Electronic structure and molecular properties of binuclear group VII pentalene metal carbonyl complexes [C₈H₆{M(CO₃)}₂] (M = Mn, Tc, Re, Bh): A relativistic density functional theory study

Homobimetallic systems where the metals are linked through a pentalenediide ligand, of the type anti-[Pn{M(CO)₃}₂] (Pn = pentalenediide), which include transition metals of the group VII with M = ₂₅Mn (1), ₄₃Tc (2), ₇₃Re (3) and ₁₀₇Bh (4), and the syn-[Pn{M(CO)₃}₂] isomer with M = ₂₅Mn (s1), ₄₃Tc (s2), ₇₃Re (s3) and ₁₀₇Bh (s4), were studied with relativistic all-electron density functional (DFT) calculations, including spin-orbit (SO) coupling via the two components ZORA Hamiltonian. The electronic structure was studied in detail in the four systems. Broken symmetry calculations were performed for all the paramagnetic systems to verify their mixed-valence character. The infrared (IR) spectra were obtained at the scalar relativistic regime and the UV-Vis was obtained by time-dependent spin-orbit DFT and compared against the experimental data available (only for 1 and 3). The relative binding energy calculations predict that the not yet reported s1, 2, s2, 4 and s4 complexes may be synthesized. Their optical and vibrational properties are described here. Due to the relativistic indirect effect acting on metal d orbitals there is an increased overlap between the nd and 2π* empty carbonyl orbitals, and hence the backbonding effect increases from 1/s1 to 4/s4. The calculated spin-dependent properties of the anionic complexes reveals that the isotropic g tensor decreases down the group, showing that the quenching of the total angular momentum of the unpaired electron increases along the group.