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 heterobimetallic complexes [CH3Si{SiMe2N(4-CH3C6H4)}3M−Co(CO)3(L)] (M = Ti, Zr; L = CO, PPh3, PTol3) with unsupported metal−metal
bonds between cobalt atoms and titanium or zirconium atoms is being reported. The synthesis of the dinuclear
compounds was achieved by salt metathesis of the chlorotitanium and zirconium complexes and the alkalimetal
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) Å; M = Ti, L = PTol3 (4b):  2.473(4) Å;
M = Zr, L = CO (5):  2.705(1) Å; M = Zr, L = PPh3 (6a):  2.617(1) Å] as well as the 3-fold molecular
symmetries. Upon axial phosphine substitution, a metal−metal bond contraction of ca. 0.08 Å is observed,
which also results in the quantum chemical structure optimizations performed on the model compounds [(H2N)3Ti−Co(CO)4] (3x) and [(H2N)3Ti−Co(CO)3(PH3)] (4x) using gradient-corrected and hybrid density functionals.
A theoretical study of the homolytic dissociation 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 thermodynamic contribution to the stability of the experimentally investigated complexes. The central
question of the polarity of the metal−metal bond is addressed by detailed analysis of the calculated electron
charge distribution using natural population analysis (NPA), charge decomposition analysis (CDA), Bader's
atoms in molecules (AIM) theory, and the electron localization function (ELF). Both the orbital-based NPA
and CDA schemes and the essentially orbital-independent AIM and ELF analysis 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 first time to analyze metal−metal bond polarity and appears to be a powerful theoretical tool for the
description of bond polarity in potentially ambiguous situations.