CHEM 1311A Syllabus - Georgia Institute of Technologyww2.chemistry.gatech.edu/~barefield/1311/coordination_complexes.pdf · CHEM 1311A Syllabus • Transition metals and Coordination

CHEM 1311A Syllabus

• Transition metals and Coordination Chemistry– Introduction to coordination compounds; stereochemistry,

isomerism and nomenclature– Coordination compounds: bonding models and energetics– Coordination compounds: equilibria and substitution reactions

• Bioinorganic chemistryFourth Exam – Friday, April 17

What do these have in common?

• Hemoglobin• Myoglobin• Automobile paints• Anti-cancer drugs (some)• Industrial catalysts (many)• Arthritis drugs• Vitamin B12

• Cytochromes

• “Blue blood”• Ferredoxins• Rubies• Emeralds• Legumes (nitrogen fixers)• Radiopharmaceuticals (some)• MRI contrast agents

All contain a transition metal!!All are coordination compounds

Many are colored

Transition metal complex (coordination compound) terminology

• Coordination compound, coordination complex, complex - a compound containing a metal ion and appended groups, which are Lewis bases and may be monatomic or polyatomic, neutral or anionic.

• Ligand - Lewis base bonded (coordinated) to a metal ion in a coordination complex.– Those with only one point of attachment are monodentate

ligands. – Ligands that can be bonded to the metal through more than one

donor atom are termed bidentate (two points of attachment), tridentate, etc. Such ligands are termed chelating ligands.

Some examples of ligands (with abbreviations)

N N

NH2

NH2

SCH3

SCH3

C

C

O-O

O O-

O

O-

PR2

PR2

H2NN

NH2

H

N

NH2 NH2H2NH2N

SNH2

N N

O- -O

N N

N N

H

HH

H

N NCO2

-

CO2-

-O2C

-O2C

I- Br- Cl- F- OH- H2O SCN- NH3 NO2- PR3 P(OR)3 C2H4 PF3 CO CN-

bipy

en

acac

ox

R = Ph, diphosR = Me, dmpe

dien

EDTA

tren

salen

cyclam

HN NH2

HN NH2

trien


• Coordination number - number of ligands coordinated to a metal ion, 2-12.

• Coordination geometry or stereochemistry (octahedral, tetrahedral, square planar) - geometrical arrangements of ligands (donor groups) about a metal ion.

Examples of coordination complexes

PPh3

Ni PPh3

BrBr

PPh3

Ni PPh3

ClCl

NiCl PEt3

ClEt3P

Comparison of space filling models of triphenylphosphine, P(C6H5)3 = PPh3, and

triethylphosphine, P(CH2CH3)3 = PEt3

Space filling models depict Van der Waals radii for atoms and reflect the effect volume that they occupy

Effect of coordination number and geometry on absorption spectrum

Comparison of electronic absorption spectra for [Co(OH2)6]2+ (octahedral) and [CoCl4]2- (tetrahedral)


H2N

H2N

NiNH2N

N NH2

H2

H2 2+OH2

Ni

OH2

H2O OH2

OH2H2O

2+ 3+

H2N

H2N

CoNH2N

N NH2

H2

H2

Effect of ligand on absorptionspectra (and color)

Formulas/structures of some Pt(II) complexes

Composition No. ions Today’s formulation

PtCl2·4NH3 3 [Pt(NH3)4]Cl2

PtCl2·3NH3 2 [Pt(NH3)3Cl]Cl

PtCl2·2NH3 0 cis-[Pt(NH3)2Cl2]trans-[Pt(NH3)2Cl2]

PtCl2·NH3·KCl 2 K[Pt(NH3)Cl3]

PtCl2·2KCl 3 K2[PtCl4]


• Isomers– Constitutional (structural) isomer - one of two or more

compounds having the same composition but differing in their atom connectivities.

– Stereoisomer - one of two or more compounds having the same atom connectivities but different spatial arrangements of atoms.

