Transition Metals and Color

1. Transition Metals and Color • Why are so many transition metal complexes colored? • Usually only metals with d0 or d10 form colorless compounds (Zn, Ag). • Colors in metal complexes (or any compound) is due to their absorption spectrum.

2. Transition Metals and Color • When a metal complex absorbs light, an electron undergoes an electronic transition from a ground state to an excited state. • Remember: ΔE = hc/λ OR λ = hc/ΔE • We see the color which is NOT absorbed, but which was reflected or transmitted. • We see the complementary color of what was absorbed.

3. Colors of Visible Light

4. Crystal Field Theory • Why do metal complexes absorb light in the Vis light spectrum? • Crystal Field Theory tries to explain this. • When a ligand approaches a free metal atom or ion in order to form a bond, e-e repulsions occur between the metal’s d-electrons and the ligands electrons. • This causes the metal’s 5 degenerate d-orbitals to increase in energy AND to split. • So they are no longer degenerate.

5. d-Orbital Splitting by Ligands

6. Octahedral Complexes and Orbitals

7. Crystal Field Theory • Different ligands cause more of an energy split in the d-orbitals. • Ligands which cause the d-orbitals to split more with a higher ΔE are called strong-field ligands. • Ligands which cause the d-orbitals to split less with a lower ΔE are called weak-field ligands. • Ligand Series from Weak to Strong: I-<Br-<Cl-<F-<H2O<NH3<en<CN-

8. Octahedral Complexes:Weak Field / Strong Field Ligands

9. Crystal Field Theory • Ligand splitting of a metal’s d-orbitals also explains why some complexes are highly paramagnetic and others are diamagnetic or weakly paramagnetic. • Highly Paramagnetic: These are weak field complexes with a low ΔE, so the d-e are easily promoted. The result is a complex with many unpaired electrons, or a high-spin complex.

10. Tetrahedral & Square Planar Complexes