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

Abstract

The 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>I </i>= 1.0 mol dm^-3 (LiClO₄) and [H⁺]= 0.2 mol dm^-3 &nbsp;(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 <i>k</i>₁, the electron transfer rate constants, are 8.90±0.07, 8.09±0.18, and 7.54±0.06 for Mo₂O_4, Mo₂O₃S, and Mo₂O₂S₂, respectively. Similarly, <i>k</i>_-1<i>/k</i>₂<i>, </i>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 Mo₂O_4, Mo₂O₃S, and Mo₂O₂S₂, respectively. Activation parameters, Δ<i>H^≠ </i>and Δ<i>S^≠, </i>are 6.64±0.72 kJ &nbsp;mol^-1 and -204±21 J K^-1 mol^-1 for oxidation of Mo₂O₄, 9.86±0.28 kJ &nbsp;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.