REACTION-KINETICS AND MECHANISM OF FORMATION OF [H4CO2MO10O38](6-) BYPEROXOMONOSULFATE OXIDATION OF CO-II IN THE PRESENCE OF MOLYBDATE

Oxidation of Co-II by HSO5- to Co-III in weakly acidic solution, in th e presence of molybdate, resulted in the formation of (primarily) the soluble dicobalt species [H4Co2Mo10O38](-)(6). The kinetics of formati on of this species was examined at 15-35 degrees C over the range pH 4 .0-5.5 and found to exhibit three separate kinetically observable step s, oxidation of Co-II to Co-III, followed by a ligand-breakdown reacti on and a slow ligand-replacement step which produces the observed prod uct(s). The first stage followed the expanded rate expression +d[Co-II I]/dt=k(ox)[Co2+][HSO5-]/[H+][HMoO4-](3). A value for k(ox) of 7.05(4) x 10(-14) mol(3) dm(-9) s(-1) at 25 degrees C was calculated using re ported formation constants describing the speciation of [MoO4](2-), [M o7O24](6-), and their protonated forms in solution. This rate expressi on may be accounted for by a mechanism arising from a series of pre-eq uilibria involving the loss of three [HMoO4](-) units and a H+ from a cobalt((II)) heteropolymolybdate, most likely [H6CoMo6O24](4-), which then allows the one-electron oxidation of Co2+ + to CO3+ by HSO5- to o ccur following co-ordination of the latter. In keeping with this propo sal, [INH4](4)[H6CoMo6O24]. 4H(2)O was crystallized from an acidic (pH 4.5) solution containing Co2+ (aq) and molybdate, and its structure d etermined by X-ray diffraction methods. The heteropolymetalate anion e xhibits a standard Anderson structure with six octahedral molybdate ed ge-sharing units surrounding the central cobalt, and all metal atoms e ffectively in a common plane. The second observable kinetic step shows no dependences on [Co3+], [oxidant], PH or [molybdate], and is interp reted as a ligand-breakdown reaction, involving loss of an OH. radical from the co-ordinated 'HSO52-' (radical) present following the actual one-electron oxidation step. Dimerization is then assumed to occur, f ollowed by slow [H4Co2Mo10O38](6-) formation stemming from further rea ction of the immediate product of the dimerization step; involving los s of co-ordinated SO42-.