Molybdenum complexes mixed metal derivatives

Substitutions at triangular trinuclear oxo-carboxylato complexes which M3(/i3-0) cores, which may be simple or may be mixed metal or mixed valence species, are often slow. The third step in water replacement in Ru(III)30(OH2)3 derivatives by acetate has a half-life of several minutes at ambient temperatures, while replacement of water by CDjOD in [Ru2Rh ( 3-0)(0Ac)g(0H2)3] has rate constants of 6.0x10 and 1.2x10 for reaction at the Ru and Rh, respectively, at 21 °C in CD30D. Further examples of kinetic studies at trinuclear centers M3O where the metal atoms are molybdenum or tungsten can be found in Section 8.2.2 earlier in this chapter. 

The molecular complexity of molybdenum (IV) alkoxides is determined by the size and ramification of the alkyl group — the polymeric (R = Me), dimeric (CN = 5, two bridging OR-groups, R = Pr ) with a double metal-metal bond, and even monomers (R = Bu ). The first representatives of W(OR)4 homologous series are tetramers with the [Ti4(OMe)16]-type structure. The derivatives of ramified or bulky alcohols are known only as mixed-ligand complexes (such as dimeric solvates of alkoxide halids with alcohols, various alkoxide hidrides and monomeric complexes with phenantroline, see Table 12.19). 

Few derivatives of molybdenum dioxide, MoOj, have been prepared, and it is doubtful whether simple salts containing tetravalent molybdenum can be formed in solution. By the electrolytic reduction of acid molybdate solutions, brownish-coloured liquids apparently containing the metal in this stage of oxidation have been obtained, but the evidence is insufficient to determine whether Mo ions are actually present, or whether the liquids merely contain mixed Mo and Mo ions. Potential measurements indicate the presence of mixed ions. The only simple substances containing tetravalent molybdenum, in addition to the oxide, are the sulphide, MoSj, the tetrachloride, M0CI4, and the tetra-bromide, MoBr. There are, however, two series of complex molybdo-eyanides, of the types R 4[Mo(OH)4(CN)J.aq. and R 4[Mo(CN)8].aq. respectively, which contain tetravalent molybdenum and yield well-cry stallised salts. Their existence is probably due to the resistance of the stable complex to hydrolytic decomposition. 

---