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, [Pt ₂(μ-HPO ₄) ₄(H ₂O) 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 K _h1 = 2.69 ± 0.04. The consecutive formation constants of the aquahalo complex (K ₁ ^X) and the dihalo complex (K ₂ ^X) were determined spectrophotometrically to be log K ₁ ^Cl = 2.36 ± 0.01 and log K ₂ ^Cl = 1.47 ± 0.01 for the reaction with Cl ^- and log K ₁ ^Br = 2.90 ± 0.04 and log K ₂ ^Br = 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 (C _X- ≫ C _Pt), 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-bridged platinum(III) binuclear complexes. The conditional first-order rate constant (k _obs) depended on C _X- as well as the acidity of the solution. From kinetic analyses, the rate-limiting step was determined to be the first substitution process that forms the monohalo species, which is in rapid equilibrium with the dihalo complex. The reaction with 4-penten-1-ol was also kinetically investigated to examine the reactivity of the lantern complex with olefin compounds.