PLANAR AND INHERENTLY NON-TETRAHEDRAL TETRACOORDINATE CARBON - A STATUS-REPORT

A variety of tetracoordinate molecules which deviate fundamentally from tetrahedral symmetry have been surveyed anew by ab initio Becke3LYP density functional computations. These refine the earlier findings that electropositive substituents (i.e. metals) drastically reduce the planar-tetrahedral energy difference and favour planar ''anti-van't Hoff'' arrangements. The lowest lying energy minima of 1,1- and 1,2-dilithioethylene, and of 1,1-dilithiocyclopropane have planar tetracoordinate carbons. Planar geometries also are favoured by pi accepters. Owing to aromatic delocalization and the small CCspiroC angle, planar 2,3-diboraspiropentane (unlike diboracyclopropane itself) is a true minimum. Planar tetracoordinate carbons also are present in 3,3-dilithio-1,2-diboracyclopropane and in a C2B2H4 isomer.