Electron delocalization in mixed-valence butadienediyl-bridged diruthenium complexes

We report electrochemical and spectroelectrochemical investigations on the butadienediyl-bridged diruthenium complexes {Ru(PPh₃)₂(CO)Cl}₂(μ-C₄H₄)] (1), {Ru(PEt₃)₃(CO)Cl}₂(μ-C₄H₄)] (2), and {Ru(PPh₃)₂(CO)Cl(NC₅H₄COOEt-4)}₂(μ-C₄H₄)] (3). All these complexes are oxidized in two consecutive one-electron steps separated by 315 to 680 mV, depending on the co-ligands. The first oxidation is a chemically and electrochemically reversible process whereas the second varies from nearly reversible to irreversible at room temperature. We have generated and investigated the mixed-valence monocations and observed CO band shifts of ca 25 cm⁻¹ and the appearance of new bands in the visible regime at ca 720 to 800 and 430 to 450 nm. The lower-energy band which tails into the near infrared has been assigned as a charge-resonance (or intervalence charge-transfer) absorption and used to estimate the electronic coupling parameter H _AB. Our investigations point to valence delocalization for 2 ⁺ , and nearly delocalized behavior for 1 ⁺ and 3 ⁺ . Even the complex with the smallest potential splitting is, however, fully delocalized on the longer ESR timescale, as is evident from the coupling pattern of the solution spectrum. Overall IR band shifts on full oxidation and the hyperfine splittings for 1 ⁺ argue for charge and spin delocalization onto the bridging C₄H₄ ligand. This issue has also been addressed by quantum chemical calculations employing DFT methods. Geometry optimizations at each oxidation level reveal inversion of the C–C bond pattern from a short–long–short to a long–short–long alteration and a bis(carbenic) structure at the dication stage. All spectroscopic features such as IR band shifts, average g-values and g-tensor anisotropies are fully reproduced by the calculations. Presented at the 3rd Chianti Electrochemistry Meeting, July 3.–9.2004, Certosa di Pontignano, Italy