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

Density functional calculations have been employed to investigate the electronic structure of [M(CO)₅X]^- species (M = Cr, Mo, W; X = NH₂, OH, halide, H, CH₃) and to compute CO ligand dissociation energies. The calculations indicate that CO loss is most facile from the cis position, and CO dissociation energies are computed to increase along the series X = NH₂ < OH < F < Cl < Br < I < CH₃ < H. These results are in agreement with available experimental data. Trends in CO dissociation are related to the ability of X to stabilize the unsaturated 16e [M(CO)₄X]^- species formed. In addition, p-destabilization of the ground-state [M(CO)₅X]^- species is equally significant. Analysis of the electronic structure of the 18e species shows that Xp-dp 4e destabilization results in hybridization at the metal center which enhances trans M-CO but reduces cis M-CO p-back-donation. Strong p-donation from X also induces s-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. © 1999 American Chemical Society.