Planar hexacoordinate carbon: a viable possibility.

The viability of molecules with planar hexacoordinate carbon atoms is demonstrated by density-functional theory (DFT) calculations for CB6(2-), a CB6H2 isomer, and three C3B4 minima. All of these species have six pi electrons and are aromatic. Although other C3B4 isomers are lower in energy, the activation barriers for the rearrangements of the three planar carbon C3B4 minima into more stable isomers are appreciable, and experimental observation should be possible. High-level ab initio calculations confirm the DFT results. The planar hexacoordination in these species does not violate the octet rule because six partial bonds to the central carbons are involved.