoichiometric mechanism, ii) analysis of each

individual steps named as intimate mechanism and lastly

iii) the magnitude of rate constants of each steps in terms

- 3 -

of bond making or bond breaking taking place in the reaction . .

Though kinetic studies pr6vide the most powerful method of

investigating detailed reaction mechanisms. However it is gene-

rally not possible to get exact information. And thus the pos­

tulated mechanism are essentially devised to explain the expe­

rimental findings.

Hughes and Ingold described organic substitution reactions

in terms of SN (Nucleophilic substitution) and SE (electrophilic

substitution) mechanism. In inorganic substitution reaction

replacement of one ligand by another in a complex, or one metal

ion by another is taking place. Following the organic termino-

logy developed by Hughes and Ingold, the inorganic substitution

reactions can be represented as follow

MXn + y MXn··l y + X • • • . .. (1)

MXn + M' ~ + M . . . • • • (2)

(Charges on metal atom are dropped for convenience)

Since our work is related only with nucleophilic substitu-

tion reaction we have not discussed the electrophilic reactions

here.

Mechanistically ligand substitution reactions can be cla­

ssified into two fundamental groups following the same termino­

logy as proposed by Hughes and Ingold. These are SNl (substitu­

:tion nucleophilic unimolecular) and SN2 (substitution nu~leophilic

- 4 -

bimolecular). These two may also be termed as dissociation and

displacement mechanism respectively. SNl.mechanism is a two

step mechanism and represented as

slow MX ::a. MXn-l + X n

fast MX l + y ) MXn-1 y ( 3) • • • n-

In the first step of the reaction leaving group is lost

and an intermediate of reduced co-ordination number is formed.

This step is rate determining step. In the second step the

intermediate species combines with incoming ligand in very fast

rate. SN2 mechanism involves a bimolecular rate determining

step in which nucleophilic reagent displaces the coordinated

ligand. Mechanism of this reaction can be represented as

slow fast MXn + y :::.. y ••• MXn-1 • • • X > MXn-1 y + X ... (4)

SN2 reactions are characterised by its intermediate with incr­

eased coordination number.

Besides, these two principle SNl and SN2 mechanism, few

ligand substitution reaction can mechanistically explained by

four centre mechanism (SF2) 30 • Practically all substitution

reactions can be conceived as acid-base reactions. and in

- 5 -

four-centre mechanisms two acid-base complexes simultaneously

exchange groups.

M - X + M' - Y--- • • • ( 5)

(Here charges and coordination numbers of metals have been

dropped for convenience). The main characteristics feature in

this mechanism is that no free X or Y ions are obtained in solu-

tion since the coordination number of each metal ion are incr-

eased by unity. Four-centre mechanism explained m~ny organic

reactions where central atom is carbon or non-metal 31 and reac-32 tions of many platinum complexes •

Even though two fundamental distinct mechanism may be

visulised for ligand substitution reaction, unequivocal assi-·

gnment of mechanistic pathway for particular reaction proves to

be extremely difficult. SN2 mechanism for base hydrolysis of

octahedral cobalt complex have been proposed by Ingold, Nyholm 80

and lobe. Again an alternative pathway which involves the for-

mation of conjugate base in a fast acid base equilibrium step

followed by rate determining dissociation of leaving group and

subsequent fast addition of solvent molecule giving the product

was suggested by Ba$olo and Pearson~ This mechanism is expressed

as SNl CB (nucleophilic substitution, unimolecular, conjugate

base) and rate expressions in the both case are same. Similarly

there are many reactions which are taking place in sever~! steps,

- 6 -

rapid formation of ion-pair equilibrium followed by dissociation

or displacement steps designated as SNl(IP) or SN2(IP) respec­

tively. The fact that a system exhibits first or second order

behaviour does not necessarily mean that an unambiguous reac­

tion mechanism can be thereby postulated. That is, it is not

correct to assume that a bimolecular reaction will show second

order kinetics and unimolecular reaction first order kinetics.

Hence the knowledge of the order of reaction must have to be

combined with additional informations before a mechanism can

be proposed with any confidence. These additional informations

are as follows :

a) Effect of charge, size and electronic structure of cen­

tral metal atom on the rate.

b) Influence of nucleophilic character of outgoing and

incoming ligand on the rate of reaction .

c) Sterle effect on reaction rate.

d) Effect of pH and ionic strength on the reaction rate.

e) Effect of temperature (i.e. heat of activation ~~

and entropy of activation 11s:F) on rate of reaction.

f) Effect of dielectric constant of the medium on the rate

of reaction.

However, it is no matter how much evidence can be supplied

to establish a particular mechanism and to say it proved. It is

very difficult to say that a reaction follows definitely through

- 7 -

33 SNl or SN2 path • Actually, mechanismsare likely to be theo-

ried which are not capable of proof but to capable of expansion

and revisions to include new experimental facts. The most pro­

bable mechanism is one that fulfills all the experimental facts.

SNl and SN2 processes of ligand substitution reactions

are characterised by virtue of their respective intermediates

in which metal atom has got decreased or increased coordination

number respectively. Now, at least, from the stereochemical

point of view SNl and SN2 reactions can be unambiguously dis­

tinguished. But_ with octahedral complexes of cobalt the forma­

tion of five,coordinated intermediates have been proposed with

a reasonable degree of certainity only in a few cases such as

( ) 3- ( - - - - )34 Co CN 5x X= Br, I , NCS, N3 , H20 , Co(NH3)4(so4)(NCS), 35

Co(CN) 4(so3)(H2o) 3- . and trans[Co(en) 2so3(0H)], trans[Co(en) 2

(H20)(so3)+] 36 • However, these intermediates are two short

lived to be isolated or directly detected. Same is applicable

for hepta-coordinated intermediates in SN2 ~eactions. Therefore

unequivocal assignment of mechanism to ligand substitution ' reaction has yet not been possible on the basis of observed

rate law. The error becomes complicated when we see that many

reactions occur following the intermediate mechanism between

SNI and SN2. Here the possibility of mixture of SNl and SN2

are not rare but~ in one, the participation of incoming group

which may be a solvent least affects the energetics of the system.

To remove this difficulties SNl and SN2 mechanism are .fur­

~her sub-grouped which are (a) SNl (lim), (b) SNl' (c) SN2

- 8 -

and SN2 (lim). These are described below in tabular form.

Criteria

Degree of bond breaking

in rate step.

Degree of bond making in

rate step.

Evidence of intermediate of reduced coordination number.

Evidence of intermediate

of increased coordination

number.

Large Large

None None to small

Definite Indefi-nite

None None

Appre­ciable

Appre­ciable

None

None

Large

None

Inde- Definite finite

37 Langford and Stengle further classified ligand substitu-

tion reaction into three stoichiometrically distinct pathway

and this classification were based on the mode of activation.

These are as follows:

Dissociation path (D) in which an intermediate with reduced

coordination number is produced. In the first step of this path

leaving group is lost. The total path can be described as

-X +Y MXn ---::.... ~ MX 1Y • • • ( 6)

-.::-- MX l ~ n-+X n- -Y

\where X is the leaving group and y is the entering group.

- 9 -

Associative path (A) is characterised by producing an inter-

mediate of increased coordination number. Ip the first step

entering ligand is bonded and process can be represented as

+Y

-Y

-X "< +X

• • • (7)

Lastly in the interchange path (I) or concerted path the leav­

·ing group moves from inner coordination sphere to outer coordi­

nation sphere and incoming group follows the reverse. No inter-

mediate is observed in the process and this interchange mechanism

~an be stated as

• • • • ( 8)

·It has been told earlier that thPee distinct path namely

D, A and I paths are based on the mode of activation. The modes

of activation of D path and A path are designated by 'd' and

'a' respectively. The term mode of activation means how the

activation energy is affected by the nature of entering group.

