The following successive axial ligand substitution reactions of the head-to-head (HH) 2-pyridonato-bridged cis-diammineplatium(III) dinuclear complex ([(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(H2O)] 4+) with halide ions X- (X- = Cl- and Br-) to give monohalogeno and dihalogeno complexes were studied kinetically: [(H2O)Pt(NH3)2(μC5H 4NO)2Pt(NH3)2 (H2O)]4+ + X- ⇄ Kobs1 [(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(X)]3+ + H2O, [(H2O)Pt(NH3)2(μ-C5H 4NO)2Pt(NH3)2(X)]2 (X)]3+ + X- ⇄ Kobs2 [(X)Pt(NH3)2(μ-C5H4NO) 2Pt(NH3)2(X)]2+ +H2O. The acid dissociation constant (Kh1) of the axial aqua ligand in the HH dimer and the formation constants (K1X and K2X of the monohalogeno and dihalogeno complexes were determined spectrophotometrically to be -log(Kh1/M) = 1.71 ± 0.04; log(K1Cl/M-1) = 5.93 ± 0.02, log(K2Cl/M-1) = 3.71 ± 0.00 for the reaction with Cl- and log(K1Br/M-1) = 6.20 ± 0.05, log(K2Br/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(N4) 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.
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