Investigation of Metal-dπ-Butadiynyl-π Interactions in (η⁵-C₅H₅)(CO)₂FeCCCCH Using Photoelectron Spectroscopy

The electronic structure of (η⁵-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 π orbitals, the cyclopentadienyl e₁″ orbitals, and the iron-butadiynyl σ bond. Metal-dπ-butadiynyl-π 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 π donor ligand, and in this case the occupied butadiynyl π orbitals interact with occupied metal dπ orbitals in filled/filled type interactions. The mixing between occupied metal dπ and butadiynyl π orbitals is extensive, and the π system of 1 can be thought of as a five-centered poly-yne. Results from Fenske-Hall and extended Hückel calculations agree with the information obtained from PES and also give additional information about orbital coefficients and charges of the CCCCH π system. The spectrum of 1 is also compared to that of (η⁵-C₅H₅)(CO)₂FeCCH, and the separate σ and π bonding components of the CCH and CCCCH ligands are evaluated.