A number of industrially important homogeneous and heterogeneous reactions are catalysed by vanadium, molybdenum and tungsten compounds. Such reactions include the isomerization of allyl alcohols, olefin metathesis, oxo transfer reactions and oxidation of propene in SOHIO process. The versatile chemistry of such metals is also an essential part of a variety of biological systems. Specifically, the activities of various enzymes are based on the reactions of a metal centre. Both artificial and biological catalysts involve such metal ions in high oxidation states, therefore many active systems can be modelled with isolated metal-oxo species which are stabilised by hard donors, such as alkoxido and aryloxido ligands.