In d mode of activation the activation energy is solely de­

p~nds on the dissociation of M - X bond (where X is a leaving

group). Consequently rate is independent of the nature of the

incoming ligand. Again in a\mode of activation entering group

affects the activation energy. Because in the transition state

the entering group partially forms a new bond (MXn ••• Y) and

tHis bond formation hence the bonding energy contributed to I

the activation energy depends on the incoming ligand nature. I .

- 10 -

As a result the variation of rate is observed with the varia-

tion of entering ligand • It is interesting to note that two

modes of activation are possible in I-path. They are designated

as Id (dissociative interchange) and Ia (associative inter­

change) paths. In Id path there is a weak bonding with entering

group and activation energy is not appreciably affected by the

nature of incoming ligand. Whereas in Ia path there will be

substantial bonding to both the leaving and incoming group in

the intermediate state and bond making and bond breaking have

got the equal priority in the intermediate state. Thus entering

ligand plays a significant role in determining the activation

energy. However it is obvious that operationally clear cut

assignment of Id and Ia mechanistic labels to the substitution

r~actions would be most difficult. Nonetheless, for ligand

substitution reactions of most octahedral Co(III) complexes are

and Cr(III) complexes experimental evidences/strongly in favour

of Id and Ia mechanism respectively.

If we try to corelate the four mechanism namely SNl(lim)

SNl' SN2 and SN2(lim) developed in ligand. substitution reac­

tiom by following Hughes and Ingold scheme applicable for

organic reaction with the mechanismsproposed by Langford and

Stengle which are based on mode of activation, we find SNl(lim)

·corresponds to D path as because in both cases the evidence of

intermediate with reduced coordination number is their prime

,requirement. SN2(lim) parallels to A-mechanism because in.·both

- 11 -

rrite step cases/involves bond making and evidence for intermediate of

increased coordination number is definite. Id and Ia mechanism

can be labelled SNl and SN2 path respectively. The Id mecha­

nism in which the evidence of intermediate with reduced coor-

dination number can not be presented, but other requirements

of dissociation mechanism are satisfied like SNl mechanism.

Both in Ia or SN2 process rate determining step depends on

bond breaking and bond making equally. But intermediate of

increased coordination number cannot be evidenced.

Electrostatic approach toward the interaction between a

metal ion and ligands is known as crystal field theory. Place­

ment of donor atoms at different steriochemical position~will

perturb the d-orbital of first transtion metal ions to differ-

ent extent. Relative labilities of complexes of metal ions.

hence the rate of substitution reactions are now being expla-38 39 ined in terms of crystal field theory •. Orgel and Jorgenson

qualitatively applied crystal field stabilisation energy (CFSE)

to determine the mechanism of ligand substitution reactions.

The energy of five different d-orbital electro~

(dx2-y2 , d22 , dxy , dyz and dzx ) uhder the influence of

crystal field can be computed and the energy will be different

for the crystal field of different geometry. The difference

between C.F.S.E. of .. ground state (octahedral) and five-

·coordinated square pyramidal transition state (for

seven-coordinated pentagonal bipyramid transition state (for

SN2) is the loss in crystal field stabilisation energy ( ~Ea).

- 12 -

~Ea is also known as crystal field activation energy (CFAE).

In ligand substitution process 6Ea is a part energy contribu­

ted to the total activation energy. High value of 6Ea means

greater is the contribution to total activation energy and it

implies a slow reactions0 whereas a zero or negative value

means a relatively fast reaction without any additional con­

tribution to the normal activation energy. ~Ea values for SNl

and SN2 mechanism for octahedral complexes of dn (n = 1 to 9)

like system have been calculated and it was found experimentally 0 1 2 that in strong field d , d and d systems are relatively

1 b .l th d5, d4 , d3 and d6 t D · d f a 1 e an sys em. ecreas1ng or er o

lability is d5 > d4 > d3 > d6 and its true for SNl or SN2

mechanism. But in the weak fields~ which correspond to outer­

orbital complexes (d4 to d10 ) , the only system for which reac-

tion are predicted to be slow by either of these mechanism are

d3 and d8 • It should be remembered that crystal field activation

energy are only a part of bonding energy contributed to the

total activation energy. C.F.A.E. of Ni(II) (d8 ) and Cr(III)

(d3) are equal to 4.26 Dq. for pentagonal bipyramid geometry40 .

'But Ni(II) reacts faster than Cr(III). Actually larger contri­

bution to the total activation energy is due to the change in

metal ligand attractions, ligand-ligand repulsions etc.

The main characteristic features of all catagories of

mechanism discussed so far for ligand substitution reaction

are factually different degrees of bond breaking and bond ·for­

mation. In some cases bond breaking are getting much importance

- 13 ·-

where bond making dominates the other type of reaction and

in some cases both are given equal importance. These bond

making and bond breaking proces~are coherently related with

the sizes and charges of central metal atoms and incoming

ligands. Effect of sizes and charges of central metal atom

and incoming ligand on the rate of ligand substitution reac­

tions involving SNl and SN2 path have been discussed nicely

by Basolo and Pearson41 • According to them an SN2 mechanism

is favoured when the size and charge of the central metal atom

increases whereas increased size of leaving group and other

ligands helps to follow the SNl path.

Now following discussion is mainly concerned with Co(III)

(d6) system. Octahedral Co(III) complexes are relatively inert

and kinetics can be followed by conventional techniques. Ligand

substitution reactions for octahedral cobalt(III) complex have

been studied extensively and reaction may be represented as

••• ( 9)

(Charges are dropped for convenience)

Here Y be a solvent or any other nucleophile. In aqueous

medium and in the absence of any other nucleophile H20 and

-IOH may be regarded as competing nucleophiles present in the

system and then the reactions is known as hydrolysis. The rate I hydrolysis reaction may be written texpress ion for the as

d[Cornplex]T ------ • • • (10) dt

- 14 -

, When the reaction is studied in aqueous medium below pH 3 the

reaction product is usually aquo-complex and it is designated

as aquation or acid hydrolysis. If the reaction is carried out

in pH range 3 < pH < 10 it is termed as simple hydrolysis and

the reaction products will be the equilibrium mixture of aquo

anrl hydroxo complexes. Above pH 10 the reaction is known as

base hydrolysis and the product is usually hydroxo complex.

However,if X is water andY is any other nucleophile, the reac­

tion is called anation reaction. Now we will discuss i) Acid \iii)

hydrolysis, (ii) Base hydrolysis and/Anation reactions of

Co(III) complexes successively.

Acid Hydrolysis - Aquation of pentammine cobalt(III) complex

may be represented as follows :

3+ m-+ H20 ~ [Co(NH3) 5(H20)] + X • • • ( 11)

Different mechanisms have been suggested by different authors

with the variation of xm- in complex. Chan 41 proposed SN2 . m- - - - -mechan1sm when X is F , Cl , Br and No3 . Again Banerjea

and Dasgupta 42 suggested SN2 mechanism when X m- is s2_o 3-. On . 43

the other hand Lan~ford found out the possibility 6f five

coordinated intermediate which reacts with water in a very

rapid step. Langford carried out experiment with anions like

F-, Cl-, Br-, N03- and H2Po4- • The same reaction was also 44 studied by Pearson and Moor and they proposed solvent assisted

pissociation mechanism when xm- is N03-. According to th~m

- 15 -

lthe reaction proceeds through SNl path because they did not

observed any true evidence of five coordinated intermediate. \

Acid hydrolysis of univalent dichloro-complexes occurs in two

steps and can be represented as

• • • ( 12)

• • • ( 13)

The observed rate data 45 show that the second step (13) is 100

·times slower than first step (12). This is due to the effect of

·positive charge of complex on the rate and it clearly indicates

that the removal of negative group in form of xm- is more diff­

icult the greater the remaining positive charges on complexes.

