Rates of Electron Self-Exchange Reactions between Oxo-Centered Ruthenium Clusters Are Determined by Orbital Overlap

We report rate constants for electron self-exchange of trinuclear ruthenium clusters of the type [Ru3O(OAc)(6)(CO) (L)(2)](0/-), where L is 4-cyanopyridine, pyridine, or 4-(dimethylamino)pyridine. Rate constants were determined by H-1 NMR line-broadening experiments in CD3CN, CD2Cl2, and THF-d(8), and range from 6.5 x 10(6) to 2.5 x 10(6) s(-1) M-1. Faster self-exchange is observed with more electron-withdrawing substituents on the ancillary pyridine ligands. This effect is attributed to increased orbital overlap between the donor and acceptor as more electron density is drawn onto the pyridine ring. This view is supported by measured NMR contact shifts of the pyridyl protons which reflect increased electron spin density with increasing pyridine ligand electron-withdrawing ability. Normal solvent dependence is also observed, where higher outer-sphere reorganization energies lead to slower exchange.