The boron–boron triple bond in NHC→BB←NHC

Abstract

Quantum chemical calculations of the compound B₂(NHC^Me)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(NHC^R)₂ of Braunschweig which has an even slightly shorter B–B bond. The bonding situation in the molecule is best described in terms of NHC^Me→B₂←NHC^Me donor–acceptor interactions and concomitant π-backdonation NHC^Me←B₂→NHC^Me 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 NHC^Me⇄BB⇄NHC^Me. 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(NHC^Me)₂ has the value of 2.34. The suggestion by Köppe and Schnöckel that the strength of the boron–boron bond in B₂(NHC^H)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(NHC^Me)₂ is a mistaken application of the principle of isolable relationship.