INTRODUCTORY

Vanadium, named after the Nordic Goddess “Vanadis”, is a trace element that occurs in concentrations ranging from 0.1 – 3.0 nmol/g in most mammalian cells. Its concentration is about 136 ppm in the earth’s crust and is nineteenth element in the order of abundance. Anthropogenic sources include the combustion of fossil fuels, particularly residual fuel oils, which make up the single largest overall release of vanadium to the atmosphere. Vanadium has been reported to be an essential bio-element [1] for certain organisms, including tunicates, bacteria and some fungi. The physiological role of vanadium is not known but its importance has been indicated for the normal growth and development [2]. Crans and coworkers have outlined five specific ways by which vanadium interacts with proteins [3, 4]. Vanadium with atomic number 23 and electronic configuration [Ar]3d34s2, can exits in at least six oxidation states. Oxidation states +III, +IV and +V are most common but +III oxidation state is reducing in nature and difficult to exist in aqueous (pH~ 7.0) solution. Oxidation states +IV and +V are generally stabilized through V-O bond, and oxocations [VO]2+, [VO]3+ and [VO2]+ are most common for biological systems.
The presence of vanadium in vanadium based enzymes e.g. vanadate-dependent haloperoxidases and vanadium nitrogenase attracted attention of researchers to develop coordination chemistry of vanadium in search of good models for these enzymes. Studies on the metabolism and detoxification of vanadium compounds under physiological conditions, stability and speciation of vanadium complexes in biofluides, and potential therapeutic and catalytic applications have further influenced the coordination chemistry of vanadium.