Stability and Bonding Situation of Electron-Deficient Transition-Metal Complexes. Theoretical Study of the CO-Labilizing Effect of Ligands L in [W(CO)₅L] (L = C₂H₂, NCH, N₂, C₂H₄, OH₂, SH₂, NH₃, F^-, Cl^-, OH^-, SH^-) and [W(CO)₄L]²^- (L²^- = O₂C₂H₂²^-, S₂C₂H₂²^-) and the Structure of the 16-Valence-Electron Complexes [W(CO)₄L] and [W(CO)₃L]^2-

Abstract

The geometries of the formal 18-valence-electron (VE) complexes [W(CO)5L] with the π- and σ-bonded monodentate ligands L = N2, NCH, C2H2, C2H4, OH2, SH2, NH3, F-, Cl-, OH-, SH- and those of [W(CO)4L]2- with the bidentate ligands L2- = O2C2H22-, S2C2H22- have been calculated at the B3LYP level of theory. The structures of the 16-VE complexes [W(CO)4L] and [W(CO)3L]2- have also been optimized. The bonding situation of the 16- and 18-VE complexes was analyzed with the help of the CDA and NBO partitioning schemes. The goal of the study was to investigate the labilizing influence of the ligand L on the W−CO bonds in the 18-VE complexes and the stabilizing effect on the 16-VE species. Three different structural isomers of the 16-VE species with monodentate ligands have been found as energy minima. All complexes [W(CO)4L] have either distorted-trigonal-bipyramidal structures with L in the equatorial position (cis1) or square-based-pyramidal forms with L in the basal position (cis2) as the global energy minimum. Square-based-pyramidal structures with L in the apical position (trans) are energetically high-lying minima which have a different electronic state than the cis1 form. The ligand HCCH becomes a 4-electron donor in [W(CO)4(HCCH)](cis1) via donation from the out-of-plane π-orbital of acetylene, which significantly stabilizes the 16-VE complex. This mechanism is clearly weaker in the π-NCH complex, and it is very weak in the π-bonded dinitrogen and ethylene complexes. The negatively charged ligands F-, Cl-, OH-, and SH- have also a strong CO-labilizing effect in [W(CO)5L]- because the ligands stabilize the formal 16-VE species [W(CO)4L]- by electron donation from the p(π) lone-pair donor orbital. The stabilization by the negatively charged ligands is slightly weaker than that of HCCH. The σ-bonded ligands SH2, NH3, and N2 stabilize [W(CO)4L] very poorly, and the ligands OH2 and σ-NCH are only weakly stabilizing. The high stability of the 16-VE complexes with bidentate ligands [W(CO)3(X2C2H2)]2- (X = O, S) cannot solely be explained with strong W←X2C2H22- π-donation, which is already operative and even stronger in the 18-VE parent complexes [W(CO)4(X2C2H2)]2-. An important additional reason for the stability of the complexes [W(CO)3(X2C2H2)]2- lies in the ability of the ligands X2C2H22- to enhance the bond strength of the three W−CO bonds.