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General Chemistry I 1
BONDING IN TRANSITIONMETAL COMPOUNDS ANDCOORDINATION COMPLEXES
8.1 Chemistry of the Transition Metals
8.2 Introduction to Coordination Chemistry
8.3 Structures of Coordination Complexes
8.4 Crystal Field Theory: Optical and Magnetic
Properties
8.5 Optical Properties and the Spectrochemical
Series
8.6 Bonding in Coordination Complexes
8CHAPTER
General Chemistry I
General Chemistry I 2
347
Emerald
3BeO∙Al2O3∙6SiO2
with someAl3+ replaced
by Cr3+
General Chemistry I 3
8.1 CHEMISTRY OF THE TRANSITION METALS
348
General Chemistry I 4
- Decreasing radii for small Z transition atoms
→ Increase in Zeff
- Increasing radii for large Z transition atoms
→ Increase in electron-electron repulsion
349
General Chemistry I 5
Lanthanide contraction: bad shielding by 4f orbitals
→ the radii of the 6th period ~ the 5th period
→ decrease in atomic and ionic radii by increasing Z
along the 6th period
349
General Chemistry I 6
350
melting point: function of the bond strength in solids
- roughly correlated with the number of unpaired e-
General Chemistry I 7
351
Enthalpy of hydration of M2+ ions
M2+(g) → M2+(aq): Hhyd
= Hof(M2+(aq)) – Ho
f(M2+(g))
Lowering of Hhyd from a line
→ due to crystal field stabilization
Anomalies of Mn
→ due to the stable half-filled d shell
General Chemistry I 8
Oxidation states
more common oxidation state
Increasing tendency toward higher oxidation states among
heavier transition elements in the same group:
Fe (2,3) → Ru (2,3,4,6,8), Ni(2,3) → Pd(2,4)
351
General Chemistry I 9
Hard and Soft Acids and Bases
Pearson (1963)
~ Extension of Lewis’ definition –
electron pair acceptor (acid) and donor
(base) – by adding categories ‘hard’
and ‘soft.’
~ 'Hard' species: small, high charge
states, low electronegativities, weakly
polarizable
~ 'Soft' species: large, low charge
states, high electronegativities, strongly
polarizable
~ ‘Borderline’ species
Ralph Pearson (US, 1919 - )
353
General Chemistry I 10
354
General Chemistry I 11
HgF2(g) + BeI2(g) → BeF2(g) + HgI2(g) s/h h/s h/h s/s
354
Prediction of chemical reactivities of inorganic reactions
~ Preferred direction: hard acid/hard base or soft acid/soft
base
AgBr(s) + I–(aq) → AgI(s) + Br–(aq) s/b s s/s b
EXAMPLE 8.2 Predict whether the following reactions will occur.
(a)CaF2(s) + CdI2(s) → CaI2(s) + CdF2(s)
(b)Cr(CN)2(s) + Cd(OH)2(s) → Cd(CN)2(s) + Cr(OH)2(s)
NO
YES
General Chemistry I 12
8.2 INTRODUCTION TO COORDINATION CHEMISTRY
355
Formation of Coordination Complexes
Werner’s investigation:
Compound 1: CoCl36NH3 (orange-yellow)
Compound 2: CoCl35NH3 (purple)
Compound 3: CoCl34NH3 (green)
Compound 4: CoCl33NH3 (green)
Alfred Werner (Swiss,1866-1919) Nobel prize in chemistry(’13)
Treatment with HCl → did not remove NH3
AgNO3 + Cl- → AgCl(s) in the ratio of 3 : 2 : 1 : 0
General Chemistry I 13
Conductivity measurements:
Compound 1: [Co(NH3)6]3+(Cl–)3 → Conductivity of Al(NO3)3
Compound 2: [Co(NH3)5Cl]2+(Cl–)2 → Conductivity of Mg(NO3)2
Compound 3: [Co(NH3)4Cl2]+(Cl–) → Conductivity of NaNO3
Compound 4: [Co(NH3)3Cl3] → Nonelectrolyte
→ Concept of “coordination sphere”
around the central metal ion
inner and outer coordination sphere
→ Formation of an octahedral complex
356
In the above complexes, NH3 and Cl- that are attached to Co are called LIGANDS
General Chemistry I 14
357
anhydrous CuSO4CuSO4∙5H2O
→ [Cu(OH2)4]SO4∙H2O
General Chemistry I 15
Monodentate ligands
mono “one” and dens “tooth”
357
General Chemistry I 16
Bidentate ligands
Chelating ligands: chelate (G. chele, “claw”)
[Co(EDTA)]– ~ 1 hexadentate[Pt(en)3]4+ ~ 3 bidentates
358
(‘en’)(‘ox’)
