THERMOCHEMISTRY OF TRANSITION-METAL BENZENE COMPLEXES - BINDING-ENERGIES OF M(C6H6)(X)(+) (X=1, 2) FOR M=TI TO CU

The sequential bond energies of the mono- and bis-benzene complexes of the first-row transition metal ions, M(+) = Ti+-Cu+, are determined by collision-induced dissociation with Xe in a guided ion beam tandem mass spectrometer. Values for the 0 K M(+)-C6H6 bond energies are found to be in good agreement with theoretical predictions and with most previous experimental values in those cases where they are available. Such agreement can be obtained only with ions prepared at a controlled internal energy and with careful consideration of the effects of multiple collisions, the internal energy of the ions, and their dissociation Lifetimes, which require a knowledge of the vibrational frequencies of the ions and transition states leading to dissociation. Thresholds for dissociation of the benzene Ligand from the bis-benzene complexes are calculated to have large kinetic shifts that may limit the absolute accuracy of the bond energies determined for (C6H6)M(+)-C6H6. For the case of M = Cr, the sum of the first and second bond energies compares reasonably well with literature values; but for M = V, the agreement is poor, a result attributed to an inaccurate heat of formation for V(C6H6)(2). Revised heats of formation for gas-phase M(C6H6)(2) species where M = V and Cr are derived from the present results. The trends in the metal-benzene bond energies across the periodic table are discussed.