DELOCALIZED ELECTRONIC INTERACTIONS IN CHIRAL CYCLOPENTADIENYLRHENIUM HALIDE-COMPLEXES - VALENCE PHOTOELECTRON-SPECTRA OF CPRE(NO)(L)X (CP = ETA-5-C5H5, ETA-5-C5(CH3)5 L = CO, P(C6H5)3 X = CL, BR, I)
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The electron energies and distributions of the title complexes are investigated by valence photoelectron spectroscopy. Assignment and characterization of the ionization bands are assisted by the trends in He I and He II cross sections for ionization intensities, as well as by the widths and vibrational progressions of certain ionization bands. The shifts of the ionizations, which are key to revealing the nature of the electronic structure interactions in a system, are caused by halogen substitutions on the metal, methyl substitutions on the cyclopentadienyl rings, and phosphine substitutions for the carbonyls. The first three ionizations, corresponding to the three occupied d orbitals of the d6 Re(I) metal center, are widely separated. This is because each of the three metal d orbitals have very different interactions with the three different CO,NO, and halogen ligands. The ionizations indicate extensive metal-halogen mixing and a very delocalized electronic structure throughout the metal-ligand system. The first two ionizations correspond to the metal-halogen ir* interaction, which results from the filled-filled interaction, with the first ionization including back-bonding to the carbonyl and the second ionization including back-bonding to the nitrosyl. The third ionization involves back-bonding to both the carbonyl and the nitrosyl. The first two ionizations of the iodide complexes show short vibrational progressions corresponding to the CO and NO stretches. The difference in stabilization of the metal levels by metal-carbonyl and metal-nitrosyl back-bonding is determined from the splitting between the first two ionization bands. The M-X * orbitals have increasing halogen character from the chloride complex to the iodide complex, but unlike for the previously studied Re(CO)5X complexes, where the first ionization shifted from predominantly metal to predominantly halogen in proceeding from X = Cl to X = I, in this case the first three ionizations remain predominantly metal throughout. There is indication in one case of further delocalized interaction of the M-X electrons with Cp-based electrons. The extent of delocalized and fluid electron density in these systems is in contrast to other systems like (5-C5H5)Fe(CO)2X and (CO)sReX. 1991, American Chemical Society. All rights reserved.
