Attila Kovács and Gernot Frenking

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-electron complexes [W(CO)4L] and [W(CO)3L]2-

Organometallics, 20 (2001) 2510-2524


The geometries of the formal 18 valence electron (VE) complexes [W(CO)5L] with the s and p bonded monodentate ligands L = N2, NCH, C2H2, C2H4, OH2, SH2, NH3, F-, Cl-, OH-, SH- and [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 VE 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 trigonal bipyramidal structures with L in the equatorial position (cis1) or square-planar forms with L in the basal position (cis2) as global energy minimum. Square-planar 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 four-electron donor in [W(CO)4(HCCH)](cis1) via donation from the out-of-plane p-orbital of acetylene which significantly stabilizes the 16 VE complex. This mechanism is clearly weaker in the p-NCH complex and it is very weak in the p-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(p) lone-pair donor orbital. The stabilization by the negatively charged ligands is slightly weaker than that of HCCH. The  s-bonded ligands SH2, NH3 and N2 stabilize [W(CO)4L] very poorly, and the ligands OH2 and s-NCH are only weakly stabilizing. The high stability of the 16 VE complexes with bidentate ligands [W(CO)3(X2C2H2)]2- (X = O, S) can not solely be explained with strong W to X2C2H22- s-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.