Jansen, Georg; Schubart, Martin; Findeis, Bernd; Gade, Lutz H.; Scowen, Ian J.; McPartlin, Mary:

Unsupported Ti-Co and Zr-Co Bonds in Heterobimetallic Complexes: A Theoretical Description of Metal-Metal Bond Polarity.

In: Journal of the American Chemical Society (J.Am.Chem.Soc.), Jg. 120 (1998) ; Nr. 29, S. 7239-7251
ISSN: 0002-7863
Zeitschriftenaufsatz / Fach: Chemie
The synthesis, structural, and theor. characterization of heterobimetallic complexes [CH3Si{SiMe2N(4-CH3C6H4)}3M-Co(CO)3(L)] (M = Ti, Zr; L = CO, PPh3, PTol3) with unsupported metal-metal bonds between Co atoms and Ti or Zr atoms is reported. The synthesis of the dinuclear compds. was achieved by salt metathesis of the chlorotitanium and Zr complexes and the alkali metal carbonylates. X-ray crystal structure analyses of four of these heterobimetallic complexes established the unsupported metal-metal bonds [M = Ti, L = CO (3): 2.554(1) .ANG.; M = Ti, L = PTol3 (4b): 2.473(4) .ANG.; M = Zr, L = CO (5): 2.705(1) .ANG.; M = Zr, L = PPh3 (6a): 2.617(1) .ANG.] as well as the 3-fold mol. symmetries. Upon axial phosphine substitution, a metal-metal bond contraction of .apprx.0.08 .ANG. is obsd., which also results in the quantum chem. structure optimizations performed on the model compds. [(H2N)3Ti-Co(CO)4] (3x) and [(H2N)3Ti-Co(CO)3(PH3)] (4x) using gradient-cor. and hybrid d. functionals. A theor. study of the homolytic dissocn. of the metal-metal bonds focuses on the relaxation energies of the complex fragments and indicates that the geometrical constraints imposed by the tripod ligand lead to a major thermodn. contribution to the stability of the exptl. studied complexes. The central question of the polarity of the metal-metal bond is addressed by detailed anal. of the calcd. electron charge distribution using natural population anal. (NPA), charge decompn. anal. (CDA), Bader's atoms in mols. (AIM) theory, and the electron localization function (ELF). Both the orbital-based NPA and CDA schemes and the essentially orbital-independent AIM and ELF anal. suggest a description of the Ti-Co bond as being a highly polar covalent single bond. The combination of AIM and ELF is employed for the 1st time to analyze metal-metal bond polarity and appears to be a powerful theor. tool for the description of bond polarity in potentially ambiguous situations.