Reactions of molybdenum(VI) with metal ion reductants.

The reactions of aqueous H2MoO4 at low pH with titanium(II), titanium(III), europium(II), vanadium(II), and germanium(II), as monitored at 430 nm, give biphasic profiles featuring a sharp rise in absorbance followed by a marked decrease (Fig. 1). The final product is the dimeric Mo(V) cation, [Mo2O4]2+, and the strongly absorbing intermediate is taken as a monomeric Mo(V) species. The molar absorbances of the transients from different reductants are not the same, nor are the rate laws governing the fadings. None of the decay curves exhibits evidence of a second order dependence on the transient. The kinetic behaviors of these systems are consistent with the intervention of successor complexes of the type [Chemical structure: see text], (formed by inner sphere reductions of Mo(VI)), which decompose, via first-order processes, to a monomeric Mo(V) species. The latter then experiences rapid dimerization, which is kinetically silent. The possibility that Ge(II) bypasses the unstable tripositive state by reducing Mo(VI) to Mo(IV) (which then undergoes rapid Mo(VI)-Mo(IV) comproportionation) is considered.