An €Xtreme example of the charge effect is the relative rate of

hydrolysis of RuC1 63- and Ru(H2o) 5cl2+ which differs by a factor

108 . However effect of charge does not always confirm the trend 4~

Another point to be noted here that increasing chelation

decrease the rate of acid-hydrolysis. It can be explained on

the basis of solvation theory. In the transition state the

intermediate of chelated complex is less stabilized by solvation

due to its relatively larger size. Of course bond formation with

water does not change the charge of the complex and extent of

solvation, what is important here is the bond breaking in the

transition state.

- 16 -

It should be anticipated that crowding of a reaction centre

might favour a dissociation mechanism (sNi) by effectively

relasing some degree of steric strain- but reduce the chance

of association by resisting the attack of an approaching nu-

cleophile.

. 47 Study of aquation of [Co(AA) 2(Cl) 2 ]+ (where AA represen-

tating a bidentate ligand ethylenediamine or substituted ethyle­

nediamine) reveals a correlation between increase in rate with

increased substitution on the ligand ethylenediamine. This

. steric effect suggests the dissociation mechanism in which in-1 :creasing steric strain assist the formation of an intermediate

of reauced coordination number. The increase in rate of hydroly­

sis of complex type Co(AA) 2cl2+ with increasing crowding may

also be explained as due to inductive effect of substituted

alkyl group which increase the electron density on the cobalt

atom resulting easy dissociation of Co-Cl bond. Basolo and his

co-workers 48 studied the aquation taking a series of substituted

pyridine complexes ofthe type [Co(en) 2 (x-Py)Cl] 2+ and found in­

ductive effect on the aquation rate is very small. Hence, incr-

ease of aquation rate with the increase of alkyl substitution

on the ligand AA in the complex Co(AA) 2cl2+ can not be explained

by inductive effect of the substituted alkyl group. An extreme

example of steric crowding which decrease the aquation rate is

the hydrolysis of CoLco3+ complex49 (L = hexamethylcyclam)

and it was observed that hydrolysis of above complex is slawer

by a factor 1015 then the hydrolysis rate of Co(en) 2co3+ •

I

- 17 -

50-53 Tobe and his workers studied the aquation of cis and

trans-Co(en) 2LCln+ where L is the non-re~eable ligand which

can be placed either cis or trans to leaving chloride ion. It

was found as L was varied in sequence L = OH-, N3-, Cl-, NCS-,

CN- and N02- of decreasing electron pair donor ability through

metal to ligand n-bonding, the rate first falls, reaches a

minimum and then again shows a rise with a maximum at N02

- It

has been found that electron donating groups (capable of form­

ing n-bonds) like OH-, N3-, Cl-, Br- and NCS- favour the SNl

promoted reaction because these ligands through their electron

donating (n-bonding) ability increase the electron density on

I cobalt(III) which leads to weakening the Co-Cl bond arid facili-I

i tating the rate of dissociation of chloride ion in an SNl

mechanism.

·~ ?' HO - Oo - Cl .. . • • ( 14)

·(Here the pentacoordinated intermediate formed are stabilized

by n-bonding.)

On the other hand electron withdrawing groups like CN- and

,No 2- which are n-acceptors reduce the electron density on metaL

atom and consequently invite the nucleophilic attack of the

water molecule, hence facilitating at least some bond formation

with water molecule in transition state by SN2 path. Thus we

find both SNl and SN2 mechanism are possible in the aquation

hrocess of Co(en) 2 LC1(J-n)~ and actual mechanism would depend

bn the nature of L i.e. non-participating ligand. It was also I

- 18 -

observed that in SNl promoted reaction cis-isomer reacts faster .

than trans-isomer (cis effect) with retention of its configura-

tion. Rearrangement of configuration occur in case of trans­

isomer. Again in SN2 promoted reaction trans-isomer reacts

faster than cis-isomer and here both cis and trans isome~re-

tain their configurations.

Another criterion for predicting mechanism developed by

Marcus 54 is the linear free energy relationship (LFER). For

the aquation reactions

(3-n)+ [Co(NH3) 5X]

3+ [Co(NH3) 5(H20)]

n­+ X • • • ( 1 5 )

the plot of log k vs pK, where K is equilibrium constant for.

n- -the reaction,is a straight line of unit slope for X = NCS ,

F-, H2Po4-, Cl-, Br-, I- and N03- • This lenear free energy

relation with unit slope suggests that the nature of x- in

transition state is the same as in the product (i.e. solvated

x-). This also indicates a dissociative mechanism in which

Co-X b~d rupture is virtually complete in the transition state.

However this does not reflect correctly the role of entering

water molecule. To confirm a distinct and stable intermediate

of Co(III) species in the transition state. Posey and Taube 55

investigated the metal ion catalysed aquation of [Co(NH3) 5x] 2+

(where X= Cl-, Br- and I-). This aquation reaction can be

represented as follow

. I

- 19 -

2+ [Co(NH3) 5X] + H20

3+ [Co(NH3 ) 5H20~ + X • • • ( 16)

The metal ions used in this aquation follow the sequence :

Hg2+ > Ag+ > Tl 3+ of their reactivity. It was found that Hg2+ ratio of 18 induced aquation/ 0 content of solvent to that of the

product complex is always 1.012 which is independent of X. The

high values of fractionation effects and identity in size for

- - - 2+ three complexes (X- = Cl , Br , I ) with Hg suggest that from

all three reactants

bly [Co(NH3) 5] 3+ is

ble preference for

less reactive Ag+

a common and distinct intermediate, presum-

formed and this intermediate has reproduci­

o16 isotope in the water. The data for the 3+ and T1 suggest that water participation

in the transition state is significant and varies with each

case and thus ruling out a distinct intermediate.

Effect of pressure on the aquation rate gave another valua-

ble information regarding mechanism through the interpretation

of volume of activation. Jones and Swaddle56 has reported that

for the aquation of Co(NH3) 5x( 3-n)+ (X= Cl-, Br-, N0 3- and

SO 4 2-) the volume of activation ( IJ. vi:) are equal to the molar

volume change ( fjv) for the respective completed reaction57

supporting the Langford's conclusion based on LFER. Chan and

his co-workers 58- 62 studied the aquation rates taking a series

of cobalt complex of the type [Co(en) 2(R-NH2)Cl] 2+ (where R-NH2=

Me-NH2 , Et-NH2 , alyl~NH2 , Prop-NH~:' Isoprop-NH2 ) and found

rates of aquation decrease first,attaining a minimum at·'

- 20 -

R-NH2 = Prop-NH2 and then increase. This was explained on the

basis of solvolytic approach to aquation. The decreasing part

of the curve was expl~~byassuming bimolecular interchange

of ligand between solvation shell and coordination shell and

the interchange rate decrease with increasing steric crowding.

When steric crowding is maximum then seven coordinated transi-

tion state is energetically unfavourable and consequently reac-

tion then proceed through an SNl path. Hence increase of rate

is further observed. Actually solvation effect and steric effect

on aquation rate acts in opposite direction in a dissociative

;mechanism and it may be thought that observed rate is the net

'effect of two opposing factors and that the mechanism could

have been dissociative one throughout.

·Anion induced aquation of Co(NH3)5N32+ in presence of

n- - - - - - -various anions e.g. X = F , Cl , Br , No3 , SCN , H2Po4 and

so42- was studied by Haim and Taube 63 and they found [Co(NH3)

X](J-n)+ are formed in addition to [Co(NH3) 5H2o] 3+ • It was

suggested that Co(NH3) 53+ is an intermediate in these reactions

and competition of added anions and water for Co(NH3) 53+ species

accounts the products formed in the reaction. From the study

of aquation of Co(en) 2LC12+ (L = aniline or p-toludine) 64 in

mixed aqueous solvent it was found that rate differ much in

solvents of indentical dielectric constant and proportional

~o water concentration. and possible bimolecular mechanism for t the reaction was proposed.

