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The structural and bonding properties of the dinitrogen-bridged diniobium [(Cy2N)3Nb(μ-NN)Nb(NCy2)3] complex experimentally characterized by Berno and Gambarotta, which exhibits a strongly activated N–N bond of 134 pm, have been explored using density functional methods and compared with those of a series of related [(R2N)3Nb(μ-NN)Nb(NR2)3] (R = H, Me, iPr, tBu, Cy) model species and other experimentally relevant [Nb(μ-NN)Nb] systems, in order to rationalize the unusually long N–N distance. Geometry optimizations of [(Cy2N)3Nb(μ-NN)Nb(NCy2)3] and three other known systems indicate that the most favourable N–N distance lies within the range of commonly reported results for end-on bound dinitrogen–diniobium complexes, between 123 and 128 pm. However, structures exhibiting appreciably longer N–N distances, close to 134 pm, are found to be only weakly disfavoured, and may represent the preferred geometry in cases where lengthening of the N–N bond counteracts the effects of highly repulsive steric interactions between terminal fragments. Calculations on model complexes, in which small-sized [R = H, Me] terminal groups are involved, support the finding that N–N bond lengths within the 123–128 pm range are most favoured, whereas calculations on larger [R = iPr, tBu] model species indicate that the presence of excessively repulsive intramolecular interactions can lead to substantial changes in the geometric properties of the [Nb–NN–Nb] core, including significant increase in N–N bond length and activation. The preference for N–N distances ranging between 123 and 128 pm, irrespective of ligand size and identity, can be understood on the basis that the principal bonding mechanisms across the central [Nb–NN–Nb] core are largely unaffected by changes in the chemical composition and properties of terminal fragments. However, the balance between repulsive (steric) and attractive (electrostatic plus orbital) bonding contributions can be altered by the presence of geometrically rigid and oversized peripheral groups and, in these cases, the interplay between repulsive and attractive bonding effects is dominated by the former and can result in abnormally elongated N–N distances. The present calculations thus provide a rationale for the observed structural properties of the [(Cy2N)3Nb(μ-NN)Nb(NCy2)3] system on the basis of the interplay between electronic and steric factors.
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