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 degrees. The ground electronic states of these conformers are (1)A(1) (C-2v, 0 degrees), (1)A (C-2, 60 and 120 degrees), and (1)A(g), (C-2h, 180 degrees) in the neutral form and (2)A(1) (C-2v, 0 degrees), (2)A (C-2, 60 and 120 degrees), and (2)A(g) (C-2h, 180 degrees) in the singly charged cationic form. For Mo- and W-bis(toluene), the four rotamers are resolved into three (0, 60/120, and 180 degrees) and two (0 and 60/120/180 degrees) groups, respectively. For all three metal sandwiches, the most stable conformer is in the complete eclipsed configuration (0 degrees) and has the highest ionization energy. The conversion from 60/120/180 degrees to 0 degrees rotamer is observed from helium to argon supersonic expansions and is more efficient for the heavier Mo and W species.