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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, November 30

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

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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.

• 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.

Effect of coordination number and geometry on absorption spectrum

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

(octahedral) and [CoCl4]2- (tetrahedral)

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Transition metal complex (coordination compound) terminology

• 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

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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.

Stereoisomers: Diastereoisomers

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

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

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Stereoisomers: Diastereoisomers

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

X

M

X

L L

XL

LL

X

XL

X

M mer

X

ML X

XLL

LL

X

XX

L

M fac

ML3X3

Rotate by 180E

Stereoisomers: Enantiomers

C ClBr

F

H

CCl Br

F

H

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

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Stereoisomers: Enantiomers

3+

H2N

H2N

CoNH2N

N NH2

H2

H2

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

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

How many of these diastereoisomers have nonsuperimposable mirror image forms?

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

N

NH2 H2N

H

H2NNH

NH2 =dien =

How many of the diastereoisomers that can exist for [Co(dien)(Cl)(NO2)2] have non-superimposable mirror images, i.e., are enantiomeric?

N

Co

NO2

N Cl

NO2N

N

Co

NO2

N NO2

NCl

N

Co

Cl

N NO2

NO2N

N

Co

Cl

N NO2

NO2N

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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.

HN NH2

HN NH2

N

N

N

N

=

How many diastereoisomers can be formed? How many are enantiomeric?

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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• High spin – maximum number of unpaired electrons for dn

– Spin pairing energy is greater than ∆E (∆o)• Low spin – minimum number of unpaired electrons for dn

– Spin pairing energy is less than ∆E (∆o)

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Dependence of magnetic and spectral properties on ligand type

• Spectrochemical series– I– < Br– < Cl– < SCN– < F–, OH– < NO2

– < H2O < SCN– < NH3 < en < NO2

– < PR3 < P(OR)3, C2H4< PF3, CO, CN–

Energy level diagram for complexwith F donor ligands

(n-1) d

n s

n p

)

L orbitals

F*

FFF

F*

F*

nt2g

eg*

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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*

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(n-1) d

n s

n p

)

L orbitals

Comparison of level diagrams for complexeswith F only and F plus B donor ligands

(n-1) d

n s

n p

)

L orbitals

(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*

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(n-1) d

n s

n p

)

L pi acceptororbitals

L orbitals

(n-1) d

n s

n p

)

L orbitals

Comparison of level diagrams for complexeswith F only and F plus B acceptor ligands

(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 F only, F plus B donor, and F plus B acceptor ligands

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HOMO and LUMO for cyanide ion

E

s

s

p

p

)

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*

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Dependence of magnetic and spectral properties on ligand type

• Spectrochemical series– I– < Br– < Cl– < SCN– < F–, OH– < NO2

– < H2O < SCN– < NH3 < en < NO2

– < PR3 < P(OR)3, C2H4< PF3, CO, CN–

• Strong field ligands = low-spin complexes– have B-acceptor orbitals: B* as in CO or CN–, B*as in

CH2=CH2, low lying d as in PR3, PF3

• Weak field ligands = high-spin complexes– have B-donor orbitals: usually multiple p orbitals as in X

• Intermediate field ligands = usually high spin for +2 ions, low-spin for +3 ions– have few, or no, B -donor or acceptor orbitals, or there is a

poor match in energy of available B orbitals: NH3, H2O, pyridine

Variation of )O in octahedral Ti(III) complexes

Ti(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

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Electronic absorption spectra

• Selection rules– Transitions that occur without change in number of

unpaired electrons (spin multiplicity) are allowed– Transitions that involve a change in the number of

unpaired spins are “forbidden” and are therefore of low intensity.> solutions of high-spin d5, e.g., Mn(II), complexes are

lightly colored• 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 ∆o.

• 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

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

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Effect of ligand on absorptionspectra (and color)

Number of d electrons and spectral intensity

[Mn(OH2)6]2+

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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-y21dxy

1 dxz1dz21 dyz

1dz21

Transitions in d1 and d2 complexes

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

ligand fields

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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 and dxy is identical and the interaction with these orbitals is considerably greater than with the dz

2 and dx2

- y2 orbitals; therefore the dyz, dxz

and dxy orbitals are higher in energy than dz

2 and dx2

- y2 .

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

Factors affecting the magnitude of )(Crystal Field Splitting)

• Charge on the metal. For first row transition elements )O varies 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.• Geometry and coordination number. For similar ligands )t will

be about 4/9 )O. This is a result of the reduced number of ligandsand 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.

• Identity of the ligand. The dependence of ) on the nature of the ligand follows a regular order, known as the spectrochemicalseries, for all metals in all oxidation states and geometries.

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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.• 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

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

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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

The Chelate Effect is largely 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

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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– 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 ∆G = 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

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Water exchange rates in aquo metal ions

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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

• 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.

– There is minimal change in bond distance in either oxidant or reductantupon electron transfer.

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+

# 10-92.114[Co(NH3)6]2+

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