Chemical bonding in "early-late" transition metal complexes [(H₂N)₃M-M′(CO)₄] (M = Ti, Zr, Hf; M′ = Co, Rh, Ir)

Quantum chemical DFT calculations at the BP86/TZ2P level have been carried out for the complex [HSi(SiH₂NH)₃Ti-Co(CO)₄], which is a model for the experimentally observed compound [MeSi{SiMe₂N(4-MeC₆H₄)}₃Ti-Co(CO)₄] and for the series of model systems [(H₂N)₃M-M′(CO)₄] (M = Ti, Zr, Hf; M′ = Co, Rh, Ir). The Ti-Co bond in [HSi(SiH₂NH)₃Ti-Co(CO)₄] has a theoretically predicted BDE of D_e = 59.3 kcal/mol. The bonding analysis suggests that the titanium atom carries a large positive charge, while the cobalt atom is nearly neutral. The covalent and electrostatic contributions to the Ti-Co attraction have similar strength. The Ti-Co bond can be classified as a polar single bond, which has only little π contribution. Calculations of the model compound (H₂N)₃Ti-Co(CO)₄ show that the rotation of the amino groups has a very large influence on the length and on the strength of the Ti-Co bond. The M-M′ bond in the series [(H₂N)₃M-M′(CO)₄] becomes clearly stronger with Ti < Zr < Hf, while the differences between the bond strengths due to change of the atoms M′ are much smaller. The strongest M-M′ bond is predicted for [(H₂N)₃Hf-Ir(CO)₄].