Experimental measure of metal-alkynyl electronic structure interactions by photoelectron spectroscopy: (η⁵-C₅H₅)Ru(CO)₂CCMe and [(η⁵-C₅H₅)Ru(CO)₂]₂(μ-CC)

The gas-phase He I and He II photoelectron spectra of the propynylruthenium molecule CpRu(CO)₂CCMe (Cp = η⁵-C₅H₅) and the ethynediyldiruthenium molecule [CpRu(CO)₂]₂(μ-CC) are compared with the spectrum of CpRu(CO)₂Cl to experimentally determine electronic structure interactions of the alkynyl ligands with the metal. The spectra indicate that the interaction between the filled metal-dπ and filled alkynyl-π orbitals dominates the metal-alkynyl π electronic structure, mirroring previously characterized CpFe(CO)₂ alkynyls. All valence ionizations of the Ru molecules are stabilized with respect to similar Fe compounds, contrary to the common expectation of lower ionization energies with atomic substitution down a column of the periodic table. Ab initio electronic structure calculations suggest that this stabilization traces to the greater inherent electronic relaxation energy associated with removal of Fe 3d electrons compared to removal of Ru 4d electrons. Destabilization of the first two ionization bands of the diruthenium molecule are a result of filled-filled interactions between alkynyl π-bonds with the symmetric combination of metal-metal-dπ orbitals, showing electronic communication between the metals through the alkynyl bridge. From the photoelectron spectrum, this communication was calculated to have a minimum electron-transfer integral of 0.56 eV. The stabilization of the antisymmetric combination of the metal-metal-dπ orbitals gives a direct and unique experimental measure of the interaction with the alkynyl π^∗ orbitals. The stabilization caused by the alkynyl π^∗ orbitals was found to be approximately one-third of the destabilization caused by the filled-filled interaction with the alkynyl π-bonds and about one-fourth to one-third the stabilization provided by back-bonding to a carbonyl ligand.