Reactivity and binding energies of transition metal halide ions with benzene.

We have generated novel halogen-ligated transition metal ions MX(n)+ (M = Sc, Ti, V, and Fe, X = Cl, Br and I, n = 1-3). We have explored their reactions with benzene, a typical aromatic hydrocarbon. Attachment of one benzene molecule is usually rapid, whereas attachment of a second benzene molecule is generally much slower. The kinetics were analyzed to estimate binding energies, modeling the attachment reaction as a radiative association process. In all cases the Standard Hydrocarbon semiquantitative estimation approach was employed, and in some cases the more accurate variational transition state (VTST) kinetic modeling approach was also applied. Density functional (DFT) quantum calculations were also performed to give computed binding energies for some of the complexes. Taking previously determined binding energies for halogen-ligated alkaline-earth ions as benchmarks, it is concluded that binding of the first benzene molecule to the transition-metal species is strongly enhanced by specific chemical interactions, while binding of the second benzene molecule is more nearly electrostatic. The binding energies are not strongly dependent on the identity of the transition metal ion, and the metal-ion dependences can be rationalized in terms of valence-orbital occupations of the metals. The binding energies are nearly independent of the identity of the halogen ligands.