Synthesis, Structure, and Bonding of d³ Molybdenum-Oxo Complexes

Reduction of d² metal-oxo ions of the form [MO(PP)₂ Cl]⁺ (M=Mo, W; PP=chelating diphosphine) produces d³ MO(PP)₂ Cl complexes, which include the first isolated examples in group 6. The stability and reactivity of the MO(PP)₂ Cl compounds are found to depend upon the steric bulk of the phosphine ligands: derivatives with bulky phosphines that shield the oxo ligand are stable enough to be isolated, whereas those with phosphines that leave the oxo ligand exposed are more reactive and observed transiently. Magnetic measurements and DFT calculations on MoO(dppe)₂ Cl indicate the d³ compounds are low spin with a ² [(d_xy )² (π*(MoO))¹ ] configuration. X-ray crystallographic and vibrational-spectroscopic studies on d² and d³ [MoO(dppe)₂ Cl]^0/+ establish that the d³ compound possesses a reduced M-O bond order and significantly longer Mo-O bond, accounting for its greater reactivity. These results indicate that the oxo-centered reactivity of d³ complexes may be controlled through ligand variation.