KINETICS AND MECHANISMS OF LIGAND-SUBSTITUTION REACTIONS AT TETRAHEDRAL BIS(DIIMINE)COPPER(I) COMPLEXES IN ACETONE AND METHANOL

The equilibrium constants for the stepwise substitution of biquinolyl (biq) by 2,9-dimethylphenanthroline (dmp) in Cu(biq)2+ were determined in acetone and methanol solutions by UV-visible spectroscopy. The kinetics of the mixed-ligand complex formation in these two solvents were studied by means of the stopped-flow method. The formation of Cu(biq)(dmp)+ follows two parallel pathways: a direct reaction via an outer-sphere encounter complex and a solvent-assisted path via an intermediate in which one biq ligand is fully dissociated. Unexpectedly, the rate behavior is quite different in these two solvents. In acetone the overall rate constant for the direct reaction, k12 = K(os)k(I), is 1.8 x 10(3) M-1 s-1 with k(I) = 2.9 x 10(2) s-1. The solvent-assisted pathway is of minor importance. In methanol k12 is 7 M-1 s-1 and k(I) = 1.1 s-1. The rates for the direct pathway and the solvent pathway are similar. The ligand dependence (dmp, phen) of k(I) in acetone shows that an associative activation mode is preferred.