Restricted Metal-Arene and Phosphorus-Carbon Bond Rotation in (arene)Cr(CO)₂L Complexes (L = PPh₃, P(o-tolyl)₃)

(1,4-Di-tert-butylbenzene)Cr(CO)₂PPh₃ exhibits restricted areneCr bond rotation in solution (ΔG^ǂ₂₀₃ = 31.6 kJ mol⁻¹). A series of (arene)Cr(CO)₂P(o-tolyl)₃ complexes have been prepared which exhibit restricted PC rotation in solution; in cases where planar chirality is present, diastereoisomers may be observed which differ in the helicity of the exo₂ phosphine conformation. Diastereoisomer interconversion and o-tolyl ring exchange occur with very similar activation energies. Crystal structure data: (1,4-di-tert-butylbenzene)Cr(CO)₂PPh₃, monoclinic, space group P2₁/n, a = 10.137(1) Å, b = 13.992(2) Å, c = 20.882(3) Å, β = 96.16(2)°, Z = 4, R_w = 9.18% for 345 refined parameters and 3643 observed reflections; (p-xylene)Cr(CO)₂P(o-tolyl)₃, monoclinic, space group P2₁/n, a = 8.552(1) Å, b = 17.410(2) Å, c = 17.558(2) Å, β = 91.95(2)°, Z = 4, R_w = 9.96% for 317 defined parameters and 3392 observed reflections; (benzene)Cr-(CO)₂P(m-tolyl)₃, monoclinic, space group P2₁/n, a = 11.120(3) Å, b = 11.227(1) Å, c = 20.289(4) Å, β = 100.63(2)°, Z = 4, R_w = 6.49% for 299 defined parameters and 3127 observed reflections.