(C<sub>5</sub>M<sub>2</sub><sub>-</sub><i><sub>n</sub></i>)<i><sup>n</sup></i><sup>-</sup> (M = Li, Na, K, and <i>n</i> = 0, 1, 2). A New Family of Molecules Containing Planar Tetracoordinate Carbons

By highly correlated ab initio methods and DFT calculations, we have shown that alkaline metals can stabilize planar tetracoordinate carbon-containing molecules with the C(C<sub>4</sub>) skeleton. This family of molecules is C<sub>5</sub>M<sub>2</sub>, where M is an alkaline metal. The stability of these compounds is rationalized in terms of the delocalization of the p-orbital perpendicular to the molecular plane, the global hardness, and the electrophilicity. The analysis of several molecular scalar fields shows that the bonding between the C<sub>5</sub><sup>2-</sup> dianion and the metals is strongly ionic. The structures reported are the first examples with a planar tetracoordinate carbon, surrounded by carbon atoms, and stabilized, only, by electronic factors.