The boron-boron triple bond in NHC→B≡B←NHC

Quantum chemical calculations of the compound B-2(NHCMe)(2) and a thorough examination of the electronic structure with an energy decomposition analysis provide strong evidence for the appearance of boron-boron triple bond character. This holds for the model compound and for the isolated diboryne B-2(NHCR)(2) of Braunschweig which has an even slightly shorter B-B bond. The bonding situation in the molecule is best described in terms of NHCMe-->B-2<--NHCMe donor-acceptor interactions and concomitant pi-backdonation NHCMe-->B-2<--NHCMe which weakens the B-B bond, but the essential features of a triple bond are preserved. An appropriate formula which depicts both interactions is the sketch NHCMe reversible arrow B=B reversible arrow NHCMe. Calculations of the stretching force constants F-BB which take molecules that have genuine single, double and triple bonds as references suggest that the effective bond order of B-2(NHCMe)(2) has the value of 2.34. The suggestion by Koppe and Schnockel that the strength of the boron-boron bond in B-2(NHCH)(2) is only between a single and a double bond is repudiated. It misleadingly takes the force constant F-BB of OBBO as the reference value for a B-B single bond which ignores pi bonding contributions. The alleged similarity between the B-O bonds in OBBO and the B-C bonds in B-2(NHCMe)(2) is a mistaken application of the principle of isolable relationship.