Probing the structural transformation and bonding of metal–boride clusters MB3 (M = La, Ta, Re, Ir)

Abstract

Theoretical investigations using density functional theory (DFT) and ab initio wavefunction theory (WFT) have been performed to understand the geometric and electronic structures, chemical bonding, and structural transformation of 5d^x6s²-metal doped triboron clusters MB₃ (M = La, Ta, Re, Ir; x = 1, 3, 5, 7). Global-minimum structural searches find that early-metal doped MB₃ (M = La, Ta) clusters adopt a two-dimensional (2D) planar structure, with σ- and π-type delocalized molecular orbitals (MOs) consisting of M-5d and B-2p atomic orbitals (AOs) identified by chemical bonding analysis. In contrast, late-metal doped MB₃ (M = Re, Ir) clusters prefer three-dimensional (3D) structures of near-pyramidal and triangular pyramid geometries, respectively, which exhibit enhanced stability involving σ- and δ-type M(5d)–B(2p) interactions. The M−B bonding in the Re and Ir borides is more covalent than the La and Ta ones due to less charge transfer and similar orbital energies of late 5d-metals and boron. Moreover, the Jahn–Teller effect leads to MO mixing and electron redistribution, thus enlarging the HOMO–LUMO gaps. This work provides insights into the nature of the structural stability in triboron clusters induced by metal doping.