Boron Nanowheels with Axles Containing Noble Gas Atoms: Viable Noble Gas Bound M©B Clusters (M=Nb, Ta).

The viability of noble gas axled boron nanowheels Ng M©B (Ng=Ar-Rn; M=Nb, Ta; n=1, 2) is explored by ab initio computations. In the resulting Ng -M complexes, the Ng-M-Ng nanorod passes through the center of the B ring, providing them with an inverse sandwich-like structure. While in the singly Ng bound analogue, the Ng binding enthalpy H at 298 K ranges from 2.5 to 10.6 kcal mol , in doubly Ng bound cases it becomes very low for the Ng M©B →Ng+NgM©B dissociation channel, except for the case of Rn, for which the corresponding H values are 3.4 (Nb) and 4.0 kcal mol (Ta). For a given Ng, Ta has slightly higher Ng-binding ability than Nb. The corresponding free-energy changes indicate that these systems, particularly the Xe and Rn complexes, are good candidates for experimental realization in a low-temperature matrix. The Ng-M bonds were found to be covalent in nature, as reflected in their large Wiberg bond indices, formation of a 2c-2e σ orbital between Ng and M centers in natural bond orbital and adaptive natural density partitioning (AdNDP) analyses, and the short Ng-M distances. Energy decomposition analysis and a study on the natural orbitals for chemical valence show that the Ng-M contact is supported mainly by the orbital and electrostatic interactions, with almost equal contributions. Although both the Ng→M σ donation and Ng←M π backdonation play roles in the origin of orbital interaction, the former is significantly dominant over the latter. Further, AdNDP analysis indicates that the doubly aromatic character (both σ and π) in MB clusters is not perturbed by the interaction with Ng atoms.