Substituent effects on the frontier molecular orbitais of aryl isonitriles

An investigation of the frontier molecular orbitais ofo- and p-RC₆H₄NC (R=H, CH₃, NO₂, F, Cl, CF₃, OCH₃) was carried out so that a thorough understanding of the intricacies of σ donation and π acceptance could be developed and used to modify subtly the electron density on metal centers. The results of this study-Indicate that the substituent position (ortho vs. para ) does alter the electron density in the ligand appreciably and that substitution of the phenyl ring with the groups indicated has a smaller effect on the σ-donating ability than it does on the π-accepting ability of the isonitrile ligand.

The π-accepting abilities of the isonitrile ligands increase in the order o-, p-CH₃OC₆H₄NC, o-, p-CH₃C₆H₄NC, o-, p-C₆H₅NC, o-, p-FC₆H₄NC, o-, p-CF₃C₆NC, o-, p-ClC₆H₄NC, o-, p-NO₂C₆H₄NC while the σ-donating ability decreases in this order. The energies of the σ-donor and π-acceptor orbitais are shown to correlate well with observed E values of Cr(RC₆H₄NC)₆ and Mn(RC₆H₄NC)₆⁺¹ complexes. This demonstrates how the theoretical results can be useful in understanding the observed physical properties of isonitrile-metal complexes.