- 21 -

From the views of aquation kinetics of octahedral Co(III)

jcomplexes discussed so far it is reasonable to conclude that

!the major aquation reactio~of Co(III) complexes follow the

•SNl or diss~ciation path~ but SNl vs SN2 controversy regarding

'mechanism still persists.

Base hydrolysis - The mechanistic aspects of base hydrolysis

of several octahedral cobalt-amine com~ have been thoroughly

investigated and the reaction can be illustrated as

2+ [Co(NH3 )

5Cl] + OH- ~

2+ [Co(NH

3)

50H] . + Cl- . . . ( 17)

The rate of reaction found to be second order65- 66 , first order \ I ~o complex concentration an~ first order to OH- concentration. l .

From the experimental rate law and on the basis of the known

neuc1eophilicity of OH- Ingold and his co-workers 67 proposed

a simple SN2 mechanism for the above reaction (17).

slow:> HO Co ••• Cl fast

(18)

1he nucleophilic activity of the OH- only in these octahedral

complex is not clear. Co(III) being the moderately hard acid

may have tendency to bind other polar ligands. The fact that ;

' ~he rate of base hydrolysis are even high for chelated and

Jterically crowded Co(III) diamine complex can b~. presente~. ', .. : i

'.jf' '

a;s an evidence against SN2 mechanism and it wa.s..!:~u~~~e:r f '6-'~- t

:- __ : .--.~- ~- P) r-_, : -~ .... . -- ;~ : ·=~~_:..:: ...::::."'l:7.

- 22 -

1 found that rate of base hydrolysis of Co(III) complexes is

generally faster (several powers of 10) than acid hydrolysis

'which goes against the SN2 process.

68 ) 2+ Bronsted observed that hydrolysis of Co(NH3 5No3 is

independent of pH below 3; but the rate of hydrolysis of

[Co(NH3)4 (H20)(N03 )2+] was dependent on hydrogen ion concen­

tration even below pH 3. He explained the fact by assuming a

rapid acid ba~e equilibrium step in which a reactive hydroxo

.complex as an intermediate is formed and the concentration of I

\this reactive hydroxo species is inversly ,proportional to hy-

ldrogen ion concentration. This equilibrium can be represented

+ [Co(NH3)4(0H)(N03)] + H+

• • • ( 19)

Therefore rate will show the same reverse proportionality.

Base. hydrolysis of Cr(H2o) 5Cl2+ has similarly been explained

by assuming Cr(H2o) 4(0H)Cl+ intermediate69 .

Based on the early works of Bronsted an alternate mecha-. 70

nism was proposed by Garrick • This mechanism involves a rapid

~stablishment of pre-equilibrium in which the hydroxide ion

form an amide conjugate base by abstracting a proton from one

of amine nitrogens. The rate determining step being the disso­

ciation of the conjugate base to give a penta coordinated in-

termediate which then adds on solvent to give the hydroxo product.

- 23 -

. 2+ 2+ (Co(NH

3)

5X) + OH- (Co(NH3 ) 4 (NH2)X) + H20 ••• fast

CB kCB 2+ --~>~ Co ( NH 3) 4 ( NH2) + X • • • r a t e de term in-

ing

. . . (20)

This overall mechanism is called SNl CB mechanism and the

final rate expression is as follows

d[Co(NH3 ) 5x2+J

dt • • • ( 21)

where Ka is the acid dissociation constant of the complex,

Kw is the ionic product of water. Two important points should

be noted here that establishment of pre-equilibrium is very

rapid and the concentration of amido conjugate base is to be

low.

In support of the SNl CB mechanism variety of indirect

evidence and arguments have been discussed below.

To proceed through SNl CB path complex must have at least

one acidic proton to form its conjugate base. It was observed 3--71

that the hydrolysis rates of the complexes Co(CN) 5cr·· 72

trans- Co(2,2-bipyridine) 2 (N02 )2+ , trans-Co(p,p,p,p-tetra-. +73 ~thylethylenediphosphine) 2 (Cl) 2 which have no acidic pro-

tons are independent of pH over alkaline range.

- 24 -

The optically active ethylenediaminetetraacetato Co(III)

~complex has no acidic proton to loose, however,Bug:h 74 observed

the complex undergoes second order ~ase hydrolysis~ first order

' ·-to OH and first order to complex ion concentration and racemi-

sed products were obtained. This can be best explained by assum­

ing bimolecular reactions in which hepta-coordinated interme­

diate is formed with the OH- in the centre of the rectangular

face formed by four acetate groups.

Pearson and his co-workers75 studied the reactions of the

yype

I I Co(en) 2 (N02)Cl+ + y-~ Co(en) 2(No2 )Y+ + Cl • • • ( 22)

(where Y-= N03 , N02- SCN-) in non aqueous medium (dimethyl­

sulfoxide) and found rate of reactions are slow, with half

lives in hours, but when traces amount of OH- are added the

half-lives are reduced to minutes. Since the reaction rate was

rapid and independent of nature of Y- the possibility of SN2

process have been ruled out. These results support the forma-

tion of pentacoordinated intermediate which is formed in a

rate determining step by loss of chloride ion from the conju­

g,te base of parent complex and picks up the ligand Y- rapidly.

76 Basolo and Pearson extended the SNl CB mechanism by in-

cluding the possibility of a rate controlling bimolecular reac-'

t~on of conjugate base (SN2 CB) as

- 25 -

2+ Co(NH

3)5X fast~ Co(NH

3)4

NH2xt + H+

CB

+ slow \II fast_ Co(NH3 )4NH2X + H2o ~ H20 n. ~~ ••• X-· ~

2+ - ( ) Co(NH3) 50H +X ••• 23

77 and this mechanism was strongly supported by Green and Taube .

They show OH - need not be a entering group in base hydrolysis

reaction. If the reaction is carried out in 180 entiched

-water and OH behaves as entering group, the 160/180 ratio

of hydroxide ion should correspond to the l6011s0 ratio

in the hydrolysis product, barring relatively small kinetic

isotope effect.

From the studies of base hydrolysis of the complexes

( 2+ ) 2+ 2+ . Co NH3) 5Cl , Co(NH3 5Br and Co(NH3 ) 5No 3 1t was found that

( ) 2+ 77 . 18 the common intermediate Co NH3 4NH2 produced 1n 0 en-

riched water has the same ratio of 16o;18o as the water and

therefore the reactions are consistant with SNl CB mechanism.

The ratio was some what different for Co(NH3) 5F2+ species is

explained by SN2 CB mechanism in which water attacks the con­

jugate base before losing the F- (Eq.23). It is reasonable

since F- would be group very difficult to displace of all

other groups used.

In presence of anions like n- - - - 2-y ; N3 , N02 , SCN , so4 3- • and P04 , the base hydrolys1s reactions of Co(NH3) 5x

2+ (i =

Cl-, Br-, I- and N03-) in aqueous solution gave the products

- 26 -

78 Co(NH3 )

5y ( 3-n)+ · t' 1 t · d d t f 1 · 1n proper 1on a mos 1n epen en o eav1ng

( 2+ group in addition to Co NH3) 50H • This example is strongly in

favour of conjugate base mechanism which proceeds to pentacoor-

dinated intermediate.

Base hydrolysis of complexes of the type Co(en) 2Ax( 3-n)+

was studied and found the reaction proceeds through a common

intermediate79 and hence the concentration of isomeric products

are independent of the nature of leaving group n-X • This

result also is in good agreement with SNl CB mechanism.

In some cases it was found that SNl CB mechanism cannot

alone explain the rate of base hydrolysis and an alternative

SN2 mechanism can be applied equally well to explain the rate

data 80 . Actually detection of conjugate base intermediate is

the direct proof of SNl CB mechanism and attempts made to detect

it in aqueous solution were failed due to formation of ion-pairs.

