Tetrel Bonding of the Carbenium Ion Forms a Pentacoordinate Carbon Atom

As a flat trigonal species, the CR3+ carbenium ion contains a pair of deep π-holes above and below its molecular plane. In the case of CH3+ a first base will form a covalent bond with the central C, making the combined species tetrahedral. Approach of a second base to the opposite side results in a longer but rather strong noncovalent tetrel bond (TB). While the CMe3+ can also form a similar asymmetric complex with a pair of bases, it also has the capacity to form a pair of nearly equivalent TBs, such that the resulting symmetric trigonal bipyramid configuration is only slightly higher in energy. When the three substituents on the central C are phenyl rings, the symmetric configuration with two TBs predominates. These tetrel bonds are quite strong, reaching up to 20 kcal/mol. Adding OPH2 or OCH substituents to the phenyl rings permits the formation of intramolecular C··O TBs to the central C, very similar in many respects to the case where these TBs are intermolecular.