Background Chemistry of Molybdenum
Molybdenum is the fourth member of the second transition series and is placed with chromium and tungsten in Group 6 of the Periodic Table. In its chemical properties molybdenum resembles tungsten and vanadium, the first member of Group 5, rather than chromium. Chemically molybdenum is versatile, forming compounds in a range of readily interconvertible oxidation states, complexes with many inorganic and organic ligands including physiologically important compounds, binuclear and polynuclear species involving bridging ligands (oxide, hydroxide, sulfide) or direct metal-metal bonds between molybdenum atoms, and compounds in which the molybdenum coordination number ranges from four to eight.
P.C.H. Mitchell, Chemical and Engineering News, 2003, 81, 108: Molybdenum.
Molybdenum : an outline of its chemistry and uses, ed. E.R. Braithwaite, J. Haber. Amsterdam ; Oxford : Elsevier, 1994.
Mitchell, P. C. H. et al., (eds.), Proceedings of the Climax International Conferences on the Chemistry and Uses of Molybdenum, Climax Molybdenum Co. Ltd., London and Ann Arbor: First 1973, Second 1976, Third 1979, Fourth 1982, Fifth 1985.
Mitchell, P.C.H., in Ullmann’s Encyclopedia of Industrial Chemistry, 5th Ed., 1990, A 16, Chap. 7, pp 675 - 682 and references therein.
In biological systems molybdenum is an essential constituent of enzymes [Stiefel, 2002; Williams and da Silva, 2002] which catalyse redox reactions, e.g. oxidation of aldehydes, xanthine and other purines [Stiefel, 1996; Schindelin et al., 1996], and reduction of nitrate and molecular nitrogen. [Sellman, 1993; Kim et al., 1993; Burris and Roberts, 1993]. The biochemical importance of molybdenum is due to its ability (a) to provide facile electron-transfer pathways, a consequence of the easy interconvertibility of different oxidation states, (b) to form bonds with nitrogen-, oxygen-, and sulfur-donors which are sufficiently strong to permit the existence of stable complexes but also sufficiently labile to permit facile ligand exchange reactions or changes of molybdenum co-ordination number (as when interacting directly with a substrate).