Theoretical study of the electronic structure of group 6 [M(CO)(5)X](-) species (X = NH2, OH, halide, H, CH3) and a reinvestigation of the role of pi-donation in CO lability

Density functional calculations have been employed to investigate the elect ronic structure of [M(CO)(5)X](-) species (M = Cr, Mo, W; X = NH2, OH, hali de, H, CH3) and to compute CO ligand dissociation energies. The calculation s indicate that CO loss is most facile from the cis position, and CO dissoc iation energies are computed to increase along the series X = NH2 < OH < F < Cl < Br < I < CH3 < H. These results are in agreement with available expe rimental data. Trends in CO dissociation are related to the ability of X to stabilize the unsaturated 16e [M(CO)(4)X](-) species formed. In addition, pi-destabilization of the ground-state [M(CO)(5)X](-) species is equally si gnificant. Analysis of the electronic structure of the 18e species shows th at X-pi 4e destabilization results in hybridization at the metal center whi ch enhances trans M-CO but reduces cis M-CO pi-back-donation. Strong pi-don ation from X also induces sigma-antibonding interactions between the metal and the cis CO ligands. A fragment analysis reveals that these effects are strongest for the "hard" fluoride, hydroxide, and amide ligands.