Interconversion of MeReO(dithiolate)(NC5H4-X) and MeReO(dithiolate)(PAr3) complexes: The equilibrium constants follow the Hammett equation but the rate constants do not

Equilibration occurs among the species MeReO(dithiolate)Py, MeReO(dithiolate)(PZ3), Py, and PZ3 where the chelating dithiolate ligand is 1,2-ethanedithiol (edt) or 1,3-propanedithiol (pdt), Py stands for NC5H4-4-X and PZ3 for both P(C6H4-4-Y)3 and P(alkyl)n(Ph) 3-n. Equilibrium constants in the pdt series were evaluated directly; values of K generally favor phosphane coordination and range from 4.8 × 10^-2(X = NMe2, Y = Cl) to 3.2 × 10^4(X = CN, Y = OMe). The values of K are well correlated by the Hammett equation with (rho)XK= 2.7(3) and (rho)YK=-2.0(3). Kinetic data were determined with the stopped-flow method for 65 reactions of the edt and pdt complexes, and resolved into forward and reverse components by use of the equilibrium constants. Values of kfor deviate markedly from Hammett behavior, especially along any series with a given X substituent, where plots of log kfor against 3(sigma)Y take on a Vshaped appearance. This pattern has been interpreted in terms of a two step mechanism for ligand substitution reactions of these complexes. The rate constants for those phosphanes that are the better Lewis bases are governed by Re-P bond formation. The rate constants for those phosphanes that are weaker Lewis bases, on the other hand, are governed by the second step in which an initial (psi)-octahedral complex rotates towards a transition state that is an approximate trigonal prism. In so doing, the prior Re-P interaction is weakened, which gives rise to an increase in log kfor with (sigma)Y.