PHOTOELECTRON SPECTROSCOPIC INVESTIGATION OF ISOMERIC METHYL (ISOCYANIDE), ETA(2)-IMINOACYL AND ETA(3)-AZAALLYL COMPLEXES OF MOLYBDENUM ANDTUNGSTEN

Gas-phase ultraviolet photoelectron spectroscopy and extended-Huckel m olecular orbital calculations have been employed to investigate the va lence electronic structures of twelve compounds drawn from three gener al isomeric classes; [M(Me)(CNR)(CO)(2)(eta(5)-C(5)R(5)('))] [methyl ( isocyanide)], [M{eta(2)-C(=NR)Me}(C0)(2)(eta(5)-C(5)R(5)('))] (iminoac yl) and [M{eta(3)-H2CC(H)NR}(CO)(eta(5)-C(5)R(5)('))] (azaallyl) (M = Mo or W, R = Pr-i or Bu(t), R' = H or Me). Although the methyl (isocya nide) compounds are unstable with respect to isomerisation to the corr esponding azaallyls on the time-scale of the photoelectron spectroscop y experiment Ica. 4 h), sufficient data were acquired to support the a ssertion that RNC is a better pi donor but poorer pi acceptor than CO. The first two bands in the photoelectron spectra of all of the iminoa cyl and azaallyl compounds are assigned to the two molecular orbitals containing the four electrons of predominant metal d character. The io nisation energies of these orbitals are ca. 0.5 eV lower in the iminoa cyl than the azaallyl compounds. The energetic separation of these orb itals is traced to a destabilisation of the 2a' M(CO)(2)(eta(5)-C(5)R( 5)') fragment orbital via a filled-filled interaction with the most st able pi orbital of the iminoacyl and azaallyl ligands. The destabilisa tion is greater in the azaallyl compounds due to the additional involv ement of an azaallyl fragment orbital which has been previously termed a 'nitrogen lone pair'. Neither the photoelectron spectroscopic nor t he theoretical data are consistent with this description.