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

Abstract

Quantum chemical calculations of the compound B 2 (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 B 2 (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 →B 2 ←NHC Me donor-acceptor interactions and concomitant π-backdonation NHC Me ←B 2 →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 BBNHC 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 B 2 (NHC Me) 2 has the value of 2.34. The suggestion by Köppe and Schnöckel that the strength of the boron-boron bond in B 2 (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 B 2 (NHC Me) 2 is a mistaken application of the principle of isolable relationship.