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BRADFORD R. SOHNLEIN AND DONG-SHENG YANG, Department of Chemistry, University of Kentucky, Lexington, KY 40506-0055.
Dibenzene complexes of chromium, molybdenum, and tungsten, M(C6H6)2 (M = Cr, Mo, W), were produced by laser ablation in a supersonic jet and probed by pulsed field ionization-zero electron kinetic energy (PFI-ZEKE) electron spectroscopy. The adiabatic ionization potentials were measured to be 44081(6), 44581(11), and 43635(6) cm-1 for Cr(C6H6)2, Mo(C6H6)2, and W(C6H6)2, respectively. The chromium-dibenzene spectrum displays three quanta excitations of the 265 cm-1 metal-benzene symmetric stretch mode in the cation, whereas the molybdenum and tungsten complexes show only one excitation of the metal-ligand stretch of 277 and 370 cm-1, respectively. The shift in the ionization energy from the bare metal atom to the complex (10494, 12623 and 19792 cm-1) and metal-ligand stretch frequencies increase, as the group is descended. These observations indicate a stronger metal-benzene interaction for the heavier atoms, especially tungsten. The ground states of the neutral and ionic complexes appear to be in D6h symmetry with the hydrogen atoms slightly bent towards the metal center, as suggested by the good agreement between experiment and theory.