Xα Calculations and Spectroscopic Studies of Triruthenium and Triosmium Dodecacarbonyls

Nonempirical calculations of the Xα type are reported for Ru₃(CO)₁₂ and Os₃(CO)₁₂. Two procedures were used to model the molecular charge density for calculating the self-consistent potential: an approximate self-consistent charge scheme and the more rigorous superposition of multipolar densities. This latter type of procedure accurately calculates the splitting of occupied d orbitals in Ru₃(CO)₁₂ and Os₃(CO)₁₂. Centrally directed metal-metal σ- and π-bonding orbitals are occupied as well as their antibonding counterparts. Stability of the latter levels results from metal—carbon monoxide π bonding. A filled degenerate cyclopropane-like edge-bonding orbital furnishes four bonding electrons to the cluster framework. Difference electron density maps aid the analysis of the two lowest energy electronic transitions. The dipole-allowed transition (15e′ 6a₂′) should be strongly metal—metal antibonding, and the dipole-forbidden HOMO →LUMO excitation [formula omitted] is predominantly transition-metal—CO antibonding. Raman spectra for Ru₃(CO)₁₂ exhibit resonance enhancement of both the 185 (a₁′) and 130-cm⁻¹ (e′) Ru–Ru stretching vibrations. This behavior confirms the predicted antibonding nature of the lowest allowed transition and suggests a Jahn–Teller distortion in the excited state or B-term scattering. Spectrophotometric titrations show the metal center in Os₃(CO)₁₂ to be approximately 5 times more basic toward the proton than that in Ru₃(CO)₁₂.