Electronic Structural Effects in Self-Exchange Reactions

We report rate constants for electron self-exchange reactions of a series of trinuclear ruthenium clusters substituted with three pyridines [Ru3O(OAc)6(L)3]+/0 or one carbonyl and two pyridines [Ru3O(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, λis, is estimated to be 1520 cm−1 and outer sphere reorganization energy, λos is estimated to be between 1800 and 3600 cm−1 in CD2Cl2. The large range is due to uncertainty in the electron transfer distance, r. λtot 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.