Synthesis and Base Hydrolysis of the Pentaammine(dimethyl sulfide)cobalt(III) Ion

Abstract

The synthesis of the trifluoromethanesulfonate salt of the pentaarnmine (dimethy1 sulfide)-cobalt(III) ion, [NH₃)₅Co-S(CH₃)₂]³⁺, is described along with the kinetics of its hydrolysis in basic and acidic solutions. The synthesis proceeds in 44% yield from the reaction of [(NH₃)₅Co-OSO₂CF₃] (CF₃SO₃)₂ with CH₃SCH₃ in tetramethylene sulfone at 80ºC. The salt has been characterized by elemental analysis, visible-U.V. spectroscopy, and ¹H n.m.r. In basic solution the complex decomposes by Co-S cleavage to yield [(NH₃)₅CO-OH]²⁺ and non-coordinated CH₃SCH₃. The kinetics of this reaction were studied in phosphate buffers ranging from pH 8.50 to 11.67 ( µ= 1.0 M); a linear dependence of the reaction rate on [OH^-] was observed. At 25ºC, k_OH = 8.8 ± 0.2 dm³ mol^-1 s^-1. Activation parameters, determined over a temperature range from 15 to 44ºC, were ΔH^‡ = 152 ± 3 kJ mol^-1 and Δ S^‡ = 286 ± 9 J K^-1 mol^-1. In 0.01 M HClO₄ ( µ = 1.0 M, 25ºC), the cobaltsulfur bond is cleaved at a rate of 1.6×10^-6 s^-l. Activation parameters, determined over a temperature range from 25 to 60ºC, were ΔH^‡ = 106 ± 5 kJ mol^-1 and ΔS^‡= -2 ± 16 J K^-1 mol^-1.