BNg₃F₃: The first three noble gas atoms inserted into mono-centric neutral compounds-a theoretical study

Following the study of HXeOXeH and HXeCCXeH, in which two Xe atoms were inserted into H₂O and C₂H₂ theoretically and experimentally, the structures and stability of BNg₃F₃ (Ng = Ar, Kr and Xe), in which three Ng atoms are inserted into BF₃, have been explored theoretically using DFT and ab initio calculations. It is shown that BNg₃F₃ (Ng = Ar, Kr and Xe) with D_3h symmetry are local minima with short B-Ng bond lengths of 1.966, 2.027 and 2.214 Å at the CCSD(T)/aug-cc-pVTZ/LJ18 level, which are close to their covalent limits. Note that although BNg₃F₃ (Ng = Kr and Xe) are energetically higher than the dissociation products 3Ng + BF₃, they are still kinetically stable as metastable species with protecting barriers of 13.38 and 17.99 kcal mol^-1 for BKr₃F₃ and BXe₃F₃. Moreover, BKr₃F₃, the tri-Kr-inserted compound, even has comparable kinetic stability to HXeOXeH and HXeOXeF. In addition, upon the formation of BNg₃F₃, there is a large amount of charge transferred from B to Ng of at least 0.619 e. The calculated Wiberg Bond Indices (WBI) suggest that B-Ng bonds are naturally singly bonded; the large vibrational frequencies of B-Ng and Ng-F stretching modes and the negative Laplacian electron density of B-Ng bonds confirm further that BNg₃F₃ are stiff molecules with covalent B-Ng bonds. It should be noted that three Ng atoms inserted into mono-centric neutral molecules have not been reported previously. We hope that the present theoretical study may provide important evidence for the experimental synthesis of BNg₃F₃.