INVESTIGATION OF METAL-D-PI-BUTADIYNYL-PI INTERACTIONS IN (ETA(5)-C5H5)(CO)2FEC-EQUIVALENT-TO-CC-EQUIVALENT-TO-CH USING PHOTOELECTRON-SPECTROSCOPY

The electronic structure of (eta5-C5H5)(CO)2FeC=CC=CH (1) is investigated with gas-phase He I and He II photoelectron spectroscopy (PES). A central issue is the electronic communication from the metal through the poly-yne chain. The spectra show distinct ionization bands (ionizations) that originate from the metal d6 orbitals, the occupied butadiynyl pi orbitals, the cyclopentadienyle1'' orbitals, and the iron-butadiynyl sigma bond. Metal-dpi-butadiynyl-pi electronic interactions are indicated by the splitting pattern of the iron-based ionizations and from changes in ionization cross sections from He I to He II excitation, which indicate the predominant character of ionizations. The C=CC=CH (butadiynyl) ligand is best described as a net pi donor ligand, and in this case the occupied butadiynyl pi orbitals interact with occupied metal dpi orbitals in filled/filled type interactions. The mixing between occupied metal dpi and butadiynyl pi orbitals is extensive, and the pi system of 1 can be thought of as a five-centered poly-yne. Results from Fenske-Hall and extended Huckel calculations agree with the information obtained from PES and also give additional information about orbital coefficients and charges of the C=CC=CH pi system. The spectrum of 1 is also compared to that of (eta5-C5H5)(CO)2FeC=CH, and the separate sigma and pi bonding components of the C=CH and C=CC=CH ligands are evaluated.