In organic chemistry, saturated carbon is known to bond to four ligands tetrahedrally, as first recognized independently by JH van t Hoff and JA Le Bel in 1874. However, after the proposal by Hoffmann and co-workers of tetracoordinate planar carbon in 1970,[1] extensive experimental and theoretical efforts were made to search for so-called anti-van t Hoff/anti-Le Bel molecules (for recent reviews, see references [2–4]). In particular, the first experimental and theoretical realization of pentaatomic planar-coordinated carbon species in 1999 and 2000,[5–8] which confirmed earlier theoretical predictions,[9, 10] has stimulated renewed interest in designing new tetracoordinate [11, 12] and even hypercoordinate planar carbon molecules.[13–16]

Notably, a series of hypercoordinate planar carbon species with boron ligands have been proposed.[13–15, 16a, c, e] Although none of these species is the global minimum on the potentialenergy surfaces, it has been suggested that they may be viable experimentally. The two proposed hexa-and heptacoordinate