The evidence for, and the factors
determining, the formation of metal-to-metal (m-m) bonds in transition metal
oxides, sulphides, halides, and related complexes are discussed. It is proposed
that when metals of high preferred valence state are constrained to low formal
oxidation states, m-m bonds will form to allow the metal to exercise a higher
valency. It is assumed that (n-l)d orbitals are used for such bonds and
predictions of the feasibility of m-m bonding are possible by the comparison of
the size of such orbitals with the expected inter-metal distances in polymeric
metal oxides and halides. It is concluded that binary oxides and halides
(excepting fluorides) of the lower oxidation states of Zr, Hf, Nb, Ta, Mo, W, and Re and of a few neighbouring elements
will display m-m bonds.
