SUBSTITUENT EFFECTS ON THE FRONTIER MOLECULAR-ORBITALS OF ARYL ISONITRILES

An investigation of the frontier molecular orbitals of o- and p-RC6H4NC (R = H, CH3,NO2,F, Cl, CF3, OCH3) was carried out so that a thorough understanding of the intricacies of sigma-donation and pi-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 sigma-donating ability than it does on the pi-accepting ability of the isonitrile ligand. The pi-accepting abilities of the isonitrile ligands increase in the order o-, p-CH3OC6H4NC, o-, p-CH3C6H4NC, o-, p-C6H5NC, o-, p-FC6H4NC, o-, p-CF3C6H4NC, o-, p-ClC6H4NC, o-, p-NO2C6H4NC while the sigma-donating ability decreases in this order. The energies of the sigma-donor and pi-acceptor orbitals are shown to correlate well with observed E1/2 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.