• Diastereoisomer – stereoisomers not related by mirror images

• Enantiomer - one of a pair of species that are non-superimposable mirror images.

Types of IsomerismConstitutional

(structural) Stereo

Diastereomers(geometric)

Enantiomers(optical)

Linkage

Ionization

Hydration

● Constitutional (structural) isomers – same composition, different atom connectivities

● Stereoisomers – same composition, same atom connectivities, different spatial arrangements

Stereoisomers: Diastereoisomers

X

MX

L

L

tetrahedral square planar

ML X

LXtrans

ML X

XLcis

• Compounds that have the same atom connectivities, but which are not mirror images are diastereoisomers.


PPh3

Ni PPh3

BrBr

PPh3

Ni PPh3

ClCl

NiCl PEt3

ClEt3P


X

M

X

L L

LL

LL

X

LL

X

M trans

L

M

L

L X

LX

LX

L

LX

L

M trans

L

M

L

L X

XL

LL

L

XX

L

M cisML4X2



3+

H2N

H2N

CoNH2N

N NH2

H2

H2

H2

H2Cl

CoNN

N N

Cl

H2

H2

1+1+

H2N

H2N

CoNH2Cl

Cl NH2



X

M

X

L L

XL

LL

X

XL

X

M mer

X

ML X

XLL

LL

X

XX

L

M fac

ML3X3

Stereoisomers: Enantiomers (optical isomers)

C ClBr

F

H

CCl Br

F

H

• Compounds that have no center or plane of symmetry exist in non-superimposable, mirror-image forms.

Optical rotation

Rotate by 180E

Stereoisomers: Enantiomers

C ClBr

F

H

CCl Br

F

H



3+

H2N

H2N

CoNH2N

N NH2

H2

H2




– Even MA2B2C2 can exist in enantiomeric forms (optical isomers)

A

M

B

A C

CB

A

M

B

AC

C B

Rotate 180o about A-M-B axisA

M

B

B C

CA

How many diastereoisomers can exist for the complex ion [Co(H2NCH2CH2NH2)(NH3)2Cl2]+ ?

How many of these diastereoisomers have nonsuperimposable mirror image forms?

How many diastereoisomers can exist for [Co(dien)(Cl)(NO2)2]?

N

NH2 H2N

H

H2N NHNH2 =dien = = N N N

How many stereoisomers (diastereoisomers and enantiomeric forms) can exist for [Co(H2NCH2CH2O)3]?

The tetradentate ligand shown below forms six-coordinate complexes with Co(III) having the composition [CoLX2]+ where X is a mondentateligand. How many diastereoisomers can be formed?

HN NH2

HN NH2

N

N

N

N

=

Energy changes for formation of ML6n+

E

M + 6 Ln+ ML6

electrostaticattraction

n+

e-e replusion

differential replusionsof d orbitals

d z 2 dx - y 2 2

E)

ML n+(octahedral)6

dxydxz dyz

Magnetic properties depend upon the magnitude of Δo

• High spin – maximum number of unpaired electrons for dn

– Spin pairing energy is greater than ΔE (Δo)• Low spin – minimum number of unpaired electrons for dn

– Spin pairing energy is less than ΔE (Δo)

Dependence of magnetic and spectral properties on ligand type

I– < Br– < SCN– < Cl– < NO2– < F– < OH– < C2O4

2– ≈ H2O < NCS–

< py < NH3 < en < bipy < phen < NO2– < PR3 < CN– ≈ H– < CO

Weak FieldSmall Δ

Frequently high spinPoor σ Donors

π Bases (donors)

Strong FieldLarge Δ

Frequently low spinStrong σ Donors

π Acids (acceptors)

(n-1) d

n s

n p

L orbitals

Constructing an energy level diagram for a complex with F donor ligands

FFF

F*

F*

F*

n

)t2g

eg*

x

x

y z

x

z

y

Energy level diagram for complexwith F donor ligands

(n-1) d

n s

n p

)

