COORDINATIVE BEHAVIOR OF THE CNCN LIGAND - EXPERIMENTAL AND DENSITY-FUNCTIONAL STUDY OF SPECTROSCOPIC PROPERTIES AND BONDING IN THE CR(CO)(5)CNCN COMPLEX

The coordinative behavior of the CNCN ligand is investigated experimen tally, by means of UV-Vis, UV-photoelectron, and IR spectra of the Cr( CO)(5)CNCN complex and, theoretically, by means of density functional calculations on the same complex and on the free CNCN ligand. The dens ity functional calculations give accurate results for the spectroscopi c properties of these compounds. Special attention is paid to a compar ison of the CO and CNCN ligands. First bond dissociation energies, and various components of this bond obtained by an energy decomposition, are compared for L = CO and L = CNCN in the Cr(CO)(5)L complexes. It i s found that the sigma-base properties of CO and CNCN are quite simila r, while CNCN is found to be an only slightly stronger pi acid than CO in spite of a much lower lying pi LUMO. The pi-acceptor capability o f CNCN is not much larger than that of CO because the CNCN 3 pi has l ower amplitude than the CO 2 pi at the coordinating C, leading to sma ller overlap with the pi-donor orbital of the metal fragment.