Compounds Containing Planar-Tetracoordinate Carbon

It is a great challenge to synthesize compounds containing planar‐tetracoordinate carbon that are stable enough to be isolated, investigated, and handled under ambient conditions. There have therefore been many attempts to construct hydrocarbon frameworks that can incorporate and stabilize tetravalent carbon in this “unnatural” planar coordination geometry, for example, by steric constraints. However, success is apparently more readily achieved if the “square Planar” carbon atom is stabilized electronically. Planar‐tetracoordinate carbon is sp2‐hybridized and has an electron deficient σ system. The p orbital orthogonal to the bonding plane is occupied by two electrons. Therefore, σ‐donor/π‐accptor substituents—that is, many metals—are in principle able to stabilize this “unnatural” geometry of tetravalent carbon. Meanwhile, well‐established procedures for synthesizing very stable “anti‐van't Hoff/LeBel compounds” have been devised, in which the planar‐tetracoordinate carbon atom is stabilized, usually by the combined action of two directly bonded metals. In most cases, the square‐planar carbon is part of a double bond system. Typical stabilizing metal combinations are Zr/Al or Zr/B, but there are also examples containing two transition metals from the right‐hand side of the periodic table. Reliable estimates for the energy of stabilization of square planar carbon in such compounds are now available. Both theoretical and experimental investigations show that square planar carbon atoms that are stabilized by Zr/Zr+ and Zr/Al in dimetallic compounds are favored by about 12 and 40 kcalmol−1, respectively, over the trigonal planar alternatives. Square planar coordinated carbon is thus no longer an unusual feature in organometallic chemistry and must now be more frequently considered as an alternative structural possibility. Though no longer mere curiosities, compounds containing planar‐tetracoordinate carbon atoms (2) are not at all common. Some of the successes from the combined endeavors of different branches of chemistry in the synthesis and understanding of this “unnatural” structural unit (with respect to the usual tetrahedral 1) are presented here. Coordination to metal atoms plays a key role in this stabilization.