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

Jun 21, 2017

It is mainly due to the closeness of the energy levels of the $n s$ and $\left(n - 1\right) d$ orbitals.

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 $\boldsymbol{\left(n - 1\right) d}$ electrons into bonding.

And the general ease of incorporating the $\left(n - 1\right) d$ electrons into bonding is mainly due to the closeness of the energy levels of the $\boldsymbol{n s}$ and $\boldsymbol{\left(n - 1\right) d}$ 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 $\left(n - 1\right) d$ and $n s$ orbitals for the first, second, and third row transition metals.

For the most part, the difference in energies are within $\text{3 eV}$ or so, which allows the $d$ orbitals to act as valence orbitals in addition to the $s$ orbitals.