Cation-aromatic bonding in group 14 organometallics

The geometric and electronic structure of (para)toluene-XY₃⁺ complexes have been determined by ab initio, gradient optimization methods at the MP2 level of theory in a valence double-zeta + polarization basis set. The Y substituents are CH₃, H and Cl, and X is any one of the Group 14 atoms; carbon, silicon, germanium, tin and lead. Comparison is made with a recently reported crystal structure of the toluene-Si(Et)₃⁺ complex and gas phase mass spectrometry reactions of tert-butyl and silynium cations with arenes. The carbenium complexes have an essentially classical four-coordinate ring (ipso) carbon atom. In passing from X = Si to X = Pb the XY₃ group moves in tandem with the geminally bound hydrogen atom to a position where the X atom is almost directly above the ipso carbon atom and the hydrogen atom is almost coplanar with the benzene ring. In general, one X-Y bond lies parallel to the long axis of toluene to give approximate C_s symmetry complexes. The toluene-XY₃⁺ binding energies are found to increase from the Sn to the Pb complexes for each Y substituent, indicating a special stabilizing interaction for the lead complexes. The binding energies, geometric structures and charge distribution of the complexes are interpreted in terms of classical σ-bonded and localized π-bonding models.