Electronic Structural Effects in Self-Exchange Reactions

Abstract

We report rate constants for electron self-exchange reactions of a series of trinuclear ruthenium clusters substituted with three pyridines [Ru(3)O(OAc)(6)(L)(3)](+/0) or one carbonyl and two pyridines [Ru(3)O(OAc)(6)(CO)(L)(2)](0/-). For the 0/- couple in the latter series, the observed rate constant is determined by orbital overlap. More electron-withdrawing pyridine ligands increase the donor-acceptor overlap and more effectively reduce a large reorganization barrier (λ ≈ 10,000 cm(-1)), leading to faster exchange. Larger aromatic ligands also increase the rate by increasing the coupling. For the +/0 couple in tris-pyridyl clusters, there is no observable trend based on pyridine electron-withdrawing ability, and we conclude that charge density on peripheral ligands is not a determining factor for this exchanging pair. From structural and vibrational data, the inner sphere reorganization barrier, λ(is), is estimated to be 1520 cm(-1) and outer sphere reorganization energy, λ(os) is estimated to be between 1800 and 3600 cm(-1) in CD(2)Cl(2). The large range is due to uncertainty in the electron transfer distance, r. λ(tot) for the +/0 pair is then estimated to be between 3320 and 5120 cm(-1), well less than the 10,000 cm(-1) estimated for the 0/- pair. The similar rate constants observed despite very different reorganization energies is explained by donor-acceptor orbital overlap in the 0/- pair, which is consistent with greater delocalization allowed by donor-acceptor orbital symmetry.