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

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 5d6s²-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.