Donor-acceptor properties of isonitriles studied by photoelectron spectroscopy and electron transmission spectroscopy.

Some alkyl- and aryl-isonitriles, considered as CO analogue s-donor and p-acceptor ligands in transition metal chemistry, have been studied by means of HeI photoelectron spectroscopy and electron transmission spectroscopy, in order to evaluate their donor-acceptor properties from the measured ionisation energies (IEs) and vertical electron attachment energies (VAEs). The investigated molecules included 2-propyl-, 1-butyl-, t-butyl-, 1-pentyl, cyclohexyl-, as well as 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 co-ordination and back donation were identified. The results show that the IE (10.62-10.95 eV) of the s electron pair (nc) responsible for the s-donor capability is substantially lower than that of CO. The VAEs of the empty p* orbitals involved in the d/p* back donation indicate that aryl-isonitriles are better acceptors (VAEs <0.3 eV ) than their aliphatic counterparts (VAEs > 2.7 eV). In the case of aryl derivatives the p-donor ability could also play some role in metal-ligand bonding (IEs 8.74-9.34 eV). Isonitrile coordination characteristics are compared also to those of CO, N2 and CH3CN.