However some evidenc~for the five coordinated intermediate

were obtained in non-aqueous medium (Dimethyl sulfoxide).

The conception of ion-pair formation was successfully

applied by Chan and Tobe81 in both base hydrolysis and aquation

of the compounds of the type Co(en) 2Lcl2+. The following inter­

change scheme avoids the assumption of intermedidate of altered

coordination number. In this process at first parent complex

mblecule is solvated and the solvent molecules form a zone which

iJ termed as solvation shell. In both base hydrolysis or aquation !

I

- 27 -

process OH- and H2o respectively replaces one solvent molecule

from solvation shell in a very fast equilibrium step. Then in

a rate determining step interchange of OH- (for base hydrolysis)

or H2o (for acid hydrolysis) from solvation shell to inner coor­

dination shell is occured. The process can be illustrated as

follows :

+ y

slow.,. k . . .

(Here Y = OH-, H2o , charges on metal atom are dropped for

convenience).

(24)

Attack of OH- on the solvation shell may be explained by Grothus

Chain transfer82 •

Ion pair formation could be detected spectrophotometrically

;which is a strong evidence in favour of this mechanism. but un­

fortunately the attempts to detect conjugate base formation

were failed83 • There are two types of ion pairing mechanisms

which are designated as SNl IP and SN2 IP depending whether the

exchange of ligand is unimoleculer or bimolecular. Lalor and

Long84 studied the base hydrolysis of Co(NH3) 5x2+ (X = Cl-,

~r-, N3- ) and proposed SNl CB mechanism based on activation

was performed by Banerjea i parameters. Whereas similar study ' ' 85 1and Dasgupta ! and they proposed

on Go(NH3) 5(NCS) 2+ and Co(NH3) 5(No2 )2+ complexes

SN2 IP mechanism. However a very recentstudy86

- 28 -

'on base hydrolysis of Co(NH3) 5(NCS) 2+, Cr~NH3 ) 5 (NCS) 2+ and

Rh(NH3) 5(NCS) 2+ in aqueous organic media concludes that reac­

!tion occurs through the formation of an ion pair in a pre­

equilibrium step involving the substrate and OH- and not with

a conjugate base87 • The rate of formation of ion pair to product

indicates a significant associative character with OH- acting

as the attacking nucleophile.

An electron transfer mechanism of base hydrolysis of com-2+176

plexes of the type Co(NH3) 5x was developed by Gillard.

In the rate determining step of this mechanism OH- reduces I Co(III) to labile Co(II) by transfering one of its electron.

Anation Reaction - Reaction involving replacement of water

molecule from an aqua-complex is termed as anation reaction.

Anation reaction on octahedral cobalt( III) complex can be

illustrated as follows:

3+ n- (3-n)+ CoL 5(H2o) + y ~ CoL 5Y + H20 . . . (25)

In anation reactions, kinetic dependence on the anating

ligands (generally anionic) can be explained by associative

mechanism assuming nucleophilic attack by the anion towards

the central metal atom, on the other hand dissociative mecha­

~ism is also applicable to explain the experimental observation.

- 29 -

The dissociative mechanism of anation reaction may be illus-

trated as follows :

n+ kl slow n+ CoL 5H20 ::a. CoL5 + H20

k_l fast (26) ...

CoL5

n+ - k2 C L (n-1)+ + X > 0 5 fast

In this case a five coordinated intermediate is formed.

By applying steady state approximation the rate of formation

of the final produtt is given by

( n-1)+ d[CoL 5X ]

= dt

n+ k1k2 (CoL 5H20 ](X-]

k_1 + k2 [x-]

n+ then rate = (k1k2/k_1)[CoL 5H2o ][X-]

then rate =

• • • ( 27)

••• (28)

• • • ( 2 9)

1hus depending on the conditions applying, the rate may or may

not be dependent on the concentration of the entering substi­

tuent. In principle, a gradual change from second to first order

kinetics may be revealed as the concentration of substituent is

increased. Once this limit is reached the rate should then be I

- 3'C) -

independent on the nature of the incoming group:· and the

observed rate constant should be equal to that for isotopic

exchange between water and aquo complex. Attempts to detect

these effects, in reactions of aquo complexes of cobalt(III)

ammines, were not successful due to outer-sphere association

between oppositely charged reactants at the high anionic con-

centrations required to achieve the anion-independent limit.

Under these conditions an ion pair path, which is kinetically

·second order, competes ag~inst reaction via a dissociative

process

n+ KE n+ CoL 5H20 - [CoL 5H20 ) ,X - (ion pair) +X ~

-.:e--

. . . ( 30) n+ k~ (n-1)+

[CoL 5H20] X - [CoL 5X] + H20

To avoid the ion-pairing, Haim and Wilmarth87 used anionic 2-

complex like Co(CN) 5H2o in their anation study with N3-

and SCN- ions. The kinetic results of Haim and Wilmarth showed

that the reactions proceed by a limiting SNl mechanism with

( ) 2- 18 the intermediate Co CN 5 • The rate.of OH2 exchange of

Co(CN) 5H2o2- was found to be identical with the rate of N3 and SCN- substitution under similar experimental condition.

This is a further support in favour of five coordinated inter­

mediate formed in the reaction. The values of k_1/k2 revealed

that azide is better nucleophile to Co(CN) 52- than thiocyanate.

- 31 -

The extension of this work88 to a number of ligands established

the following order of nucleophilic reaciivity for the subs-

: trate Co(CN) 52-

0H- > N3- > SCN- > I > NH 3 > Br- > s2o~- > CNO- > H20

. In this series, the position of water is that calculated for

a concentration equal to that of the other ligands.

In case of Co(CN) 5H2o2- , the five cyanide groups would i :give rise to a high electrondensity of the cobalt centre, re-

. sulting in a relatively weak Co-OH2 bond. Further, n-bonding

and an increase in bond angle would be expected to stabilize

i the five coordinated intermediate Co(CN) 52- • However, accord­

. ing to Basolo and Pearson89 the inert metal ions such as Co(IIJ)

and Cr(III) will never form a true five coordinated intermediate

'unless special stabilizing ligands are present. Thus Co(NH3) 53+

and Cr(H2o) 53+ would be too high in energy to exist, the diff­

erence in the apparent stability of Co(CN) 52- and Co(NH3)5

3+

:lies in the special nature of the cyano ligand. First, strong

electron donation from the ligand to the Co(III) centre in the

6 bond causes an accumulation of negative charges on the metal

;which is only partly removed by back n bonding. This weake~s

the Co-OH2 bond and promotes SNl mechanism. Second, in the

higher energy five coordinated species, n bonding becomes more

: important. In particular, there is a strong depletion of the

electron density in the t 29 orbitals. This probably allows

- 32 -

rearrangement to a trigonal bipyramid to occur with little or

no less of ligand field stabilization enetgy.

In general if the complex is cationic and the entering

ligand is anionic in nature, anation may occur via an ion pair

path e.g.,

3+ K 3+ Co(NH3) 5H20 - E::::.. Co(NH3) 5H2o -+X ' X

fast (ion pair) (31) ... 3+ k

Co(NH3) 5X2+ + H20 Co(NH3) 5H2o - a> ' X slow

In such a case the rate equation for anation reaction is

d[Co(NH3) 5x2+]

dt =

3+ kaKE[Co(NH3)5H20 Jtotal[X-]

1 + KE[X-] • • • ( 32)

substrate, K = E ion pair equilibrium constant, and ka =rate

constant for interchange of outer sphere complex to inner sphere

complex.

90 91 . Eigen and cooworkers ' proposed the 1on pair mechanism

to explain the major kinetic data for the anation reactions of

labile aqua-complexes. Such a mechanism involving

(a) Free ions -- Ion pair ~

(b) Ion pair ~ Inner sphere complex

seems applicable to most of the anation reactions.

