Design of neutral hydrocarbons having a planar tetracoordinate carbon

Cyclopropyne (C 3 H 2 ) and related spiro bicyclic structures show unusual structural preference for the planar tetracoordinate structure over the conventional tetrahedral arrangement. C 5 H 4 has been proposed as the smallest neutral hydrocarbon with planar tetracoordinate carbon. NICS values indicate substantial stabilization of the planar arrangements over the tetrahedral counterparts.