L orbitals

F*

FFF

F*

F*

nt2g

eg*

Metal-ligand B-bonding interactions

dB-pB donor interactions; halide, hydroxide

dB-pB acceptor interactions (rare)

dB-dB acceptor interactions; phosphorus, arsenic

dB-B* acceptor interactions; CO, CN-, NO, RNC

dB-B* acceptor interactions; olefins (C=C)

(n-1) d

n s

n p

)

L orbitals

Energy level diagram for complexwith F and B donor ligands

B

B*

F*

F*

F*

n

FFF

t2g

eg* t2g

x

x

y z

x

z

y

HOMO and LUMO for cyanide ion

E

s

s

p

p

(n-1) d

n s

n p

)

L pi acceptororbitals

L orbitals

Energy level diagram for complexwith F and B acceptor ligands

FFF

B

B*

F*

F*

F*

n

t2g

eg*

(n-1) d

n s

n p

)

L pi acceptororbitals

L orbitals

(n-1) d

n s

n p

)

L orbitals

(n-1) d

n s

n p

)

L orbitals

Energy level diagrams for complexes with Fonly, F plus B donor, and F plus B acceptor

ligands

)

F + B acceptor

strong field ligands

eg*

t2g

Effect of B-donor and B-acceptor interactions on ) in octahedral complexes

energy of d-orbitalsprior to interaction with ligands

)

F + B donor

weak field ligands

t2g*

eg*

)

F bonding only

intermediate field ligands

t2g

eg*

Dependence of magnetic properties on L

● All 4d and 5d transition metal complexes are low spin.● All CN– and CO complexes are low spin.● All aqua ions of 3d metals are high spin.● d4 Cr2+, Mn3+ High spin except for very strong field ligands.● d5 Mn2+ Almost always high spin, t2g

3eg2.

Fe3+ Low spin for bpy or stronger field ligands.

● d6 Fe2+ High spin for NH3 and weaker field ligandsLow spin for bpy and stronger field ligands.

Co3+ Low spin for NH3 and stronger field ligands.

● d7 Co2+ Usually high spin.


2– ≈ H2O < NCS–


Weak FieldPoor σ Donors

π Bases (donors)

Strong FieldStrong σ Donors


Variation of M-O distance in [M(OH2)6]3+ with number of M d-electrons

Data are for {[Cs(OH2)6][M(OH2)6]}(SO4)2 J. Chem. Soc., Dalton Trans. 1981, 2105

1.86

1.88

1.90

1.92

1.94

1.96

1.98

2.00

2.02

2.04

0 1 2 3 4 5 6 7

No. of d electrons

M-O

dis

tanc

e/Ǻ

Ti

V

Cr

Mn Fe

Co

Mn(III),Fe(III) are h.s.

Co(III) is l.s.

Electronic absorption spectra

• Selection rules

– Electronic transitions that occur without change in number of unpaired electrons (spin multiplicity) are allowed

– Electronic transitions that involve a change in the number of unpaired spins are “forbidden” and are therefore of low intensity.

> e.g., solutions of high-spin d5, e.g., Mn(II), complexes are lightly colored

dxz dxy dyz

dx2-y2 dz2

E dxz dxy dyz

dx2-y2 dz2

dxz dxy dyz

dx2-y2 dz2

dxz dxy dyz

dx2-y2 dz2

Allowed vs forbidden transitions

Number of d electrons and spectral intensity

[Mn(OH2)6]2+

Electronic absorption spectra

– Electronic transitions are symmetry forbidden in complexes with a center of symmetry (octahedral), but are not symmetry forbidden in complexes without a center of symmetry (tetrahedral)

Comparison of electronic absorption spectral intensities for [Co(OH2)6]2+

and [CoCl4]2- (symmetry)

Comparison of electronic absorption spectral intensities for [Mn(OH2)6]2+

and [MnBr4]2- (symmetry)

Electronic absorption spectra, cont’d • Absorption bands are broad because metal-ligand bonds are

constantly changing distance (vibration) and since electronic transitions occur faster than atomic motions this means that there are effectively many values of Δo.

