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

DAFF, PJ; KALTSOYANNIS, N; (1995) PHOTOELECTRON SPECTROSCOPIC INVESTIGATION OF ISOMERIC METHYL (ISOCYANIDE), ETA(2)-IMINOACYL AND ETA(3)-AZAALLYL COMPLEXES OF MOLYBDENUM AND TUNGSTEN. J CHEM SOC DALTON (12) 1925 - 1932.

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Abstract

Gas-phase ultraviolet photoelectron spectroscopy and extended-Huckel molecular orbital calculations have been employed to investigate the valence electronic structures of twelve compounds drawn from three general 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)('))] (iminoacyl) 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 (isocyanide) compounds are unstable with respect to isomerisation to the corresponding azaallyls on the time-scale of the photoelectron spectroscopy experiment Ica. 4 h), sufficient data were acquired to support the assertion 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 iminoacyl and azaallyl compounds are assigned to the two molecular orbitals containing the four electrons of predominant metal d character. The ionisation energies of these orbitals are ca. 0.5 eV lower in the iminoacyl than the azaallyl compounds. The energetic separation of these orbitals 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 stable pi orbital of the iminoacyl and azaallyl ligands. The destabilisation is greater in the azaallyl compounds due to the additional involvement of an azaallyl fragment orbital which has been previously termed a 'nitrogen lone pair'. Neither the photoelectron spectroscopic nor the theoretical data are consistent with this description.

Type:Article
Title:PHOTOELECTRON SPECTROSCOPIC INVESTIGATION OF ISOMERIC METHYL (ISOCYANIDE), ETA(2)-IMINOACYL AND ETA(3)-AZAALLYL COMPLEXES OF MOLYBDENUM AND TUNGSTEN
Keywords:TRANSITION-METAL DIMERS, SPECTRA
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry

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