Shifting paradigms: electrostatic interactions and covalent bonding.

The role of electrostatic interactions in covalent bonding of heavier main group elements has been evaluated for the exemplary set of molecules X(2)H(2) (X=C, Si, Ge, Sn, Pb). Density functional calculations at PBE/QZ4P combined with energy decomposition procedures and kinetic energy density analyses have been carried out for a variety of different structures, and two factors are responsible for the fact that the heavier homologues of acetylene exhibit doubly hydrogen-bridged local minimum geometries. For one, the extended electronic core with at least one set of p orbitals of the Group 14 elements beyond the first long period is responsible for favorable electrostatic E-H interactions. This electrostatic interaction is the strongest for the isomer with two bridging hydrogen atoms. Secondly, the H substituent does not posses an electronic core or any bonding-inactive electrons, which would give rise to a significant amount of Pauli repulsion, disfavoring the doubly bridged isomer. When one of two criteria is not met the unusual doubly bridged structure no longer constitutes the energetically preferred geometry. The bonding model is validated in calculations of different structures of Si(2)(CH(3))(2).