Ethylene Rotation in Chiral Octahedral Rhenium(I) Complexes

A series of complexes of the type [TpRe(CO)(L_D)(η²-ethylene)] has been synthesized, where L_D is ^tBuNC, PMe₃, pyridine, 1-methylimidazole, or NH₃ (increasing in electron-donating ability of L_D). The rates of the propeller-like rotation of ethylene about the ethylene−rhenium bond have been determined using spin saturation transfer experiments at low temperatures (−85 to −60 °C). It was found that the ΔG^⧧ values correlate with both the C⋮O stretching frequencies and Re(II/I) reduction potentials for these complexes, indicating that the barrier to rotation is primarily electronic in nature.