UNSUPPORTED TI-CO AND ZR-CO BONDS IN HETEROBIMETALLIC COMPLEXES - A THEORETICAL DESCRIPTION OF METAL-METAL BOND POLARITY

The synthesis, structural, and theoretical characterization of heterob imetallic complexes [CH3Si-{SiMe2N(4-CH3C6H4)}(3)M-Co(CO)(3)(L)] (M = Ti, Zr; L = CO, PPh3, PTol(3)) with unsupported metal-metal bonds betw een cobalt atoms and titanium or zirconium atoms is being reported. Th e synthesis of the dinuclear compounds was achieved by salt metathesis of the chlorotitanium and zirconium complexes and the alkalimetal car bonylates. X-ray crystal structure analyses of four of these heterobim etallic complexes established the unsupported metal-metal bonds [M = T i, L = CO (3): 2.554(1) Angstrom; M = Ti, L = PTol(3) (4b): 2.473(4) A ngstrom; M = Zr, L = CO (5): 2.705(1) Angstrom; M = Zr, L = PPh3 (6a): 2.617(1) Angstrom] as well as the 3-fold molecular symmetries. :Upon axial phosphine substitution, a metal-metal bond contraction of ca. 0. 08 Angstrom is observed, which also results in the quantum chemical st ructure optimizations performed on the model compounds [(H2N)(3)-Ti-Co (CO)(4)] (3x) and [(H2N)(3)Ti-Co(CO)(3)(PH3)] (4x) using gradient-corr ected and hybrid density functionals. A theoretical study of the homol ytic dissociation of the metal-metal bonds focuses on the relaxation e nergies of the complex fragments and indicates that the geometrical co nstraints imposed by the tripod ligand lead to a major thermodynamic c ontribution to the stability of the experimentally investigated comple xes. The central question of the polarity of the metal-metal bond is a ddressed by detailed analysis of the calculated electron charge distri bution using natural population analysis (NPA), charge decomposition a nalysis (CDA), Bader's atoms in molecules (AIM) theory, and the electr on localization function(ELF). Both the orbital-based NPA and CDA sche mes and the essentially orbital-independent AIM and ELF analysis sugge st a description of the Ti-Co bond as being a highly polar covalent si ngle bond. The combination of AIM and ELF is employed for the first ti me to analyze metal-metal bond polarity and appears to be a powerful t heoretical tool for the description of bond polarity in potentially am biguous situations.