Bonding and reactivity in π-allyl—metal compounds

The π-allyl group of π-C₃H₅Co(CO)₃ has two angles of tilt, one of which (from semi-empirical molecular orbital calculations) is stabilised principally by the influence of the C(p_v) orbitals of the terminal carbon atoms, which form part of the σ-framework of the π-allyl group, and the other of which is stabilised by a balance between bonding orbital components of the central and terminal carbon atoms. The Co(CO)₃ moiety has asymmetric bonding, with one CO group more weakly bonded to the metal atom. The asymmetric bonding of the Co(CO)₃ moiety is primarily caused by the electronic character of the π-allyl group, but is significantly influenced by the magnitude of the τ-tilt angle of the π-allyl group. The relatively high reactivity of π-C₃H₅Co(CO)₃, compared with the reactivity of π-C₃H₅Fe(CO)₂NO, Co(NO)(CO)₃, or Ni(CO)₄, is explained by the relatively weak bonding of a CO group to the metal atom and a possible explanation of the anomalous relative rates of the reactions of π-C₃H₄RCo(CO)₃ (R = H, 1-CH₃, 2-CH₃, 1-Cl, 2-Cl) with P(C₆H₅)₃ is indicated.The angles of tilt of the π-allyl group and the asymmetric bonding of the π-cyclopentadienyl moiety in π-C₃H₄Ni(π-C₅H₅)]₂ are caused by factors similar to those in π-C₃H₅Co(CO)₃.