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

The bonding structures of CO 3 Li 3 + and CS 3 Li 3 + are studied by means of oriented quasi-atomic orbitals (QUAOs) to assess the possibility of these molecules being planar hexacoordinated carbon (phC) systems. CH 3 Li and CO 3 2- 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 CO 3 Li 3 + . Upon the transition from CO 3 Li 3 + to CS 3 Li 3 + , 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 CS 3 Li 3 + makes C-Li attraction possible. Upon comparison with the C-Li bond in methyllithium, it is found that the C-Li covalent interactions in CO 3 Li 3 + and CS 3 Li 3 + have about 14% and 6% of the strength of the C-Li covalent interaction in CH 3 Li, respectively. Consequently, it is concluded that only CS 3 Li 3 + may be considered to be a phC system.