The boron-boron triple bond in NHC→B[triple bond, length as m-dash]B←NHC.

Abstract

Quantum chemical calculations of the compound B2(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 B2(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→B2←NHCMe donor-acceptor interactions and concomitant π-backdonation NHCMe←B2→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⇄B[triple bond, length as m-dash]B⇄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 B2(NHCMe)2 has the value of 2.34. The suggestion by Köppe and Schnöckel that the strength of the boron-boron bond in B2(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 π bonding contributions. The alleged similarity between the B-O bonds in OBBO and the B-C bonds in B2(NHCMe)2 is a mistaken application of the principle of isolable relationship.