Nature of Metal–Metal Bond in Group‐V Dinuclear Metallaborane Compounds: Open‐Shell–Open‐Shell Vs Closed‐Shell–Closed‐Shell Interaction

Abstract The investigation of metal–metal bonding is interesting due to the captivating structural features, unique chemical reactivity and physical properties of this class of complexes. The synthesis of the complexes having metal–metal bonds started to explor after the isolation of the dianionic Re2Cl82−, possessing quadruple bonds between two Re atoms. After that, several complexes containing multiple metal‐metal bonds were synthesized, isolated and characterised. The bonding situation in the metallaboranes [(Cp*)2M2(B2H6)2] of group‐V elements (M) in the +3 oxidation state [M = V (1), Ta (2)] was investigated by the DFT, NBO, QTAIM calculations and further with Energy Decomposition Analyses coupled with Natural Orbital for Chemical Valence (EDA–NOCV). Even though the metallaboranes are isostructural, the nature of metal–metal bonding interaction was found to be different, revealing the presence of open‐shell–open‐shell interaction in divanadium compound while its Ta‐analogue possesses closed‐shell–closed‐shell interaction between two tantalum atoms. However, EDA–NOCV analyses suggest that TaIII−TaIII bonding interactions are stronger than those of the divanadaborane analogue, having an intrinsic interaction energy of −247.6 kcal/mol (2), and the interaction energy leading to the formation of a V−V bond between two molecular fragments, is −192.5 kcal/mol (1). The bonding analyses indicate that the strength of the metal–metal bond becomes stronger as the metal becomes heavier which is due to the higher contribution from electrostatic stabilization energy. The pairwise orbital analysis of the metal–metal bond denotes significant dative interaction between two tantalum centres, contributing 52.6 % (2) to the total orbital interaction energy. The electron‐sharing interaction energy has been computed to be 44.3 % for the divanadaborane analogue (1). Two Cp*− ligands do not play significant role in the stabilization/destabilization of the V−V bond while the Ta−Ta bond is slightly destabilized by Cp*− ligands. The electronic π‐clound mostly fills empty orbitals on the metals, M = V and Ta. Further analyses show that the B2H62− unit drags out some amount of electron densities from the central M−M bond of both complexes, which are visible in the deformation densities. The chemical bonding between two VIII ions of two triplet V‐complexes (3–4) was also investigated via EDA‐NOCV analysis. The analysis concluded the existance an one electron bond between two vanadium centres (VIII−VIII). Also, the other unpaired electron of triplet fragments in 3/4 prefers to reside on the non‐bonding orbital on one of the V‐containing molecular fragments.