Abstract
Some alkyl and aryl isonitriles, considered as CO analogue σ‐donor and π‐acceptor ligands in transition metal chemistry, were studied by means of HeI photoelectron spectroscopy and electron transmission spectroscopy, in order to evaluate their donor–acceptor properties from the measured ionization energies (IE) and vertical electron attachment energies (VAE). The investigated molecules were 2‐propyl, 1‐butyl, tert‐butyl, 1‐pentyl, cyclohexyl, 2,6‐dimethylphenyl, 4‐methoxyphenyl and 4‐chorophenyl isonitrile. By interpreting the spectra on the basis of literature data and quantum chemical calculations, the spectral features associated with the molecular orbitals mainly involved in coordination and back‐donation were identified. The results show that the IE (10.62–10.95 eV) of the σ electron pair (n~c~) responsible for the σ‐donor capability is substantially lower than that of CO. The __VAE__s of the empty π* orbitals involved in the d/π* back‐donation indicate that aryl isonitriles are better acceptors (VAE <0.3 eV) than their aliphatic counterparts (VAE >2.7 eV). In the case of aryl derivatives, the π‐donor ability could also play some role in metal–ligand bonding (IE 8.74–9.34 eV). Isonitrile coordination characteristics are also compared with those of CO, N~2~ and CH~3~CN. Copyright © 2004 John Wiley & Sons, Ltd.