Precise Investigation of the Axial Ligand Substitution Mechanism on a Hydrogenphosphato-Bridged Lantern-Type Platinum(III) Binuclear Complex in Acidic Aqueous Solution

Detailed equilibrium and kinetic studies on axial water ligand substitution reactions of the “lantern-type” platinum(III) binuclear complex, [Pt2(μ-HPO4)4(H2O)2]2-, with halide and pseudo-halide ions (X- = Cl-, Br-, and SCN-) were carried out in acidic aqueous solution at 25 °C with I = 1.0 M. The diaqua Pt(III) dimer complex is in acid dissociation equilibrium in aqueous solution with −log Kh1 = 2.69 ± 0.04. The consecutive formation constants of the aquahalo complex ( ) and the dihalo complex ( ) were determined spectrophotometrically to be log = 2.36 ± 0.01 and log = 1.47 ± 0.01 for the reaction with Cl- and log = 2.90 ± 0.04 and log = 2.28 ± 0.01 for the reaction with Br-, respectively. In the kinetic measurements carried out under the pseudo-first-order conditions with a large excess concentration of halide ion compared to that of Pt(III) dimer (CX− ≫ CPt), all of the reactions proceeded via a one-step first-order reaction, which is a contrast to the consecutive two-step reaction for the amidato-brid...