Kinetics and mechanisms of the axial ligand substitution reaction of the head-to-head 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex

The following successive axial ligand substitution reactions of the head-to-head (HH) 2-pyridonato-bridged cis-diammineplatinum(III) dinuclear complex ([(H2O)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(H2O)](4+)) With halide ions X- (X- = Cl- and Br-) to give monohalogeno and dihalogeno complexes were studied kinetically: [GRAPHICS] The acid dissociation constant (K-h1) Of the axial aqua ligand in the HH dimer and the formation constants (K-1(X) and K-2(X)) of the monohalogeno and dihalogeno complexes were determined spectrophotometrically to be -log(K-h1/M) = 1.71 +/- 0.04; log(K-1(Cl)/M-1) = 5.93 +/- 0.02, log(K-2(Cl)/M-1) = 3.71 +/- 0.00 for the reaction with Cl-, and log(K-1(Br)/M-1) = 6.20 +/- 0.05, log(K-2(Br)/M-1) = 4.55 +/- 0.01 for the reaction with Br-. The two platinum atoms are nonequivalent in the HH dimer, in which the first deprotonation occurs selectively to the water molecule on the Pt(N-4) atom and the first nucleophilic substitution with X- occurs preferentially to the Pt(N2O2) atom, where the atoms in parentheses are the coordinating atoms. The first water substitution with X- proceeds via two similar substitution paths, whereas the second substitution proceeds through two dissimilar paths: one is a simple substitution path and the other is via dissociation of the water molecule, providing a coordinatively unsaturated complex. These reaction paths are reasonably explained by the relative strength of the trans effect of the water, hydroxide, and halide ions in the monohalogeno complexes.