Stability and bonding situation of electron-deficient transition-metal complexes. Theoretical study of the CO-labilizing effect of ligands L in [W(CO)(5)L] (L = C2H2, NCH, N-2, C2H4, OH2, SH2, NH3, F-, Cl-, OH-, SH-) and [W(CO)(4)L](2-) (L2- = O2C2H22-, S2C2H22-) and the structure of the 16-valence-electron complexes [W(CO)(4)L] and [W(CO)(3)L](2-)

The geometries of the formal 18-valence-electron (VE) complexes [W(CO)(5)L] with the and sigma -bonded monodentate ligands L = N-2, NCH, C2H4, C2H4, O H2, SH2, NH3, F-, Cl-, OH-, SH- and those of [W(CO)(4)L](2-) with the biden tate ligands L2- = O2C2H22-, S2C2H22- have been calculated at the B3LYP lev el of theory. The structures of the 16-VE complexes [W(CO)(4)L] and [W(CO)( 3)L](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 sch emes. 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 e ffect on the 16-VE species. Three different structural isomers of the 16-VE species with monodentate ligands have been found as energy minima. All com plexes TW(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-py ramidal structures with L in the apical position (trans) are energetically high-lying minima which have a different electronic state than the cis1 for m. The ligand HCCH becomes a 4-electron donor in [W(CO)(4)-(HCCH)](cis1) vi a donation from the out-of-plane Jr-orbital of acetylene, which significant ly stabilizes the 16-VE complex. This mechanism is clearly weaker in the pi -NCH complex, and it is very weak in the rr-bonded dinitrogen and ethylene complexes. The negatively charged ligands F-, Cl-, OH-, and SH- have also a strong CO-labilizing effect in [W(CO)(5)L](-) because the ligands stabili ze the formal 16-VE species [W(CO)(4)L](-) by electron donation from the p( Jr) lone-pair donor orbital. The stabilization by the negatively charged li gands is slightly weaker than that of HCCH. The a-bonded ligands SH2, NH3, and Nz stabilize [W(CO)(4)L] very poorly, and the ligands OH2 and sigma -NC H are only weakly stabilizing. The high stability of the 16-VE complexes wi th bidentate ligands [W(CO)(3)(X2C2H2)](2-) (X = O, S) cannot solely be exp lained with strong W <--X2C2H22- rr-donation, which is already operative an d even stronger in the 18-VE parent complexes [W(CO)(4)(X2C2H2)](2-). An im portant additional reason for the stability of the complexes [W(CO)(3)(X2C2 H2)](2-) lies in the ability of the ligands X2C2H22- to enhance the bond st rength of the three W-CO bonds.