Axial water substitution kinetics of sulphato- and hydrogenphosphato-bridged binuclear platinum(III) complexes

The kinetics of the axial water substitution reactions for [Pt2(B–B)4(H2O)2]2− (B–B=SO4 2−, HPO4 2−) with Cl−, Br− and SCN− are reported in acidic aqueous solution. With a large excess of entering ligand and hydrogen ion, only the disubstituted product complex was formed. The reaction rates are first order with respect to the substrate complex and entering ligand and decrease with increasing H+ concentration. In the presence of 0.10 M H+, the rate constants for the replacement of the first water ligand by Cl−, Br− and SCN− are (7.84±0.46)×10−2, (6.65±0.41)×10−2 and (20.5±1.1)×10−2 M−1 s−1 in [Pt2(SO4)4(H2O)2]2−; (35.5±3.7)×10−2, (30.3±1.4)×10−2 and (84.9±13.4)×10−2 M−1 s−1 in [Pt2(HPO4)4(H2O)2]2−, respectively, at 24 °C and a constant ionic strength of 0.66. The entropies of activation for the reactions are negative and, in the range of −141 and −59 J K−1 mol−1. These results support an associative-interchange, Ia, mechanism.