SYNTHESIS AND BASE HYDROLYSIS OF THE PENTAAMMINE(DIMETHYL SULFIDE)COBALT(III) ION

The synthesis of the trifluoromethanesulfonate salt of the pentaammine(dimethyl 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 80-degrees-C. The salt has been characterized by elemental analysis, visible-u.v. spectroscopy, and H-1 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 (mu = 1.0 M); a linear dependence of the reaction rate on [OH-] was observed. At 25-degrees-C, k(OH) = 8.8+/-0.2 dm3 mol-1 s-1. Activation parameters, determined over a temperature range from 15 to 44-degrees-C, were DELTAH(double dagger) = 152+/-3 kJ mol-1 and DELTAS(double dagger) = 286+/-9 J K-1 mol-1. In 0.01 M HClO4 (mu = 1.0 M, 25-degrees-C), the cobalt-sulfur bond is cleaved at a rate of 1.6x10(-6) s-1. Activation parameters, determined over a temperature range from 25 to 60-degrees-C, were DELTAH(double dagger) = 106+/-5 kJ mol-1 and DELTAS(double dagger) = -2+/-16 J K-1 mol-1.