The stabilisation of low valent states of the transition metals: Introductory lecture

Abstract

The ligands most effective in stabilising low valent states are uncharged ligands where carbon is the ligand atom, e. g. CO, PhNC and C 6 H 6, tertiary phosphines, arsines and chelate heterocyclic amines. Anionic ligands, even CN − and RC C −, are much less effective. All the factors which affect the stabilities of complexes must influence the stabilisation of unusual valent states of the transition metals. The most important is the formation of π-type molecular orbitals between the metal and ligand atoms When the ligand orbitals are of low energy and are vacant the bonding π-type molecular orbitals can contain the electrons from the d-orbitals of the metal, and those added during the reduction of the complex until they are all filled. In this way very low valent states of the metal atom can be stabilised. On the other hand, if the ligand orbitals are filled they provide enough electrons to fill the π-type molecular orbitals and the metal electrons are forced into higher energy antibonding orbitals from which some may be readily removed by oxidation, and so higher valent states are stabilised. The electronegativities of the ligand atoms are an important secondary factor, because high electronegativity causes the above processes to be most effective in valency stabilisation. Thus the most electronegative ligand atoms of their types, viz. C and F −, stabilise the lowest and the highest valent states respectively.