- :33 -

In this mechanism anation of aquo complex proceeds through

a outersphere association between the aquo complex and the

incoming ligand, prior to the slower rate determining M-DH2 bond dissociation, whereupon the nearby ligand slips into the

position vacated by leaving water molecule. This interchange

process can occur either through dissociative or associative

path.

Anation reaction was first studied by taking aquo-pentammine

92 cobalt complexes and the reaction can be represented as

3+ n-[Co(NH3)5H2o] + Y ~

(3-n)+ [Co(NH3) 5Y] +H2o ••• (33)

n-(here, Y = Cl-, Br-, N03-, NCS-, so

42-, H2Po

4-)

All thes~ ligands replace the water molecule from the com­

plex by dissociative interchange (Id) mechanism with the prior

formation of ion-pair between the complex cation and incoming

anonic ligand. In all cases rate determining steps involve the

, dissociation of Co-OH2 bond.

Anation of aquopentaammine cobalt(III) complex ion by bi­

chromate ion was investigated by Haight93 . He proposed a mecha­

'nis~ which involves the attack of HOCr03- to Co(NH3) 5(H2o) 3+

and rate law involving proton attachment to the incoming ligand

(acid catalysis) was suggested.

3+ Replacement of water molecule from Co(NH3) 5H2o by acetic I i 94 acid and acetate ion was observed in the pH range 1 < pH<5.5.

- ~4 -

It was observed that anation by HOAc follows the second order

kinetics and there was no evidence for ion-pair formation.

Whereas in case of acetate ion anation, kinetic results provi-

des the evidence for ion pairing with the ion pair equilibrium

constant value 5.37 mol-l.

In the anation reaction of aquo·-pentaammine cobalt(III)

complex by aliphatic monocarboxylic acids i.e. formic acid,

acetic acid, propionic acids over a pH range in which ligands

can exist as acidic or basic form in the solution. Joubert

et a1 95 found that the anations by these carboxylic acids

follow second order kinetics in which participation of ion

pair could not be detected, whereas, anations by carboxylate

ions proceed through the ion pair formation and familiar Id

mechanism have been suggested.

The anations of aquopentaammine cobalt(III) by dicarboxylic

acids and their . anidn$" were studied with much interest.

Eldik and Harris studied96 , 97 the anation of Co(NH3) 5(H2o) 3+

ion by HC2o4- in aqueous medium and found that the reaction

proceed through ion pair formation. But considering the low

values of activation parameters they proposed bimolecular

mechanism with no evidence of ion-pair formation for the ana­

tion of Co(NH3) 5H2o3+ by H2c2o4 • A four centre transition

state was visualized which can achieve oxalate addition with-

out Co-o bond cleva~e.

- J5 -

Eldik et a1 98 and Dash et a1 99 took malonic acid and the

corresponding malonate ion for investigating the kinetics of

anation on Co(NH3) 5H2o3+ • It was observed that complex cation

form ion pair with hydrogen malonate or malonate ion and de-

aquation process· was believed to occur through Id mechanism.

Both the group of workers could not find the evidence of ion

pair formation in the anation by malonic acid. They suggested

that malonic acid anation reaction proceed through bimolecular

mechanism and it was found in oxalic acid anation and malonic

acid anation the second order rate constant data for both the

anation are nearly same. From the study of anation of

Co(NH3) 5H2o3+ by malonic acid and oxalic acid in o2o solvent

and from relation between pK1 of anating carboxylic acids with rate

the second order/constant) they suggested that 0-H bond fission

on the - COOH group of carboxylic acid is of significant im­

portance in the rate determining step of the process.

Das et a1 100 observed hydrogenphthalate and phthalate ions

anate Co(NH3) 5(H2o) 3+ by the same Id mechanism and formation of

:ion-pair between the complex cation with hydrogen phthalate

·and phthalate were suggested. However it was also observed that

the phthalic acid in the concentration range studied could

not anate the said complex.

I 101

I Das and Das further showed that hydrogen succinate and

) 3+ succinate ion could anate Co(NH3 5H20 • Succinate ion anates I 3+ ~o(NH3 ) 5H20 according to Id mechanism involving rapid formation

- ~:6 -

of ion pair equilibrium prior to the rate determinating step.

But it was found succinic acid replaced the water molecule

) 3+ from Co(NH3 5H2o following second order kinetics without

forming any ion pair species.

3+ Salicylate102 anation of Co(NH3 ) 5H2o was found to follow

conventional Id mechanism and it was also observed salicylic

acid could not form any ion pair with the complex cation.

Thiocyanate anation of Co(NH3) 5H2o3+ in aqueous alcohol

103 or dioxane mixture was studied by Holba et al • They sugges-

ted electrostatic theory of polar effect of the medium on the

( 3+ anation of Co NH3 ) 5H2o by using methyl alcohol and ethyl-

alcohol as co-solvents.

Formation of pentaammineglycinecobalt(III) 104 ion from

aquo pentaammine cobalt(III) complex ion was believed to occur

through Id mechanism involving the prior formation of ion pair

between Co(NH3) 5H2o3+ and glycine.

Kelm et a1 105 failed to detect the kinetic evidences of

ion pair participation in the anation reaction of Co(NH3) 5H2o3

+

- 2-by Cl and so4 • But low positive value of volume of acti-

vation which is comparable to that of the corresponding water ' 3+ exchange process of Co(NH3) 5H2o led them suggest the Id

mechanism for the above said anation reactions.

- 37 -

3+ Water replacement reactions in Co(NH3)5H2o by dimethyl

sulfoxide106 w~s studied in acidic aqueous DMSO medium. By

varying the DMSO component in the mixt~e it was shown that

there are dominating outer s~here interaction involve in the

solvent DMSO and co-ordinated water molecule.

3+ Replacement of water molecule from Co(NH3) 4(H2o) 2 by

polyphosphate ion107 was followed by cation exchange resin

and dissociative interchange mechanism was predicted in which

dissociation of first coordinated water molecule requires

27.3 kcal/mole.

Reports on oxalate anation of diaquotatraamine108 cobalt(III)

complex were obtained and found at a pH range lower than 2.8,

anation occurs in a single step in which one water molecule

is replaced and monodentate oxalate complex formed undergoes

ring closure in subsequent step to form final Co(NH3) 4(c2o4)+

product. But at higher pH ( > 2.8 < 4 ) two kinetically dis­

tinct steps were observed109 which involves a fairly rapid

dissociation of one water molecule followed by slower ring

closure step.

Kinetics of deaquation o1f Co(NH3 ) 4(H2o) 2

3+ by NH3 in

1 alkaline solution was studied by Bolt110 • He proposed that

in alkaline solution Co(NH3)4(H2o) 23+ is converted to

\ Co(NH3)4(H20)(0H) 2+ and formation of Co(NH3) 5(0H) 2+ from the

- 38 -

Co(NH3)4 (H2o)(OH) 2+ occurs through dissociative interchange

mechanism in which water molecule leaves the inner coordina-111

tion sphere to outer~sphere zone. It was shown that seN-replaces the water molecule from Co(NH3)4(H2o)(OH) 2+ in a

similar way where competition ratio of H20 / NCS- for the

intermediate in the outersphere is approximately five. Rece­

ntly/ Ghosh and Banerjee112 studied the catalysing effect of

N0 3- on the anation reaction of Co(NH3)4(H2o) 23+ by SCN- and

N3 . It was assumed that N03- pulls off the aquo ligand

thereby forms a reactive five coordinated intermediate. A

mechanism involving ion pair formation was suggested.

