The electronic structure of metal acetylides

Abstract

Three series of acetylide complexes have been examined by gas phase ultraviolet photoelectron spectroscopy to elucidate the electronic structure and bonding of the acetylide to the metal. The electronic properties of the metal fragments and the acetylides were varied in these systems to understand how the acetylide σ and π systems bond to a metal center. The first series of complexes probes the extent of metal-metal electronic communication through the conjugated, acetylide bridged ruthenium dimer compound [(η⁵-C₅H₅)Ru(CO)₂]₂(μ-C≡C). The broad envelope of overlapping metal ionizations seen in the dimer compound compared to the "monomer" (η⁵-C₅H₅)Ru(CO)₂C≡C-CH₃ revealed extensive metal-metal communication through the acetylide bridge. Also observed was a stabilization of metal ionizations in the ruthenium compounds compared to analogous iron compounds. The second series of acetylide complexes probes the bonding effects of increasing the electron richness at the metal center coupled with increasing electron withdrawing capability on the acetylide. The complexes under examination were of the general formula (η⁵-C₅H₅)ML₂C≡C-R (M = Fe, Ru, L = CO, R = p-C₆H₄-NO₂; M = Ru, L = P(CH₃)₃, R = C₆H₅, p-C₆H₄-NO₂] . Ancillary ligand substitution of trimethylphosphine for carbonyl increased the electron richness at the metal center, and the electron withdrawing capability of the acetylide was increased by para substitution of a nitro group on the phenylacetylide. The filled/filled interaction between the metal-dπ/acetylide-π orbitals dominates the metal-acetylide bonding picture in all of the compounds. Nitro substitution on the phenylacetylide resulted in a substantial inductive charge shift, but minimal π effects were observed. Nitro substituted phenyl derivitives showed similar bonding to the acetylides. The third series of complexes probes acetylide bonding with the M₂R₄P₄ core [M = Mo, P = PMe₃, and R = C≡C-Si(CH₃)₃, C≡C-C(CH₃)₃]. A different bonding picture was revealed in the molybdenum series, with the metal-metal- δ orbital having a filled/filled interaction with the acetylide π orbital, as well as donating electron density into the empty acetylide π* orbital. The effects of silicon substitution on the acetylide is probed by comparing ᵗButylacetylene with trimethylsilylacetylene.