(NHC) 3 →B + B - ←(CO) 2 : a allotrope of boron in ionic form stabilized by Lewis base

In this paper, we divide allotropes into two categories,  one is the zero-oxidation state allotrope and the other is the ionic form allotrope.  So far, many main group complexes L→M n ←L( L, Lewis base  ligands; M, main group element) have been found, which can be  regarded as zero-oxidation state allotrope of the main group elements stabilized by Lewis base  ligands. However, few compounds L→M n + M n - ←L containing ionic form allotropes of the main group elements have been reported. This class of compounds L→M n + M n - ←L is found only in  s-block alkalide salts.  In this work, we have theoretically confirmed that this class of compound L→M n + M n - ←L can be extended to P-block boron. Rigorous thermodynamic calculations based on  Born–Haber cycle confirm that (NHC) 3 →B + and (CO) 2 →B - are a compatible pair of positive and negative ions, which can form stable [(NHC) 3 →B + ][(CO) 2 →B - ] crystalline salt. More importantly, the bonding analysis of (NHC) 3 →B + and (CO) 2 →B - based on frontier orbital analysis and NBO show that (NHC) 3 →B + and (CO) 2 →B - can be regarded as positive monovalent boron B(1) and negative monovalent boron B(-1) complexes stabilized by Lewis base, respectively. So [(NHC) 3 →B + ][(CO) 2 →B - ] crystalline salt can be viewed as“B+B-”ionic salts stabilized by Lewis base ligands: an allotrope of boron in ionic form through asymmetric coordination of Lewis base.