Metal-Acetylide Bonding in (η⁵-C₅H₅)Fe(CO)₂C≡CR Compounds. Measures of Metal-dπ-Acetylide-π Interactions from Photoelectron Spectroscopy

Gas-phase He I and He II photoelectron spectroscopy is used to experimentally determine the bonding interactions of η¹-acetylide ligands in (η⁵-C₅H₅)Fe(CO)₂C≡CR compounds (R = H,^tBu, or phenyl). The spectra show a large amount of interaction between the metal dπ orbitals and the acetylide π 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 fine structure in the metal-based ionizations. The data indicate that the predominant π interactions between the acetylide ligands and the metal are filled/filled interactions between the occupied acetylide π bonds and the occupied metal dπ orbitals. The electronic interactions of the C≡CR ligands with the metal are compared with those of CH₃ (a primiarly σ donor ligand), halides (π donor ligands), and C≡N (a weak π* acceptor ligand), and are very similar to the interactions of the chloro ligand. Metal-to-acetylide-π* back-bonding is extremely small in the acetylide compounds. Varying the acetylide substituent causes significant changes in the σ and π donor properties of the C≡CR ligand. Compared to C≡CH, the C≡C^tBu ligand is a stronger σ donor ligand and also has a stronger filled/filled interaction between the metal dπ and acetylide π orbitals. The electronic mixing with the C≡CPh ligand is even more extensive, since the metal dπ orbitals and the C≡C π bonds are further mixed with the phenyl ring π orbitals. The nature of these π interactions helps to explain the observed reactivity of electrophiles with ML_nC≡CR compounds and the observed electronic communication along the metal-carbon-carbon atom chain in transition metal-acetylide compounds.