1. Coordination Chemistry: Bonding Theories Coordination Chemistry: Bonding Theories Chapter 20 Chapter 20 1

2. 2 1. Chemistry of the d-orbitals

3. 2. Crystal Field Theory 3 A purely electrostatic consideration  Ligand electrons create an electric field around the metal center  Ligands are point charges and we do not take their orbitals into consideration  No metal-ligand covalent interactions

4. 3. Energy of the d-orbitals 4 Degenerate 3d atomic orbitals

5. 4. The Octahedral Crystal Field 5 Octahedral coordination environment:  Point Group: O h  d z 2 and d x 2 – y 2 orbitals: e g symmetry  d xy, d xz, and d yz : t 2g symmetry Destabilized Stabilized

6. A. The Magnitude of Δ oct 6 Determined by the strength of the crystal field:  Weak field  Strong field Δ oct (weak field) < Δ oct (strong field) d 5 electron configuration

7. B. Crystal Field Stabilization Energy (CFSE) 7 Consider a d 1 electron configuration: i.e. [Ti(H 2 O) 6 ] 3+ Ti 3+ Δ oct Electronic absorbance  The single electron will be in less energetic ground state. Ground State

8. C. CFSE and High vs Low Spin 8 Consider a d 4 electron configuration (Cr 2+ ):  Pairing has to be invoked Δ oct < p (pairing energy) Weak Field, High spin Δ oct > p Strong Field, Low spin t 2g 3 e g 1 t 2g 4 CFSE = [ 3 x 0.4Δ oct ] - [ 1 x 0.6Δ oct ] = 0.6Δ oct CFSE = [ 4 x 0.4Δ oct ] = 1.6Δ oct – 1 p

9. I. What determines Δ? 9 i.Oxidation state of the metal ion  Δ with ionic charge ii.Nature of M 3d < 4d < 5d Really big Δ, normally low spin (As you go down the periodic table, Δ) iii.Number and geometry of ligands Δ tetrahedral only ~50% of Δ octahedral

10. II. What determines Δ? 10 iv. Nature of ligands  Spectrochemical series (partial) I - < Br - < [NCS] - < Cl - < F - < [OH] - < [ox] 2- ~ H 2 O < [NCS] - < NH 3 < en < [CN] - ~ CO Weak field ligands Strong field ligands Ligands increasing Δ oct YOU CAN NOT UNDERSTAND THIS TREND WITH CRYSTAL FIELD THEORY

11. II. What determines Δ? 11 iv. Nature of ligands  Spectrochemical series (partial) The strength of a ligand determines the Δ oct - Splitting of [CoF 6 ] 3- - Splitting of [Fe(H 2 O)] 3+ I - < Br - < [NCS] - < Cl - < F - < [OH] - < [ox] 2- ~ H 2 O < [NCS] - < NH 3 < en < [CN] - ~ CO Weak field ligands Strong field ligands Ligands increasing Δ oct

12. 5. Octahedral Geometry Distortions 12 D 4h OhOh dz2dz2 d x 2 – y 2 dz2dz2 d xy d xz d yz d xz d yz d xy d xz d yz d x 2 – y 2 dz2dz2 What about a d 1 electron configuration? No YES!!! Degeneracy Removed