Pulsed-field ionization electron spectroscopy of group 6 metal (Cr, Mo, and W) bis(benzene) sandwich complexes

Group 6 metal bis(benzene) sandwich complexes (M- bz2: M=Cr, Mo, and W and bz= C6 H6) were produced with laser vaporization molecular beam techniques and studied by pulsed-field ionization zero electron kinetic energy spectroscopy and density functional theory calculations. Each sandwich complex is in a D6h eclipsed configuration with A 1g 1 and A 1g 2 as the neutral and cationic ground electronic states, respectively. The adiabatic ionization energies for Cr-, Mo-, and W- bz2 are measured to be 44 081(7), 44 581(10), and 43 634 (7) cm-1, respectively. The metal-benzene stretch and benzene torsion frequencies of the ion are measured to be 264, 277, and 370 cm-1 and 11, 21, and 45 cm-1 for Cr-, Mo-, and W- bz2, respectively. In addition, a C-H out-of-plane bending mode is measured to be 787 cm-1 for the Cr+ - bz2 complex, while a C-C in-plane bending mode is measured to be 614 cm-1 for the W+ - bz2 complex. The unusual trend in the ionization energy and metal-benzene stretch frequency indicates strong relativistic effects on tungsten binding.