Oxidation of cysteinato complexes of dimeric molybdenum(V) by hexachloroiridate(IV) in aqueous perchlorate solution

1855-1859The kinetics and mechanism of the oxidations of [Mo2(O)2(μ-O)2 (cys)2]2- abbreviated (Mo2O4) , [Mo2(O)2(μ-O) (cys)2]2- (Mo2O3S), and [Mo2(O)2(μ-O)2 (cys)2]2- (Mo2O2S2) by [IrCl6]2- have been studied in aqueous perchlorate solution at ionic strength , I = 1.0 mol dm-3 (LiClO4) and [H+]= 0.2 mol dm-3  (HClO4) . The effects of the replacement of the ethylenediaminetetraacetate (EDTA) ligand with the cysteinato (cys) ligand, as well as the progressive substitution of the bridging oxygen atoms with sulphur atoms, are discussed. At 25.0°C, the values (in dm3 mol-1 s-1) of k1, the electron transfer rate constants, are 8.90±0.07, 8.09±0.18, and 7.54±0.06 for Mo2O4, Mo2O3S, and Mo2O2S2, respectively. Similarly, k-1/k2, a measure of the stability of the intermediates formed in the reactions, have values of (1.02±0.03)×103, (1.63±0.09)×103 and ( 1.81±0.04) ×103, (dm3 mol-1) , for Mo2O4, Mo2O3S, and Mo2O2S2, respectively. Activation parameters, ΔH≠ and ΔS≠, are 6.64±0.72 kJ  mol-1 and -204±21 J K-1 mol-1 for oxidation of Mo2O4, 9.86±0.28 kJ  mol-1 and -194± 18 J K-1 mol-1 for Mo2O3S, and 10.75±0.15 kJ mol-1 and -192± 17 J K-1 mol-1 for Mo2O2S2. All reactions occur via outer- sphere electron transfer