• d0 and d10 complexes do not have d-d transitions and are colorless unless there are other types of absorptions with energies that fall in the visible region

• d1 and d9, and high-spin d4 and d6 ions have only one spin-allowed transition; high-spin d2, d3, d7 and d8 have three spin-allowed transitions

N N

NNZn Ph

Ph

Ph

Ph

+

Base

N N

NNZn PhPh

Ph

Ph

Base

dxz dxy dyz

dx2-y2 dz2

Edxz dxy dyz

dx2-y2 dz2

dxz dxy dyz

dx2-y2 dz2

dxz dxy dyz

dx2-y2 dz2

dx2-y2

dxydxz dyz

dz2

dxydxz dyz

dx2-y2 dz2

dx2-y21 dxz

1 dxy1dz21 dyz

1dx2-y21

dx2-y21 dxy

1 dxz1dz21 dyz

1dz21

Transitions in d1 and d2 complexes

Some compounds are very highly colored because of charge transfer transitions – even

some with no d electrons● There can be electronic transitions in the visible region that do

not involved d-electrons

− MnO4- (purple) and CrO4

2- (yellow) are intensely colored because electrons in filled oxygen based orbitals are excited into empty d-orbitals (LMCT)

− Ligand to Metal Charge Transfer (LMCT) bands have few selection rule restrictions and are typically very intense

● Metal to Ligand Charge Transfer (MLCT) bands may also occur for complexes with d-electrons.

− There are few selection rules and the high intensity of these bands may mask d-d transitions.

Crystal field splitting in tetrahedral complexes• Tetrahedral arrangement of four ligands

showing their orientation relative to the Cartesian axes and the dyz orbital.

• The orientation with respect to dxz, dxz and dxy is identical and the interaction with these orbitals is considerably greater than with the dz

2 and dx2- y

2 orbitals; therefore the dyz, dxz and dxy orbitals are higher in energy than dz

2 and dx2- y

2 .

• Because there are only four ligands and the ligand electron pairs do not point directly at the orbitals, Δt ~4/9 Δo. As a result the spin-pairing energy is always greater than Δ and tetrahedral complexes are always high spin.

Comparison of crystal field splittings for octahedral, square planar and tetrahedral

ligand fields

Factors affecting the magnitude of ) (Crystal Field Splitting)

● Charge on the metal. For first row transition elements )Ovaries from about 7,500 cm-1 to 12,500 cm-1 for divalent ions and 14,000 cm-1 to 25,000 cm-1 for trivalent ions.

● Position in a group. )O values for analogous complexes of metal ions in a group increase by 25% to 50% on going from one transition series to the next. This is illustrated by the complexes [M(NH3)6]3+ where ) values are 23,000 cm-1 for M=Co; 34,000 cm-1 for M=Rh and 41,000 cm-1 for M=Ir.

Larger 4d and 5d orbitalsoverlap and interact more strongly with L making the eg orbitalsmore antibonding and increasing )O.

Smaller M–L distances in more highly charged ions leads to stronger interaction with L making the eg orbitals more antibondingand increasing )O.


● Identity of the ligand. The magnitude of ΔO reflects the extent and way in which the metal interacts with the ligands.

− Better σ-donor Lewis base ligands are higher in the spectrochemical series because the eg orbitals are more antibonding and destabilized.

− Better π-donor ligands are lower in the spectrochemical series because the t2g orbitals are more destabilized since they are now π-antibonding.

− Better π-acceptor ligands are higher in the spectrochemical series because the t2g orbitals are more stabilized since they are now π-bonding.


2– ≈ H2O < NCS–


Weak FieldPoor σ Donors

π Bases (donors)

Strong FieldStrong σ Donors


Variation of )O in octahedral Ti(III) complexesTi(III) is a d1 ion and exhibits one absorption in the electronic spectrum of its metal complexes due to transition of the electron from the t2g (lower energy) orbitals to the eg (higher energy) orbitals. The energy of the absorption corresponds to )O.

