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

1 Answer
Truong-Son N.
Jun 21, 2017

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

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