Lecture 2

Coordination ChemistryCoordination Chemistry

Biological systems, e.g., heme

Coordination ChemistryCoordination Chemistry

Alfred Werner (1866-1919) 1893, age 26: coordination theory Nobel prize for Chemistry, 1913 Addition of 6 mol NH3 to CoCl3(aq)

Conductivity studies

Precipitation with AgNO3

Atkins, Jones, p. 934Chang, p. 883Miessler, Tarr, p. 278Mortimer, p. 723Whitten et al., p. 893

Werner Coordination TheoryWerner Coordination Theory

Compound Moles of ions Moles of AgCl(s)

“CoCl3.6NH3”

“CoCl3.5NH3”

“CoCl3.4NH3”

“CoCl3.3NH3”

4 3

3

2

0

2

1

0

Co

NH3

NH3

NH3

Cl

NH3 NH3 NH3 Cl

Cl

Cl– attached to NH3 may be dissociated

Werner Coordination TheoryWerner Coordination Theory

Compound Moles of ions Moles of AgCl(s)

[Co(NH3)6]Cl3

[Co(NH3)5Cl]Cl2

[Co(NH3)4Cl2]Cl

[Co(NH3)3Cl3]

4 3

3

2

0

2

1

0

Proposed six ammonia molecules to covalently bond to Co3+

Coordination ChemistryCoordination Chemistry

Definitions Coordination compounds – compounds

composed of a metal atom or ion and one or more ligands (atoms, ions, or molecules) that are formally donating electrons to the metal center

Miessler, Tarr, p. 278

Coordination ChemistryCoordination Chemistry

Definitions Coordination compounds

NH3

Co

NH3

H3N NH3

NH3H3N

3+

3Cl–

H

N

HH

M

ligand

N forms a coordinate covalent bond to the metal

(coordination sphere)

(counterion)

Coordination ChemistryCoordination Chemistry

Definitions Ligands – simple, ‘complex’ Denticity – different number of donor

atoms Chelates – compounds formed when

ligands are chelating (Gk. crab’s claw)

H3C C

O

O

M

bidentate

Coordination ChemistryCoordination Chemistry

Cr

ON

N

O O

ONCr

O -

edta4–, [(OOCCH2)2NH2NH2(CH2COO)2]4–

[Cr(edta)]–

Valence Bond TheoryValence Bond Theory

Metal or metal ion: Lewis acid Ligand: Lewis base Hybridization of s, p, d orbitals

C.N. Geometry

4 tetrahedral

56

4

Hybrids

sp3

square planar dsp2

trigonal bipyramidal dsp3 or sp3doctahedral d2sp3 or sp3d2

Valence Bond TheoryValence Bond Theory

Example 1: [Co(NH3)6]3+

Co [Ar] 3d7 4s2

Co3+ [Ar] 3d6

3d 4s 4p

if complex is diamagnetic

4d

d2sp3

octahedral

:

Valence Bond TheoryValence Bond Theory

Example 2: [CoF6]3–

Co [Ar] 3d7 4s2

Co3+ [Ar] 3d6

if complex is paramagnetic

3d 4s 4p 4d

4sp3d2

octahedral

Valence Bond TheoryValence Bond Theory

Example 3: [PtCl4]2–, diamagnetic

Pt2+ [Xe] 4f14 5d8

5d 6s 6p

dsp2

square planar

Valence Bond TheoryValence Bond Theory

Example 4: [NiCl4]2–, tetrahedral

Ni2+ [Ar] 3d8

3d 4s 4p

4sp3

paramagnetic

Valence Bond TheoryValence Bond Theory

Ligands (Lewis base) form coordinate covalent bonds with metal center (Lewis acid)

Relationship between hybridization, geometry, and magnetism

Inadequate explanation for colors of complex ions

e.g., [Cr(H2O)6]3+, [Cr(H2O)4Cl2]+

Crystal Field TheoryCrystal Field Theory

Basis: purely electrostatic interaction Spherical field: d orbitals degenerate

•

•What will happen when six ligands approach from the six vertices of an octahedron?

•

••

•

spherical field

free ion

Crystal Field TheoryCrystal Field Theory

egt2g

Crystal Field TheoryCrystal Field Theory

egt2g

eg

t2g

crystal field stabilization energy (CFSE)

Crystal Field TheoryCrystal Field Theory

eg

t2g

crystal field stabilization energy (CFSE)

Crystal Field TheoryCrystal Field Theory

Distribution of electrons

d2 d3

How is a d4 configuration distributed?

Crystal Field TheoryCrystal Field Theory

Pairing energy (P) vs. O

If O < P, weak field;

e.g., [Cr(OH2)6]2+

If O > P, strong field;

e.g., [Cr(CN)6]4–

Crystal Field TheoryCrystal Field Theory

Tetrahedral field

e

t2

e

t2

Crystal Field TheoryCrystal Field Theory

Square planar field

SP

Crystal Field TheoryCrystal Field Theory

Factors affecting magnitude of 1. Oxidation state of the metal ion

2. Nature of the metal ion

3. Number and geometry of the ligands

4. Nature of the ligands

Crystal Field TheoryCrystal Field Theory

Ligands are point charges Metal d electrons repel ligands Splitting of d orbitals Explanation for colors and magnetism of

complex ions No hybridization required