Mononuclear complexes, which contain a dianionic ene-diamido ligand bound to a group 4 metal atom in the formal d0 configuration, are analyzed by the DFT method to interpret the electronic origin of the folding at the five-membered 2,5-diazapent-3-ene metallacycle moiety. Geometry optimizations were carried out for the following models, TiCl2[o-(Me3SiN)2C6H4], Ti(OPh)2(DAD), CpTiCl(DAD), CpTiMe(DAD) and Cp2Zr(DAD) (DAD = HNCHCHNH). They show some common electronic features, the nature of the HOMO, in particular. In all cases, the latter results from the donation of a filled ene-diamido level into an empty s metal orbital, this being maximized upon the folding of the metallacycle. Such a geometric rearrangement involves the filled nitrogen pp lobes, while the CC p bond remains essentially uninvolved. The feature is confirmed by the application of the atom in molecules (AIM) theory, that provides no evidence of critical points between the metal center and the pair of two carbon atoms.
Folded 2,5-diazapent-3-ene metallacycle in ene-diamido group 4 metal compounds: DFT and AIM analyses
J. Organomet. Chem. 2004, 689, 2847-2852.