Photoelectron spectroscopy and bonding of iron and rhodium organometallic compounds with metal-carbon and metal-hydrogen bonds.

Abstract

Gas phase ultraviolet photoelectron spectroscopy was used to investigate the electronic structure of organometallic compounds. Acetylide ligands in (η⁵-C₅H₅)Fe(CO)₂CECR (R = H, ᵗBu, Ph, C=CH) compounds are found to be effective π donor ligands. There is extensive interaction between the filled acetylide π and filled dπ orbitals. The HOMO and SHOMO of the acetylide compounds are partially localized on the acetylide β carbon. Variation of the R group causes both the σ and π donor ability of the acetylide to change significantly. The dπ/pπ mixing is more extensive in the R = ᵗBu or Ph compounds than in the R = H compound. In the compound CpFe(CO)₂(C=C-C=CH), the butadiyne π system mixes extensively with the metal dπ electrons. The d1t electron density is extended across the C₄H chain. These findings explain the observed reactivity toward electrophiles and Metal/acetylide communication. The first photoelectron spectra of Rh(III) compounds were obtained. The electronic structure of (η⁵-C₅Me₅)Rh(PMe₃)(Cl)(R) compounds, where R = CI, CH₃ or C₆H₅, is sensitive to changes in the R ligand. The a donor ability of the methyl and phenyl ligands are similar, but the phenyl has an additional filled/filled interaction with a metal dn orbital. The PES of (η⁵-C₅Me₅)Rh(PMe₃)(R)₂ (R = H or CH₃) revealed an electronic structure that is not isolobal with other d⁶ organometallic compounds. The HOMO and SHOMO are ligand based, as a result of the metal orbitals mixing with the Cp ring and M-R a bond combinations. The PES spectra of the compounds (η⁵-C₅H₄X)Rh(CO)₂ (X = NO₂, CF₃, CI, H, NMe₂, CH₃) gave measures of σ (inductive) and π (resonance) effects of the X substituent. The ionization energies of the metal based orbitals correlate with the carbonyl stretching frequencies and with Hammett σ(p) parameters. The X = Cl and NMe₂ compounds have significant π overlap with the Cp ring, and also have accelerated rates of associative CO substitution. The measures of inductive and resonance effects were correlated to the rates of CO substitution. The 7t terms are crucial to the correlation and suggest stabilization of an η³-Cp slipped ring intermediate. The PES of the associative substitution products (η⁵-C₅H₅)Rh(CO)L (L = PMe₃ or PPh3) have analogous electronic structure to CpRh(CO)₂, except that the valence ionizations are destabilized due to donor properties of the phosphines. The PPh₃ ligand is a better donor than PMe₃.