Ligand )O/cm-1*Br- 11,400Cl- 13,000(H2N)2C=O 17,550NCS- 18,400F- 18,900H2O 20,100CN- 22,300*E = h< = hc/8


● Geometry and coordination number. For similar ligands )twill be about 4/9 )O. This is a result of the reduced number of ligands and their orientation relative to the d orbitals. Recall that the energy ordering of the orbitals is reversed in tetrahedral complexes relative to that in the octahedral case.

Thermodynamic vs kinetic stability

• Stability in a thermodynamic sense refers to the energetics of a formation or decomposition reaction )G = )H - T)S

• Stability in a reactivity sense refers to the rate with which a given reaction occurs )G‡ = )H‡ - T)S‡

• Complexes that undergo substitution with half-lives less than about one minute are referred to as labile; those that are less reactive are termed inert.

• Complex stability and reactivity do not necessarily correlate with ligand field strength; the latter refers to spectroscopic and magnetic properties.

• Thermodynamic and kinetic stabilities sometimes parallel but often they do not.– [Ni(CN)4]2& illustrates the latter case; the overall equilibrium

constant its formation is >1030 but the second order rate constant for CN& exchange is >5 x 105 M-1 s-1

Ligand Substitution Energetics

M–X + Y

ΔG‡

ΔGº

The nature of the M–X and M–Y bonding interactions determine ΔGº and ΔG‡.

•ΔGº is a function of the relative strengths of M–X vs. M–Y.

•ΔG‡ is a function of the lability of the M–X bond.

M–Y + X

Thermodynamic and kinetic stabilities sometimes parallel but often they do not.

Stepwise formation of [Cu(NH3)4]2+

[Cu(OH2)4]2+ + NH3 W [Cu(OH2)3(NH3)]2+ + H2O log K1 = 4.22

[Cu(OH2)3(NH3)]2+ + NH3 W [Cu(OH2)2(NH3)2]2+ + H2O log K2 = 3.50

[Cu(OH2)2(NH3)2]2+ + NH3 W [Cu(OH2)(NH3)3]2+ + H2O log K3 = 2.92

[Cu(OH2)(NH3)3]2+ + NH3 W [Cu(NH3)4]2+ + H2O log K4 = 2.18

β4[Cu(NH3)4

2+]

[Cu2 ][NH3]4= +

[Cu(OH2)4]2+ + 4 NH3 W [Cu(NH3)4]2+ + 4 H2O

Speciation is determined by ligand concentration[Cu(OH2)4]2+ + n NH3 = [Cu(OH2)4-n(NH3)n]2+

0.00.10.20.30.40.50.60.70.80.91.0

0246-log[NH3]

Frac

tion

n=0

n=1 n=2n=3

n=4

Chelating ligands have larger formation constants than comparable non-chelating ligands; the Chelate

Effect is largely (substantially?) entropic in origin

[Cu(OH2)4]2+ + en W [Cu(OH2)2(en)]2+ + 2 H2O log K1 = 10.6)H = -54 kJ mol-1, )S = 23 J K-1 mol-1

[Cu(OH2)4]2+ + 2 NH3 W [Cu(OH2)2(NH3)2]2+ + 2H2O log K2 = 7.7)H = -46 kJ mol-1, )S = -8.4 J K-1 mol -1

Ligand substitution in coordination complexes● Arguably the most important reaction of coordination complexes

is ligand substitution.● There are two limiting mechanisms for substitution reactions

– associative parallels the SN2 reaction in organic chemistry; the reaction involves an intermediate of higher coordination number. rate = k[complex][L]

> associative reactions are more important for larger metal ions and for those that have vacancies in the t2g orbitals

Ligand substitution in coordination complexes● There are two limiting mechanisms for substitution reactions

– associative (already discussed)– dissociative parallels the SN1 reaction in organic chemistry;

the reaction involves an intermediate of lower coordination number. rate = k[complex]

