Mechanisms of the axial ligand substitution reactions of dinuclear, Head-to-Head α-pyrrolidonato- and pivalamidato-bridged cis-diammineplatinum(III) complexes with halide ions

The acid dissociation constant of the axial aqua ligand in the Head-to-Head (HH) dinuclear pivalamidato-bridged cis-diammineplatinum(III) complex ([(H₂O)Pt(NH₃)₂(μ-C₅ H₁₀NO)₂- Pt(NH₃)₂(H₂O)]⁴⁺) and the formation constants of the successive substitution reactions on the axial sites of the HH α-pyrrolidonato-bridged cis-diammineplatinum(III) dinuclear complex and the HH pivalamidato-bridged cis-diammineplatinum(III) dinuclear complex with halide ions X^- (X^- = Cl^- and Br-) to give monohalo and dihalo complexes were determined spectrophotometrically at 25 °C and I = 2.0 M. A kinetic study was also performed for the monohalo and dihalo complex formations, The effect of the bridging ligand upon the axial site reactivity was compared and discussed. The ligand substitution behavior of the monohalo complex was affected by the axial ligand on the opposite Pt site and also by the bridging ligand. For both the α-pyrrolidonato- and the pivalamidato-bridged complexes, the formation of dichloro complexes proceeds only through the simple substitution path (k₂ path), whereas the formation of dibromo complexes proceeds through the two parallel reaction paths: the k₂ and k₃ paths for the former and the k₂ and k₂^# paths for the latter. The difference of the reaction paths is reasonably explained by the different trans effect of the halide ion on the opposite axial site for the former complex, but for the latter complex, the reaction paths are not easy to explain.