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.