Cage-like Ta@Bnq complexes (n=23-28,q=-1-+3) in 18-electron configurations with the highest coordination number of twenty-eight

      Inspired by recent observations of the highest coordination numbers of CN = 10 in planar wheel-type complexes in D_10h Ta@B₁₀^- and CN = 20 in double-ring tubular species in D10d Ta@B20- and theoretical prediction of the smallest endohedral metalloborospherene D2 Ta@B₂₂^- (1) with CN = 22, we present herein the possibility of larger endohedral metalloborospherenes C₂ Ta@B₂₃ (2), C₂ Ta@B₂₄⁺ (3), C_2v Ta@B₂₄^- (4), C₁ Ta@B₂₅(5), D2d Ta@B₂₆⁺ (6), C₂ Ta@B₂₇²⁺ (7), and C₂ Ta@B₂₈³⁺ (8) based on extensive first-principles theory investigations. These cage-like Ta@B^q_n complexes with B6 pentagonal or B₇ hexagonal pyramids on their surface turn out to be the global minima of the systems with CN = 23, 24, 24, 25, 26, 27, and 28, respectively, unveiling the highest coordination number of CN = 28 in spherical environments known in chemistry. Detailed bonding analyses show that 1–8 as superatoms conform to the 18-electron configuration with a universal σ + π double delocalization bonding pattern. They are effectively stabilized via spd–π coordination interactions between the Ta center and ηn–B_n ligand which match both geometrically and electronically. Such complexes may serve as embryos of novel metal–boride nanomaterials.