Characterization of an agostic bond on the basis of the electron density

Several conformations of CH3TiCl2+, C2H5TiCl2+and C3H7TiCl2+were selected for a case study on agostic bonds. Ab initio wavefunctions have been generated at Hartree-Fock, BLYP and MP2 level. Criteria solely based on a topological analysis of the electron density are proposed in order to characterize the agostic bond. These criteria are drawn from the theory of 'atoms in molecules' (AIM) and form an independent complement to conventional methods like IR, NMR and structural crystallography. The observed features systematically violate the criteria that have been proposed for hydrogen bonding. As a consequence the agostic bond is not a special type of hydrogen bond.