General Chemistry I 17
Naming coordination compounds
1) Single word for a coordination complex ~ [prefix-ligand-metal]
2) Cation first followed by anion ~ K[…] or […]Cl
3) Ending with the suffix “-o” for anionic ligand, chlorido (Cl),
no change for neutral ligands except aqua (H2O), ammine (NH3),
carbonyl (CO). Note: “chloro” for Cl in a compound ligand
4) Prefixes for the number of ligands ~ di-, tri-, tetra-, penta-, hexa-, …
(bis-, tris-, tetrakis-, … for ligands with di- (etc) in their names)
5) Alphabetical ordering for many ligands
6) Roman numeral (oxidation state) in (..) after the name of metal
~ […cobalt(III)]Cl or K[…ferrate(III)]
anionic complex ions: the ending “-ate”
359
General Chemistry I 18
359
General Chemistry I 19
Ligand substitution reactions
[Ni(OH2)6]2+(aq) + 6 NH3(aq) → [Ni(NH3)6]2+(aq) + 6 H2O
360
Another example
Cu(H2O)62+(aq)
Pale blue
HCl(aq) NH3(aq)CuCl4(aq)
_
GreenCu(NH3)6
2+(aq)Deep blue
General Chemistry I 20
‘Inert’ coordination complex:
thermodynamically unstable, kinetically
stable (inert)
‘Labile’ coordination complex:
thermodynamically unstable, kinetically
unstable (labile)
3 33 6 3 2 6 4[Co(NH ) ] ( ) 6H O ( ) [Co(H O) ] 6NH ( )aq aq aq
2 23 6 3 2 6 4[Co(NH ) ] ( ) 6H O ( ) [Co(H O) ] 6NH ( )aq aq aq
takes a week
takes a matter of seconds
361Difference between ‘inert’ and ‘labile’
Ene
rgy
Reaction
Ene
rgy
Reaction
General Chemistry I 21
8.3 STRUCTURES OF COORDINATION COMPLEXES361
Octahedral complexes with geometrical isomers(complexes of type MA2B4 (or MA2B2; B is bidentate)
cis-[Co(NH3)4Cl2]+
trans-[Co(NH3)4Cl2]+
cis-[CoCl2(en)2]+
trans-[CoCl2(en)2]+
General Chemistry I 22
Octahedral complexes with mer / fac isomers
(Complexes of type MA3B3)
mer-isomer: Similar ligands define a meridian
of the octahedron
fac-isomer: Similar ligands define a face of an octohedron
-- all three groups are 90° apart.
mer-Co(NH3)3(Cl)3 fac-Co(NH3)3(Cl)3
362
General Chemistry I 23
Tetrahedral complexes
~ Dominant for four-coordinate complexes
~ No geometrical isomers for tetrahedral
complexes of MA2B2
363
Square planar complexes
~ Au3+, Ir+, Rh+, Ni2+, Pd2+, Pt2+
~ cis-[Pt(NH3)2Cl2]
(anticancer drug, ‘cisplatin’)
~ trans-[Pt(NH3)2Cl2]
Linear geometry
~ Ions with d10 configuration: Cu+, Ag+, Au+,
Hg2+
General Chemistry I 24
Optical isomers are molecules that rotate plane polarized light
Enantiomers (Gk. άτιος, “opposite”, and μέρος, “part or porti
on”) are optical isomers whose structures are non-
superimposable mirror images (they lack reflection-rotation
symmetry)
Chiral center (chirality [G. χειρ (kheir), "hand"] ~ handedness) is
a central atom around which enantiomers are formed
A racemic mixture has equal amount of enantiomers (net
rotation of plane polarized light = 0)
Chiral Structures366
General Chemistry I 25
E.g. enantiomers of the [Pt(en)3]4+ ion
E.g. enantiomers of all-cis [Co(NH3)2(H2O)2Cl2]+
366Octahedralcomplexes oftype MA3
(A is bidentate)
Octahedralcomplexes oftype MA2B2C2
General Chemistry I 26
EDTA (ethylenediaminetetraacetate) ion
Hexadentate ligand, sequestering metal ions
Antidote for lead poisoning, preserves freshness of oil
367
General Chemistry I 27
8.4 CRYSTAL FIELD THEORY: OPTICAL AND MAGNETIC PROPERTIES
367
Crystal Field Theory
~ Ionic description of metal-ligand bonds
~ Ligands are treated as point charges approaching
the central metal ion
Octahedral coordination complexes
Degeneracy of d-orbitals lifted into two groups :
2 2 2, and , , xy yz zz x yd d d d d
General Chemistry I 28
Crystal Field Theory
• Ligands such as a halide or oxide are regarded as an electrostatic,
point charge, or point dipole type, which set up an electrostatic field.
