Thermodynamic properties of transition-metal ions as a consequence of d-orbital splitting from a molecular-orbital rather than a crystal-field viewpoint

Abstract

A simple molecular-orbital (m.o.) method is used to view the stabilisation energies of octahedrally and tetrahedrally co-ordinated transition-metal complexes. Direct comparison with the crystal-field (c.f.) theory shows why the latter approach has had its successes and failures in the past in looking at heats of hydration, tetrahedral versus octahedral co-ordination, rates of reaction, etc. as a function of d-orbital configuration. The m.o approach shows that while nd orbital effects are important in determining total (and relative) stabilisation energies, interaction of the ligand σ orbitals with metal (n+ 1)s and (n+ 1)p orbitals is generally larger and varies smoothly across the first transition-metal series. It is argued that the wholesale use of d-orbital stabilisation energies alone to quantitatively compare the chemistry of different dⁿ configurations is a dangerous and far from reliable approach.