Kinetics of aquation of hexachloroosmate(IV) and chloride anation of aquopentachloroosmate(IV) anions

Article abstract of DOI:10.1021/ic50025a017

The kinetics of the reactions OsCl₆^-2 + H₂O ?_k-1^k1 Os(OH₂)Cl₅^- + Cl^- have been investigated in HCl medium at 80°; the forward reaction has also been studied at 70-89° in HNO₃ medium, in which the aquation product and secondary products are rapidly oxidized to OsO₄ and Cl^-. In 0.00001-0.1 F HNO₃ (μ = 0.5-1.32, NaNO₃), and by deduction in HCl of the same acidity, k₁ = 3.5 × 10^-6 sec.^-1 at 79.53° in the dark; E_a = 33.1 ± 0.6 kcal., log pZ = 15.1 ± 0.4 (sec.^-1), ΔH°*₂₉₈ = 32.5 ± 0.6kcal., ΔS°*₂₉₈ = 8₄¹ ± 2 cal. deg.^-1, k₁ (calcd.) = (6 ± 1) × 10^-10sec.^-1 at25°. In 0.5-1.32 FHNO₃ (μ = 1.32, NaNO₃) at 80° the rate of disappearance of OsCl₆^-2 is up to 30% greater than for 0.00001-0.1 F H⁺, apparently due to a contribution from oxidation of OsCl₆^-2 to OsCl_6-² and Cl^-. In 0.01-1.32 F HCl (μ = 1.32, NaCl) at 80°, the previously uncharacterized Os(OH₂)Cl₅^- is the only product observed at short reaction times; the rate of loss of OsCl₆^-2 in 1.32 F H⁺ (where k = 4.17 × 10^-6 sec.^-1, close to k₁ in 0.00001-0.1 F HNO₃, μ = 1.32, at 80°) is ca. 14 times that in 0.01 F H⁺. Rate runs made in 0.016 F HCl with added low concentrations of different oxidants give k values approaching or nearly equal to k₁ for 0.00001-0.1 F HNO₃; the abnormally high rates of OsCl₆^-2 loss in HCl at low acidities may arise from reduction by Cl^- of very small amounts of OsCl₆^-2 to one or more hydrolytically more labile complexes. Chloride anation of Os(OH₂)Cl₅^- was studied at 79.53° in 2.49-3.80 F Cl^-, 2.46-3.80 F H⁺ (μ = (Cl^-), KCl). An exact first-order dependence on (Cl^-) was not observed, 10⁵k_-1 varying from 1.01 to 1.97 M^-1 sec.^-1; in 3.3-3.8 F HCl k_-1 ≈ 2 × 10^-5 M^-1 sec.^-1. The visible and near-ultraviolet absorption spectrum of Os(OH₂)Cl₅^- is reported. Compounds described in the literature as pentachlorohydroxy osmate-(IV) salts, M₂[Os(OH)Cl₅], are very probably binuclear complexes, M₄[Cl₅OsOOsCl₅]·H₂O.