A
B
o = crystalfield splittingenergy
metal d orbitals sphericalcharges
octahedralenvironment
Cr3+
367
General Chemistry I 29
368
General Chemistry I 30
369
Fig. 8.17 An octahedral crystalfield increases the energies ofall five d orbitals, but the increaseis greater for the dz and dx - y
orbitals.2 2 2
General Chemistry I 31
• Electron configuration of octahedral complexes d1-d3
by Hund’s rule
370
General Chemistry I 32
-From d4 to d7 octahedral complexes there are two possibilities,illustrated for d4 (E.g. Mn(III) complexes)
e-e repulsion low-spin configuration
ligand-ligand repulsion
If o is large (strong-field ligands), t2g4 has a lower energy.
: low-spin complex, minimum number of unpaired e-
If o is small (weak-field ligands), t2g3eg
1 has a lower energy.
: high-spin complex, maximum number of unpaired e-
370
high-spin configuration
eg
t2gt2g
eg
Low spin (t2g4) configuration High spin (t2g
3eg1) configuration
E
General Chemistry I 33
Fig. 8.18. Electron configuration for (a) high spin (large o) and (b) low spin (small o) octahedral crystal field splitting energies for Mn(III) complexes
Weak field configuration Strong field configuration H2O weak field ligand CN– strong field ligand
369- Example: d4 octahedral complexes of Mn(III)
5 x degenerated orbitals (3d4)
35
25
oo
o
eg
t2g
dxy dyz dxz
dz dx - y2 2 2Mn(H2O)63+
HIGH SPIN
5 x degenerated orbitals (3d4)
35
25
o
o
o
eg
t2g
dxy dyz dxz
dz dx - y2 2 2
Mn(CN)63-
LOW SPIN
General Chemistry I 34
370
Crystal Field Stabilization Energy (CFSE)
The amount by which the (otherwise equal) energy levels for the d electrons of a metal ion are split by the electrostatic field of the surrounding ligands in a coordination complex.
Energy difference between electrons in an octahedral crystal field and those in the hypothetical spherical crystal field.
General Chemistry I 35
Square planar crystal field
370
sp > 1.6 0
General Chemistry I 36
Tetrahedral crystal field
371
t = 4/9 o
General Chemistry I 37
Fig. 8.20. Correlation diagram showing the relationships among d-orbital energy levels in crystal fields of different symmetries.
372
General Chemistry I 38
Magnetic properties
Paramagnetic compounds
~ One or more unpaired electrons
~ Large, positive magnetic susceptibility
~ Attracted by the magnetic field
→ “weigh” more
~ Prevalent among transition-metal complexes
Diamagnetic compounds ~ All of the electrons are paired
~ Small, negative susceptibility
~ Repelled by the magnetic field
373
Magnetic susceptibility ~ Strength of a sample’s interaction with a magnetic field
General Chemistry I 39
8.5 OPTICAL PROPERTIES AND THE SPECTROCHEMICAL SERIES
374
Transition-metal complexes
~ absorb visible light → colorful
E.g. [Co(NH3)5Cl]2+ ion absorbs greenish yellow light (~530 nm)
Only red and blue light transmitted
→ purple (complementary color)
Wavelength of the strongest absorption, max
d10 complex ~ colorless (no absorption, all d-levels are filled)
High-spin d5 complex ~ weak absorption (spin flip required)
o max, so /E h h hc
General Chemistry I 40
Cr(CO)6 [Co(NH3)5(OH2)]Cl3 K3[Fe(C2O4)3] K3[Fe(CN)6] [Co(en)3]I3
Colors of the hexaaqua complexes of metal ions prepared from their nitrate salts.
E.g. [Co(H2O)6]2+
375
General Chemistry I 41
Spectrochemical series
~ An ordering of ligands according to their ability to cause
crystal field splittings.
