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