HIGHLY-REDUCED METAL-CARBONYLS

Carbonyl metalates contain metals with formally negative oxidation numbers - that has been known since the days of Walter Hieber. Nevertheless, an ion like [Cr(CO)4]4-, which contains chromium(- IV), was considered inaccessible, although it obeys the 18-electron rule. Through the activities of John E. Ellis and coworkers (see also p. 194 in this issue) the treasure-chest of mononuclear metal carbonyls was enriched by many such jewels. Further gaps were systematically filled with [M(CO)6]2-, M = Ti, Zr, Hf and [M(CO)5]3-, M = V, Nb, Ta. The highly reduced metal carbonyls are not only interesting with regard to bonding theory, but are also useful as precursors in the synthesis of multinuclear complexes.