Comparison of the one-electron oxidations of CO-bridged vs unbridged bimetallic complexes: Electron-transfer chemistry of Os₂Cp₂(CO)₄ and Os₂Cp^∗₂(μ-CO)₂(CO)₂ (Cp = η⁵-C₅H₅, Cp^∗ = η⁵-C₅Me₅)

The one-electron oxidations of two dimers of half-sandwich osmium carbonyl complexes have been examined by electrochemistry, spectro-electrochemistry, and computational methods. The all-terminal carbonyl complex Os₂Cp₂(CO)₄ (1, Cp = η⁵-C₅H₅) undergoes a reversible one-electron anodic reaction at E_1/2 = 0.41 V vs ferrocene in CH₂Cl₂/0.05 M [NBu₄][B(C₆F₅)₄], giving a rare example of a metal-metal bonded radical cation unsupported by bridging ligands. The IR spectrum of 1⁺ is consistent with an approximately 1:1 mixture of anti and gauche structures for the 33 e^- radical cation in which it has retained all-terminal bonding of the CO ligands. Density functional theory (DFT) calculations, including orbital-occupancy-perturbed Mayer bond-order analyses, show that the highest-occupied molecular orbitals (HOMOs) of anti-1 and gauche-1 are metal-ligand delocalized. Removal of an electron from 1 has very little effect on the Os-Os bond order, accounting for the resistance of 1⁺ to heterolytic cleavage. The Os-Os bond distance is calculated to decrease by 0.10 å and 0.06 å as a consequence of one-electron oxidation of anti-1 and gauche-1, respectively. The CO-bridged complex Os₂Cp^∗₂(μ-CO)₂(CO)₂ (Cp^∗ = η⁵-C₅Me₅), trans-2, undergoes a more facile oxidation, E_1/2 = -0.11 V, giving a persistent radical cation shown by solution IR analysis to preserve its bridged-carbonyl structure. However, ESR analysis of frozen solutions of 2⁺ is interpreted in terms of the presence of two isomers, most likely anti-2⁺ and trans-2⁺, at low temperature. Calculations show that the HOMO of trans-2 is highly delocalized over the metal-ligand framework, with the bridging carbonyls accounting for about half of the orbital makeup. The Os-Os bond order again changes very little with removal of an electron, and the Os-Os bond length actually undergoes minor shortening. Calculations suggest that the second isomer of 2⁺ has the anti all-terminal CO structure. (Figure Presented)