Now reviewing the anation of aquopentaammine Co(III) and ,.

diaquotetraammine Co(III) complexes it is clear that when

these complexes undergo anation reaction by anionic ligand

it occurs through the dissociative interchange mechanism with

the prior formation of ion pair between the complex cation

and negative ligand. The values of ion pair equilibrium cons­

tant is almost temperature ihdependent i.e. ion pair formation

occur in very rapid step. The nearly equal values of rate

constant data for anation reaction in aquopentaammine -

cobalt(III) complex by carboxylate anions also suggest that

cleava9e of Co-OH2 bond is almost independent of incoming

ligands. Again no significant ion pairing in the intermediate

steps was observed when carboxylic acids anate Co(NH3) 5(H2o) 3+

or Co(NH3) 4 (H2o) 23+ complex ions and bimolecular mechanism

- 29 -

instead of dissociative interchange mechanism was suggested.

Possibility of ion pair formation between charged complex

cation and neutral carboxylic acid molecule is rare. Instead,

there was a possibility of a weak hydrogen bond formation

in the outersphere zone to the coordinated water molecule.

Substitution of aquo molecule from trans-diaquodimethyl-' 113 114 glyoximato cobalt(III) complexes ' was studied by Bovykin

in aqueous and aquo-organic medium. It was found that thioure0

and its derivatives substitute the water molecules in two con-

secutive SNl steps and in each step a single water molecule

is displaced.

Samus et a1 115 have shown anionic ligands e.g. Cl-, Br ,

( 2+ I and NCS can replace water molecules from Co(DH) 2 H2o) 2 and substitution of first water molecule is independent of

the nature of anating ligands. It was further shown that Cl-,

Br can replace only one molecule of coordinated water whereas

I and NCS- able to replace the both water molecules gradually

and second order kinetic data could be fitted for the replace­

ment of second water molecule by I and NCS- • Chelating ligand

.dimethylglyoxime gives a special stability to the pe~tacoordi­

·nated intermediate which is not possible in Co(NH3 ) 53+ or in

mechanism is not' possi­/

ble in latter two cases. Replacement of second water molecule

\by thiourea and its derivative114 occured through a SNl ~echanism

- 40 -

however large and negatively charged I and SCN- (less elec-

( 3+ tronegative than Cl- and Br-) can attack the Co(DH) 2 H2o) 2 in the activated state and exhibit second order kinetics.

Though a lot of work done on the anation reaction of

aquopentaammine cobalt(III) complex, however the studies were

not limited with this specific complex. Large kinetic interest

have also been shown to the water replacement reaction of some

chelated Co(III) complexes in which two ethylene diamine ligands

strongly coordinated with the metal ion and two water molecules

occupied the site in cis position.

Dissociative interchange from outersphere to innersphere

with the prior formation of ion pair between Co(en) 2 (H2o) 23+

2- -and anating ligand so4 or H2Po4 was introduced by

Barraclough116 , and Stranks117 respectively in their work.

Brown and Harris 118 did a number of experiments regarding

anation reactions with Co(en) 2 (H2o) 23+ . Oxalate anation in

presence of N0 3- was studied in aqueous medium and observed

that the reaction proceed by a dissociative interchange mecha­

nism. The ion pair equilibrium constant values for [Co(en) 2 ) 3+ ] [ 3+ - J -1 . (H20 2 - H2c2o4 and Co(en) 2 (H2o) 2 - Hc2o4 are 11 mol

-1 and 100 mol respectively. Anation rate constant values for

bioxalate anation was found to ~be same that of for oxalic

acid anation and both the values increased with the increase - 119 of N03 concentration. Eldik et al performed the same ana-

-tion reaction in presence of No3 and ClO~ and found the

- 41 -

ion pair equilibrium constant values for [Co(en) 2 (H2o) 23

+

H2c2o4] and [Co(en) 2 (H2o) 23+ HC2o4- ] are 1.1 and

-1 -4.4 mol respectively. It was observed that N0 3 ion can

catalyse the reaction by its specefic attack on the coordina-

ted water molecules rather than water molecules which are

associated with the incoming · ligand. It was assumed a reac-

tive but stereo-retentive five coordinated intermediate 3+ Co(en) 2(H20) is formed which increase the anation rate.

( ( 3+ Oxalate anation of Co en) 2 H2o) 2

studied by Eldik120 • He reported in

by catalysed\N03- was also

his work that the volume

of activation for the N03- catalysed process can be interpreted

in terms of a dissociative mechanism in which five coordinated

intermediate is formed which is ir:dependent of pressure

< 1500 bars.

On the basis of the results obtained from pressure effect 121 study Strank et al proposed similar outersphere ion associa-

tion and interchange mechanism involving ligand water dissocia-

tion for the anation reactions. However Ia mechanism has been /

proposed for the ring closure step [from Co(en) 2(H2o)(c2o4 )+

to Co(en) 2(c2o4)+], because this step shows zero volume of

activation ( 0 ± 1.2 cm-3mol-1).

Oxalate anation of aquohydroxo-bis-ethylenediamine cobalt(III)

. ion122 was found to proceed by same Id

mation of ion-pair [Co(en) 2 (H2o)(0~) 2+

mechanism prior the for-

. . . . 2- J C204 .

- 42 -

of Replacement/aqua molecule in cis-Co(en) 2(NH 3)(H20) 3+ ion

:by salicylate 123 and oxalate124 was studi~d by Dash eta! and

observed that the substitution of water molecule proceeds

through ion pair species [Co(en) 2(H 3N)(H2o) 3+ •••• HSal-] and

,[Co(en) 2 (NH1 )(H2o) 3+ •••• Hoxal-] respectively and the conve­

nient dissociative mechanism was suggested.

3+ A few anation reactions on trans-[Co(en) 2 (H2o) 2J have

been studied; Buckingham et al 125 , 126 reported that the cis­is

'diaquo complex/more reactive than its trans isomer, whereas

the reverse is true for the hydroxo-aquo-complex. Buckingham I

:have shown in his recent work 127 that oxalate anation of trans-

Co(en)2(H2o)23+ occurs in two steps and cyclisation of trans­

'Co(en)2(H20)(c2o4)+ proceeds via cis-[Co(en) 2 (H20)(c2o4 )]+ •

128 Dutta and De used pyridine carboxylic acids as anating

ligands for the anation of cis-diaquo-bis-ethylenediamine

cobalt(III) complex ion in aqueous-ethanol mixture. They propo­

sed that picolinic acid129 anation of cis-Co(en) 2 (H2o) 23

+

follows the dissoci~tive interchange mechanism involving the

ion pair participation. However it was shown by Siddhanta

et a1 130 that anation of Co(en) 2(H2o) 23+ by benzoates and

substitute benzoates undergoes SN2 mechanism. Siddhanta et a1 131

in their earlier work on anation of Co(en) 2Cl(H2o) 2+ by benzoic

acid and substituted benzoic acids in different aqueous-organic

medium and determined the Hammett reaction constant (f) and

found the, linear relationship between f and reciprocal of

- 43 -

dielectric constant of the reaction medium and SN2 mechanism

of the anation reaction have been suggested.

Wigh et a1 132 investigated the deaquation kinetics of a­

cis-ethylenediamine-N,N'-diacetatodiaquo cobalt(III) ion by

oxalate in slightly acidic medium. The same kinetics was again

studied in strongly acidic medium by Wigh and usual dissociative

interchange mechanism for the anation reactions were proposed

in both cases. Only difference was marked is that the geometry

of the complexes changes from a-isomer to ~-isomer in the sli-

ghtly acidic medium while retention of geometry was found in I • I

1 strongly acidic medium. Catalytic effect of N03- on the same

:kinetics was studied by Eldik133 and an identical mechanism with

the participation of ion pair between the complex and oxalic

acid has been suggested.

Anation of cis-diaquo-bis(l,3-diaminepropane) coba1t(III)

ion by H2c2o4 and HC2o4- was studied134 at three different tem­

perature and found reaction proceeds through ion pair mechanism.