● The simplest substitution is ligand exchange which is not complicated by thermodynamics since ΔG = 0.– exchange rates of water have been most extensively studied– rate constants for water exchange range from 1.1x10-10 s-1 to

5x109 s-1

Observations on water exchange

● An increase in oxidation state for the metal reduces the rate ofexchange

● Early (larger) elements in a period tend to have a greater contribution from associative processes

● Heavier (larger) elements in a family have a greater contribution from associative processes; also greater bond strengths decrease rate of dissociative processes

● Occupancy of (antibonding) eg orbitals increases the rate for all oxidation states

Water exchange rates in aquo metal ions

Rate constantsa for water exchange[MLnn(OH22)]n+n+ k/s-1-1 [MLnn(OH22)]n+n+ k/s-1-1

[Ti(OH2)6]3+ 1.8 x 105

[V(OH2)6]2+ 8.7 x 101 [V(OH2)6]3+ 5.0 x 102

[Cr(OH2)6]2+ >108 [Cr(OH2)6]3+ 2.4 x 10-6

[Mn(OH2)6]2+ 2.1 x 107

[Fe(OH2)6]2+ 4.4 x 106 [Fe(OH2)6]3+ 1.2 x 102

[Ru(OH2)6]2+ 1.8 x 10-2 [Ru(OH2)6]3+ 3.5 x 10-6

[Co(OH2)6]2+ 3.2 x 106

[Ni(OH2)6]2+ 3.2 x 104

[Pd(OH2)4]2+ 5.6 x 10-2

[Pt(OH2)4]2+ 3.9 x 10-4

[Cu(OH2)6]2+ >107

[Zn(OH2)6]2+ >107

[Cr(NH3)5OH2]3+ 5.2 x 10-5

[Co(NH3)5OH2]3+ 5.7 x 10-6

[Rh(NH3)5OH2]3+ 8.4 x 10-6

[Ir(NH3)5OH2]3+ 6.1 x 10-8

aAll rate constants are expressed as first order rate constants for comparative purposes even though some reactions are associative.

Electron transfer reactions: importance of orbital occupancy and spin state on rate

t2g5

t2g6

t2g6

t2g5eg

2

e- config

8.2 x 1022.144[Ru(NH3)6]2+

2.104[Ru(NH3)6]3+

1.936[Co(NH3)6]3+

8 x 10-62.114[Co(NH3)6]2+

kex, M-1 s-1M-L BD, DComplex

● Electron transfer is second only to substitution in importance as a characteristic reaction of coordination complexes and especially in biological systems.

● Again the simplest reaction is outer-sphere electron exchange where )G=0

● Rates of electron exchange vary enormously across the transition series, but two things are invariably true:– The rate of electron transfer is greatest when electrons are transferred

from a t2g orbital on the reductant to a t2g orbital on the oxidant.– Changes in bond distance in either oxidant or reductant upon electron

transfer are minimal in these cases.– However, when electrons are lost/gained from eg orbitals reaction rates

decrease and bond distance changes increase.

Electron transfer reactions: importance of orbital occupancy and spin state on rate

1.976t2g4[Mn(CN)6]3-

1.95t2g5[Mn(CN)6]4-

61.900t2g

6[Fe(CN)6]4- *1.926t2g

5[Fe(CN)6]3- *

1.959t2g3[Cr(OH2)6]3+

≤10-52.106t2g

3eg1[Cr(OH2)6]2+

t2g5

t2g6

t2g6

t2g5eg

2

e- config

8.2 x 1022.144[Ru(NH3)6]2+

2.104[Ru(NH3)6]3+

1.936[Co(NH3)6]3+8 x 10-6

2.114[Co(NH3)6]2+

kex, M-1

s-1M-L BD,

DComplex

*νCN = 2121 cm-1 for Fe(III), 2021 cm-1 for Fe(II)

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CHEM 1311A Syllabus - Georgia Institute of Technologyww2.chemistry.gatech.edu/~barefield/1311/coordination_complexes.pdf · CHEM 1311A Syllabus • Transition metals and Coordination