Conformational isomers and isomerization of group 6 (Cr, Mo, and W) metal-bis(toluene) sandwich complexes probed by variable-temperature electron spectroscopy.

Group 6 metal (Cr, Mo, and W)-bis(toluene) sandwich complexes are synthesized in a laser-vaporization molecular beam source. Conformational isomers and isomerization of these complexes are studied by variable-temperature pulsed-field-ionization zero-electron-kinetic-energy spectroscopy and density functional theory. For Cr-bis(toluene), four rotational conformers are identified with methyl-group dihedral angles of 0, 60, 120, and 180°. The ground electronic states of these conformers are (1)A(1) (C(2v), 0°), (1)A (C(2), 60 and 120°), and (1)A(g) (C(2h), 180°) in the neutral form and (2)A(1) (C(2v), 0°), (2)A (C(2), 60 and 120°), and (2)A(g) (C(2h), 180°) in the singly charged cationic form. For Mo- and W-bis(toluene), the four rotamers are resolved into three (0, 60/120, and 180°) and two (0 and 60/120/180°) groups, respectively. For all three metal sandwiches, the most stable conformer is in the complete eclipsed configuration (0°) and has the highest ionization energy. The conversion from 60/120/180° to 0° rotamer is observed from helium to argon supersonic expansions and is more efficient for the heavier Mo and W species.