Thiomolybdates — Simple but Very Versatile Reagents

Abstract

A large number of thiometallate anions, MOxS4−xn−, and some of the seleno analogues, are known in which M is a transition metal in a high oxidation state. They are characterised by their strong colours arising from low energy S(Se)M charge transfer transitions, by unusual redox properties and by their ability to act as ligands to produce a wide range of heterometal complexes. This review outlines some of these properties associated with the thiometallate anions of molybdenum(VI). These anions are prepared by passing H2S through an alkaline molybdate solution, giving a sequential replacement of O atoms by S and ultimately MoS42− (TTM) as product. The thiomolybdates, particularly TTM, have been the most intensively studied because of their ease of preparation, their relatively greater thermal and hydrolytic stabilities and their biological roles. Unusual redox changes which involve internal electron transfer have been shown by reaction with organic disulfides and with CuII. In the latter case insoluble polymeric products are readily formed which, from EPR and EXAFS studies, involve S-bridged reduced Cu and Mo centres. Reaction of TTM with CuI, on the other hand, produces a range of complexes in which from 1 to 6 Cu ions add across the tetrahedral faces of TTM with no redox changes. Further CuI ions can be added to the [Cu6(MoS4)] moiety to produce cluster ions. This illustrates the ability of the thiometallates to act as building blocks for a very wide range of heterometal complexes ranging from simple linear ions to complex clusters, a prime example of the latter being the FeMo cofactor of the enzyme nitrogenase. TTM was identified sometime ago as the reactive intermediate in the Mo-induced Cu-deficiency that afflicts ruminants with serious consequences. Subsequently TTM, as its ammonium salt, has been successfully used in removing the excess Cu and then controlling the Cu levels of patients of Wilson’s disease. It appears to act by blocking the intestinal absorption of Cu and by converting Cu into a metabolically inert form, and in these respects is different to the mode of action of chelating agents which can be used for treating this disease.