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

Abstract

The bonding structures of CO₃Li₃⁺ and CS₃Li₃⁺ are studied by means of oriented quasi-atomic orbitals (QUAOs) to assess the possibility of these molecules being planar hexacoordinated carbon (phC) systems. CH₃Li and CO₃²⁻ 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 CO₃Li₃⁺. Upon the transition from CO₃Li₃⁺ to CS₃Li₃⁺, 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 CS₃Li₃⁺ makes C–Li attraction possible. Upon comparison with the C–Li bond in methyllithium, it is found that the C–Li covalent interactions in CO₃Li₃⁺ and CS₃Li₃⁺ have about 14% and 6% of the strength of the C–Li covalent interaction in CH₃Li, respectively. Consequently, it is concluded that only CS₃Li₃⁺ may be considered to be a phC system.