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 (<b>1</b>, Cp = η⁵-C₅H₅) undergoes a reversible one-electron anodic reaction at <i>E</i>_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 <b>1</b>⁺ is consistent with an approximately 1:1 mixture of <i>anti</i> and <i>gauche</i> 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 <i>anti</i>-<b>1</b> and <i>gauche</i>-<b>1</b> are metal–ligand delocalized. Removal of an electron from <b>1</b> has very little effect on the Os–Os bond order, accounting for the resistance of <b>1</b>⁺ 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 <i>anti</i>-<b>1</b> and <i>gauche</i>-<b>1</b>, respectively. The CO-bridged complex Os₂Cp*₂(μ-CO)₂(CO)₂ (Cp* = η⁵-C₅Me₅), <i>trans</i>-<b>2</b>, undergoes a more facile oxidation, <i>E</i>_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 <b>2</b>⁺ is interpreted in terms of the presence of two isomers, most likely <i>anti</i>-<b>2</b>⁺ and <i>trans</i>-<b>2</b>⁺, at low temperature. Calculations show that the HOMO of <i>trans</i>-<b>2</b> 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 <b>2</b>⁺ has the <i>anti</i> all-terminal CO structure

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