Question

Why do transition elements exhibit such a variety of coloured compounds, complexes and different oxidation states?

Answer 1

It is mainly due to the closeness of the energy levels of the `ns` and `(n-1)d` orbitals.

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1. The variety of colored complexes is due to the various oxidation states possible; a different oxidation state on the central metal atom alters the metal-ligand `d`-orbital splitting energy, which gives rise to varied emission wavelengths.
2. Oxidation states vary due to the general ease of incorporating the `bb((n-1)d)` electrons into bonding.

And the general ease of incorporating the `(n-1)d` electrons into bonding is mainly due to the closeness of the energy levels of the `bb(ns)` and `bb((n-1)d)` orbitals.

(You can see it's all connected.)

To illustrate this, I took orbital potential energy data from here (Appendix B.9) to graph the orbital potential energies for the `(n-1)d` and `ns` orbitals for the first, second, and third row transition metals.

For the most part, the difference in energies are within `"3 eV"` or so, which allows the `d` orbitals to act as valence orbitals in addition to the `s` orbitals.