Complex with Linear B–B–B Skeleton Trapped in Dinitrogen Matrix: Matrix Infrared Spectra and Quantum Chemical Calculations

The neutral (NN)–B–B–B–(N2) complex has been trapped in low-temperature dinitrogen matrix and identified by isotopic substitution and theoretical frequency calculations. The linear B–B–B skeleton is stabilized by two inequivalent N2, namely, one end-on and other side-on N2. The structure of linear B–B–B skeleton illustrates much difference from previously reported triangle configuration of B3 clusters. Frontier orbital analysis demonstrates that the σ orbital of end-on NN and the π-bonding orbital of side-on N2 acts as the donor orbital. π bonding character across B–B–B skeleton donates to the antibonding π* orbital of end-on NN and out of phase the B–B–B features π back-donation to antibonding π* orbital of side-on N2. The combination of strong σ-donating capacity coupled with a greater ability for accepting π-back-donation of the N2 ligand leads to the formation of (NN)–B–B–B–(N2) complex with linear B–B–B skeleton. In addition, complexes of (NN)­B­(NN), (NN)­BB­(NN), and (NN)­B4(NN) have been identified in our experiments.