The :nation rate constants for H2c2o4 and HC2o4- are virtually

same and the values of ion pair equilibrium constants were found

to be temperature independent. Dissociative interchange mechanism

was proposed for this reaction and it was found that chelating

ligand 1,3-diaminipropane is just higher homologue of 1,2-ethyJe­!

:nediamine does not alter the reac~ivity of aquo complex of Co(III)

ion.

. - 44 ..:

Deaquation of diaquo-[N,N-ethyl-bis(salicylideneiminate)]

cobalt(III) ion by thiourea, pyridine, imidazole, aniline,

- - -2 . 135 morphine, NCO , HS03 , s2o3 was stud1ed by Garlatt et al

in aqueous solutionj and found reaction rate is rapid and

nearly independent of nature of these anating ligands. It was

assumed that labilizing effect of chelating ligand salen (N,N­

ethylsalicyldeneimine) may r~sult in the stabilisation of di-

ssociative transition state for the li~and exchange process than

weakenin~· ~: Co-H20 bond in the ground state.

Saha and Banerjee136 studied the water replacement react~on

of cis-~-diaquo-(3,6-diazomethane-l,B-diamine) Co(III) complex

ion by NCS- and established a mechanism involving stepwise re­

lease of water following dissociative interchange path.

" 137 Intersting kinetic data were shown by Byrd et al in

studying the anation reactio~s of Co(CN) 4 (so4 )(H2o) 3- by NH3 ,

Py, N3- and SCN- in aqueous medium. It was shown that kinetic

data are consistent with SN1 (lim) mechanism and the order of

reactivity of ligands towards the five coordinated intermediate i

Co(CN) 4so43- are Py > 4-picoline

MeNH2 > I

4-acetyl-pyridine > NH3 > > SCN- > N

3- > N0

2-(\) CN- > HS0

3- > so

3 2-

Proteins containing metal complexes of highly unsaturated

\ligand ring systems are involved in a variety of essential I

I

lbiological functions. Compared to most other cobalt(III) com-t ' ;

~lexes, highly unsaturated macrocyclic (N4) ligands have .been I ,

- ~5 -

shown to enhance the rate of ligand substitution at the metal

centre by several orders of magnitude. Available evidence sugg­

ests that these cobalt(III) substitution reactions proceed

predominantly via either a dissociative or an interchange mecha-

138-141 . nism. Abnormally high rates have been encountered 1n

the anation reactions of [Co(P)(OH2 )2]5+ and/or-[Co(P)(NCS)

(OH2/0H)] 4+/3+ ions (H2P = 5,10,15,20 tetrakis(N-methylpyridyl)

porphine) with NCS-, pyridine, N3- and TMTU (TMTU = 1,1,3,3-

tetramethyl-2-thiourea). A strong trans-labilising effect of

NCS- and TMTU upon the remaining aqua jigand has also been

observed. A limiting D mechanism has been proposed for the

water substitution reactions of [Co(TMPyP)(OH2 ) 2] 5+ with NCS-, - 142 but with Br , I and Cl an Id mechanism is preferred • The

higher rates 143 of anation of [Co(TPPS4)(0H2) 2 ]3- with NCS-

and I- in comparison to that of the previous study142 has been

ascribed to the increased availability of electron density for

the cobalt(lii) in the later over that in the former. The kine­

tics of aqua ligand substitution of [Co(TPPS4)(0H2)2 ]3- by

'pyridine and of [Co(TMPyP)(OH2)2] 5+ by Co(CN)6

3-, Fe(CN)6

3-,

( 3- 3- 144 145 Mo CN) 8 and W(CN) 8 have also been reported ' . All

these reactions are believed to follow a limiting dissociative

mechanism.

I Recently new technique like high-pressure T-jump146- 148 \

; d 1. d . 149 h d t d f d t . . an nmr 1ne broa en1ng ave been a op e or e erm1n1ng I _± volume of activation ( ~r)of labile systems and assignmerit l

-·~6-

of reaction mechanism of coordination compounds in solution.

Kelm et a1 150 studied the anation of [Co(NH3) 5(H2o)] 3+ with

- 2- - 2-Cl , so4 and No 2 and observed Cl , so4 anates the water

molecule from Co(NH3) 5(H2o) 3+ by following Id mechanism, where­

as N02- anation proceeds through the formation of nitrite inter­

mediate, i.e. without the cleavage of Co-o bond 151 • Anation of

Co(CN) 5(H2o) 2- by Br-, I- and NCS- have been studied by high

t h . 152 d d . d . t f d . . t . pressure ec n1que an 1scusse 1n erms o 1ssoc1a 1ve 153 interchange (Id) mechanism. Tanaka et al have performed a

high pressure study on SCN- anation of diaquo-[meso-tetrakis(N­

methyl-4-pyridyl) porphinato] cobalt(III) ion and proposed a

D mechanism.

A number of studies have recently been performed on the

kinetics and mechanism ~f the reactions of Co(III) aquo- and/or . 154-159 2-160-166

hydroxo-complexes Wlth 002 , S02 as Well as S03 _167-169 _170 - 171

HSe03 , HMo04 and HW04 • All these reactions,

except that between co2 and [Co(tren)(OH) 2 ]+. have been found

to occur with half-times of the order of seconds or less. These

fast reactions require the stopped-flow technique for rate

measurements and therefore necessitate the postulation of a

'direct addition' mechanism. These reactions take place with­

out the cleavage of metal-ligand bond which is characteristic

of conventional aqua or hydroxo ligand substitution. Comparison

of the rates of these reactions clearly show that the second

prder rate constant values for so2 , HMo04- and HW0 4- falJ.

- ;q7 -

5 9 -1 3 -1 within the range of approximately 10 -10 mol dm s while 2- 2- . 3 -1 for so 3 , HSe0 3 and 00 2 the range is about 10-10 mol

3 -1 dm s • The major factor determining these rate groupings is

the increase in ~IfF and decrease in /1S=J.for the slower group,

indicative of less facile processes from the stand point of

both energy and ease of achieving the appropriate geometry

for the transition state. The mechanism proposed by the authors

. 11, th t d" 154- 171 . 1 d" t dd,'t" f t 1n a ese s u 1es 1nvo ves 1rec a 1 1on o en er-

ing ligand centre (S, Se etc.) to the oxygen of the Co-OH2 '

moiety.

Thus inspite of the considerable volume of work which have

: been reported till now from the type of mechanism operating in

substitution reactions of cobalt(III) complexes had not been

fully settled) i.e. the ligand substitution reactions in the

case of octahedral complexes of cobalt(III) do not follow any

single mechanism and the reaction path depends on th~ nature

·of the complex as weli as the incoming ligands. Various inves-

tigations reported su~gest the necessity to extend the inves­

tigations on substitution reactions of cobalt(III) complexes.

R tl . . d h b .j t. 1 . d 8 . 172 ecen y am1no ac1 s ave een useo as ana 1ng 1gan • anerJea

(used glycine as anating ligand in the anation of Cr(H2o) 63+ .)

Amino acids like glycine, alanine, ~-alanine, L-prolinet DL-

valine, L-serine can behave as a good bidentate ligands with

donor sites N,O and the ligand reactivity is varied by chang-

,ing the electron density on the dono:r sites. These ligand exist

- 49 -

;as zwitterion in a certain pH r~nge in solution depending I . tupon their i

( H 0) ] 3+ 2 2

pK 1 and pK2 values. Anation reaction of cis-[Co(en) 2

by the above said zwitterionic amino acids ligand

have not been studied so far. Viewing all these facts and keep-

:ing view of examining the mechanistic aspect of reactions, we

have studied kin~tics of anation reaction on cis-[Co(en) 2(H20)] 3+

b 1 · 173 . DL 1 ' 174 ~ 1 . L l' DL J. y g yc1ne , -a an1ne , ..,-a arnne, · -pro 1ne, -va .1ne

d L . 175 t. 1 an -ser1ne respec 1ve y.

- 49 -

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