Structure and stereodynamics of [m(CO)₅{P(C₆H ₄Me-o)₃}] (M = Cr, Mo or W) and [cr(CO) ₃{P(C₆H₄Me-o)₃}]

The crystal structure of [Cr(CO)₅{P(C₆H ₄Me-o)₃}] has been determined. It reveals an exo ₂ conformation for the phosphine ligand, in agreement with molecular mechanics calculations. Barriers to P-C rotation in the series [M(CO) ₅{P(C₆H₄Me-o)₃}] (M = Cr, Mo or W) are in the range 36-42 kJ mol^-1; restricted M-P rotation is also evident in the low-temperature ¹³CO NMR spectrum of [Cr(CO) ₅{P(C₆H₄Me-o)₃}]. The crystal structure of the π complex [Cr(CO)₃{P(C₆H ₄Me-o)}₃] is most consistent with an electron-acceptor character for the P(C₆H₄Me-o)₂ substituent. Crystal data: [Cr(CO)₅{P(C₆H₄Me-o) ₃}], monoclinic, space group P2₁/n, a = 10.774(2), b = 14.951(5), c = 14.687(7) Å, β = 91.45(2)°, Z = 4, R′ = 0.0980 for 209 variable parameters and 2326 observed reflections; [Cr(CO) ₃{P(C₆H₄Me-o)₃}], monoclinic, space group P2₁/a, a = 16.725(2), b = 7.340(1), c = 17.675(4), β = 92.66(2)°, Z = 4, R′ = 0.1187 for 263 variable parameters and 1848 observed reflections.