Kinetic and equilibrium study on the axial-ligand substitution reaction ofthe head-to-tail alpha-pyridonate-bridged cis-diammineplatinum(III) dinuclear complex: trans effect of the axial ligand through the Pt-Pt bond to theopposite axial ligand

Acid dissociation constant of the axial aqua ligand in the Head-to-Tail (HT ) alpha -pyridonate-bridged cis-diammineplatinum(III) dimer complex {[(H2O) Pt(NH3)(2)(mu -C5H4NO),Pt(NH3)(2)(H2O ](4+)} was determined spectrophotomet rically to be -log(K-h1/M) = 1.98 +/-0.01 at 25 degreesC and 1 = 2.00 M. Su ccessive 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: t(H2O)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(H2O)](4+) X(-)reversible arrow[(H2O)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(X) ](3+) + H 2O (k(obs1)) [(H2O)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(X)](3+) + X(-)revers ible arrow [(X)Pt(NH3)(2)(mu -C5H4NO)(2)Pt(NH3)(2)(X)](2+) + H2O (k(obs2)) Formation constants of the monohalo and dihalo complexes were determined sp ectrophotometrically to be log(K-1(Cl)/M-1) = 5.27 +/-0.02 and log(K-2(Cl)/ M-1) = 3.83 +/-0.01 for the reaction with Cl-, and log(K-1(Br)/M-1) = 5.33 +/-0.03 and log(K-2(Br)/M-1) = 4.44 +/-0.02 for the reaction with Br-. In t he HT dimer having two equivalent platinum atoms, the deprotonation occurs to the water molecule on one of the two Pt atoms, whereas the first nucleop hilic substitution with X- occurs to the other Pt atom. Substitution of the second water ligand with Cl- proceeds by a simple substitution path, where as the second substitution with Br- proceeds by two parallel paths: one inc ludes the dissociation of the axially coordinated water molecule followed b y the Br- coordination, and the other is the simple one-step substitution p ath. The difference of the reaction paths is reasonably explained by the di fferent trans effect of the halide ions exerted through the Pt-Pt bond to t he other terminal Pt atom in the monohalo complexes.