Analysis of bonding motifs in unusual molecules I: planar hexacoordinated carbon atoms.

The bonding structures of COLi and CSLi are studied by means of oriented quasi-atomic orbitals (QUAOs) to assess the possibility of these molecules being planar hexacoordinated carbon (phC) systems. CHLi and CO are employed as reference molecules. It is found that the introduction of Li ions into the molecular environment of carbonate has a greater effect on the orbital structure of the O atoms than it does on the C atom. Partial charges computed from QUAO populations imply repulsion between the positively charged C and Li atoms in COLi. Upon the transition from COLi to CSLi, the analysis reveals that the substitution of O atoms by S atoms inverts the polarity of the carbon-chalcogen σ bond. This is linked to the difference in s- and p-fractions of the QUAOs of C and S, as element electronegativities do not explain the observed polarity of the CSσ bond. Partial charges indicate that the larger electron population on the C atom in CSLi makes C-Li attraction possible. Upon comparison with the C-Li bond in methyllithium, it is found that the C-Li covalent interactions in COLi and CSLi have about 14% and 6% of the strength of the C-Li covalent interaction in CHLi, respectively. Consequently, it is concluded that only CSLi may be considered to be a phC system.