1CHM3400 - Lecture 31 – Mar 30

Coordination compounds (Chapter 12 469-483)

• Coordination complexes• Hard and soft ligands• Crystal field splitting• Octahedral, tetrahedral, square planar• Protein examples

2

Coordination complexesCoordination complex = metal ion with ligands forming a complex

Metal (=acid) in center surrounded by ligands

Coordination number = number of ligands

3

Hard and soft acids and bases

4

Crystal field theorySplitting of degenerate d orbitals on metal due to repulsion of valence electrons on metal and lone pair on ligands

Energy of orbital is raised due to head-on ligand interaction

5

Octahedral complexesCrystal field splitting Δ

Lowering of d(xy), d(yz), d(xz)

Raising of d(x2-y2) and d(z2)

Crystal field stabilization energy (CFSE) = n(eg)(0.6Δ) - n(t2g)(0.4Δ)

6

Crystal field splitting magnitude

I- < Br- < SCN- < Cl- < F- < OH- < H2O < NH3 < en < NO2- < CN- < CO

Energy of photon corresponds to energy level splitting

Predict energy from λ

7

High spin – low spinDepending on the crystal field splitting, high or low-spin complex is observed

Strong field ligand

Weak field ligand

What rule predicts high spin?

8

Other symmetries

Can you rationalize the different energy splittings?

Splitting for tetrahedral complexes smaller⇒………. spin complexes

9

Molecular orbital theoryCrystal field theory is easy to understand, but it completely neglects the covalent character of metal-ligand bonds.

Non-bonding orbitals

Anti-bonding orbitals

10

Coordination compounds in biological systems

Iron in hemoglobin

Why not possible?

11

High- and low-spin Fe2+

Fe2+: d6

O2 bindingNo O2 binding

Remember cooperative binding for hemoglobin

Low-spin ⇒ more compact complex⇒ fits into heme group

12

CO binding to hemoglobinWhy is CO binding to hemoglobin much stronger than O2 binding (200x stronger)?

13

Zn affinity for tetrahedral binding

Zn2+ d10 configuration

What is the crystal field stabilization energy?

Why is tetrahedral ligand chelation favored?