Spectrochemical series for ligands
2 3
Weak-field ligands (high spin) Intermediate-field ligands Strong-field ligands (low spin)
I Br Cl F ,OH : NCS en COH O N ,CNH
Spectrochemical series for metal ions
Mn2+ < Ni2+ < Co2+ < Fe2+ < Fe3+ < Co3+ < Mn4+ < Pd4+ < Ir3+ < Pt4+
Crystal field theory cannot explain the spectrochemical series!
376
General Chemistry I 42
8.6 BONDING IN COORDINATION COMPLEXES
377
Valence bond theory
dsp3 hybrid orbitals
~ linear combination of one s, three p atomic orbitals
and the dz2 atomic orbital
~ five equivalent new hybrid orbitals
~ trigonal bipyramid, PF5, CuCl5–
General Chemistry I 43
d2sp3 hybrid orbitals
~ linear combination of one s,
three p atomic orbitals
and dz2, dx2-y2 orbitals
~ six new hybrid orbitals
~ octahedron, SF6
378
General Chemistry I 44
Molecular orbital theory
Ligand field theory ~ Failure of CFT and VB theories to explain the spectrochemical
series
~ MO description for ligands
Construction of MOs for octahedral complexes (of 1st row
D-block metals)
~ Interaction between the metal 4s orbital with six ligand orbitals
→ s and s* orbitals
~ Interaction between three metal p orbitals with three ligand orbitals
→ triply degenerate p and p* orbitals
~ Interaction of the dz2 and dx2-y2 orbitals with ligand orbitals
→ a pair of d and d* orbitals
378
General Chemistry I 45
Fig. 8.27. Formation of bonding MOs from overlap of metal and ligand orbitals.
379
General Chemistry I 46
Bonding MOs
Nonbonding MOs
380
MO correlation diagramfor octahedral Cr(III)complex ([CrCl6]3-): bonding only
Antibonding MOs
General Chemistry I 47
Formation of and * bonds
(1) Interaction between an empty metal d orbital with a filled atomic
ligand p orbital. E.g. 3p orbitals of Cl–
(2) Interaction between a filled metal d orbital with an empty ligand *
antibonding molecular orbital. E.g. CO, CN–
→ metal-to-ligand (M-L) donation or backbonding
- and * MOs: M d orbital - L p orbitalor M d orbital - L * orbital
381
General Chemistry I 48
(3) Overlap of each of the metal nonbonding dxy, dyz,
and dxz orbitals with four ligand p orbitals
→ Formation of three pairs of
bonding and antibonding
MOs, t2g and t2g*.
Fig. 8.30. Bonding MO by constructive overlap of a metal dxy orbital with four ligand p orbitals.
382
General Chemistry I 49
Order of bonding strengths for different ligands
Weak-field ligands (small o)
→ Overlap between occupied p() bonding orbitals of
ligands (Br–, Cl–, CO) with t2g orbitals of metal
→ Increase in energy of t2g and decrease in o
Strong-field ligands (large o)
→ Overlap between unoccupied * antibonding orbitals of
ligands (CO, CN–) with t2g orbitals of metal
→ Lowering of energy of t2g orbitals by back-bonding (M→L) Intermediate-field ligands ~ H2O, NH3
383
General Chemistry I 50
Fig. 8.31. (a) (ML) [or (b) (ML)] donation showing a reduction (or increase) in Δo compared with that from bonding alone.
(a) Slight increase in energy of t2g electrons (in t2g* MOs)
(b) Significant lowering in energy of t2g electrons
due to back-bonding → Electrons of t2g MOs are delocalised
into unoccupied *(L)
383
eg eg*
E t2g*
t2g
Partially filledmetal d orbitals Filled ligand
p () orbitals
Empty ligandp (*) orbitals
donor (M L)ligands
eg eg*
t2g*
t2g
Partially filledmetal d orbitals
Filled ligandp () orbitals
Empty ligandp (*) orbitals
acceptor (M L)ligands
(a) (b)
General Chemistry I 51
Fig. 8.32. Effect of bonding on the energy-level structure for octahedral coordination complexes.
Summary of the MO picture (Ligand Field Theory) of bonding in octahedral coordination complexes
Cl-, Br- ligandse.g. [CrCl6]3–
IIlustrated for V2+,Cr3+,Mn4+
(d3)
CO, CN–, NO+
Ligands e.g.Mn(CN)4
384
H2O, NH3 ligands
e.g. [V(H2O)6]2+
General Chemistry I 52
10 Problem Sets
For Chapter 8,
2, 8, 18, 26, 32, 44, 46, 58, 64, 66