A molecular orbital investigation of the diamagnetism and stability of triangular 50-electron M3Ln clusters

Abstract

EHT and DFT calculations have been carried out on a series of triangular 50-electron M₃L_n clusters and of other electron-rich related species in order to understand why they are both diamagnetic and Jahn–Teller stable, although they exhibit weak M–M bonding. The six MOs associated with the M₃ skeleton are combinations of σ and π_σ frontier orbitals on the metal centers. In all these compounds the large calculated HOMO–LUMO gap is the consequence of two major factors: (i) the σ hybrid-type of the hybrid-type e′* LUMO, which maintains it at a high energy despite its weak antibonding character and (ii) the π_σ d-type character of the occupied out-of-phase a₂^′* level, which maintains it at low energy. On the other hand, at the considered internuclear separations the a₁^′ in-phase combination of the σ metal hybrids always has enough bonding character to be low-lying. The particularly low energy position of the a₂^′* level, which contains the two extra electrons, is the result of the particular orientation of the ligands around the metal centers, which controls the orientation, the hybridization and the energy of the π_σ frontier orbitals. The presence of π-donor in-plane bridging ligands strongly favors the stability of these electron-rich clusters, while terminal-only ligands or π-acceptor bridging ligands do not. The M–M bonding character is mainly contained in the a₁^′ skeletal MO. Cluster core isomerism is discussed with respect to the opening of one M–M edge of the triangle.