Kinetic and Equilibrium Study on the Axial-Ligand Substitution Reaction of the Head-to-Tail α-Pyridonate-Bridged cis-Diammineplatinum(III) Dinuclear Complex: trans Effect of the Axial Ligand through the Pt−Pt Bond to the Opposite Axial Ligand

Abstract

Acid dissociation constant of the axial aqua ligand in the Head-to-Tail (HT) α-pyridonate-bridged cis-diammineplatinum(III) dimer complex {[(H2O)Pt(NH3)2(μ-C5H4NO)2Pt(NH3)2(H2O)]4+} was determined spectrophotometrically to be −log(Kh1/M) = 1.98±0.01 at 25 °C and I = 2.00 M. Successive substitution reaction of the HT dimer with halide ions X− (X− = Cl− and Br−) at the two axial sites to give monohalo and dihalo complexes was studied kinetically: [(H2O)Pt(NH3)2(µ-C5H4NO)2Pt(NH3)2(H2O)]4+ + X− ←→ [(H2O)Pt(NH3)2(µ-C5H4NO)2Pt(NH3)2(X)]3+ + H2O (kobs1) [(H2O)Pt(NH3)2(µ-C5H4NO)2Pt(NH3)2(X)]3+ + X− ←→ [(X)Pt(NH3)2(µ-C5H4NO)2Pt(NH3)2(X)]2+ + H2O (kobs2) Formation constants of the monohalo and dihalo complexes were determined spectrophotometrically to be log(K1Cl/M−1) = 5.27±0.02 and log(K2Cl/M−1) = 3.83±0.01 for the reaction with Cl−, and log(K1Br/M−1) = 5.33±0.03 and log(K2Br/M−1) = 4.44±0.02 for the reaction with Br−. In the HT dimer having two equivalent platinum atoms, the deprotonation occurs to the water molecule on one of the two Pt atoms, whereas the first nucleophilic substitution with X− occurs to the other Pt atom. Substitution of the second water ligand with Cl− proceeds by a simple substitution path, whereas the second substitution with Br− proceeds by two parallel paths: one includes the dissociation of the axially coordinated water molecule followed by the Br− coordination, and the other is the simple one-step substitution path. The difference of the reaction paths is reasonably explained by the different trans effect of the halide ions exerted through the Pt−Pt bond to the other terminal Pt atom in the monohalo complexes.