Bonding in methyl- and silyl-cyclopentadiene compounds: a study by photoelectron spectroscopy and ab initio molecular-orbital calculations

Abstract

Photoelectron spectra of the cyclopentadiene derivatives RC₅H₅(R = 1-Me, 2-Me, and 5-SiH₃) are reported. The observed ionisation potentials correlate well with orbital energies calculated by ab initio methods. Calculated total energies of the 1-, 2-, and 5-isomers show that the order of stability is 2-Me > 1-Me > 5-Me but 5-SiH₃ > 2-SiH₃ > 1-SiH₃. The Si–C bonding orbital in the 5-SiH₃ derivative is heavily involved in the π-system of the ring, and a high overlap integral exists between silicon and the far ring-carbon atoms. It is suggested that these features of the calculated eigenvectors are related to ‘fluxional’ behaviour shown by the silyl (and related) compounds but not by the methyl derivatives.