METAL ACETYLIDE BONDING IN (ETA-5-C5H5)FE(CO)2CCR COMPOUNDS - MEASURES OF METAL-D-PI-ACETYLIDE-PI INTERACTIONS FROM PHOTOELECTRON-SPECTROSCOPY

Gas-phase He I and He II photoelectron spectroscopy is used to experim entally determine the bonding interactions of eta1-acetylide ligands i n (eta5-C5H5)Fe(CO)2C=CR compounds (R = H, (t)Bu, or phenyl). The spec tra show a large amount of interaction between the metal dpi orbitals and the acetylide pi orbitals. Evidence for this is obtained from the splitting of metal-based ionization bands, from the shifts in Cp-based and acetylide-based ionizations, from the changes in ionization cross sections between the He I and He II spectra, and from vibrational fin e structure in the metal-based ionizations. The data indicate that the predominant pi interactions between the acetylide ligands and the met al are filled/filled interactions between the occupied acetylide pi bo nds and the occupied metal dpi orbitals. The electronic interactions o f the C=CR ligands with the metal are compared with those of CH3 (a pr imiarly sigma donor ligand), halides (pi donor ligands), and C=N (a we ak pi acceptor ligand), and are very similar to the interactions of t he chloro ligand. Metal-to-acetylide-pi back-bonding is extremely sma ll in the acetylide compounds. Varying the acetylide substituent cause s significant changes in the sigma and pi donor properties of the C=CR ligand. Compared to C=CH, the C=C(t)Bu ligand is a stronger sigma don or ligand and also has a stronger filled/filled interaction between th e metal dpi and acetylide pi orbitals. The electronic mixing with the C=CPh ligand is even more extensive, since the metal dpi orbitals and the C=C pi bonds are further mixed with the phenyl ring pi orbitals. T he nature of these pi interactions helps to explain the observed react ivity of electrophiles with ML(n)C=CR compounds and the observed elect ronic communication along the metal-carbon-carbon atom chain in transi tion metal-acetylide compounds.