Density functional calculations have been employed to rationalize why the heteronuclear N2-bridged MoIIINbIII dimer, [Ar(tBu)N]3Mo(µ-N2)Nb[N(iPr)Ar]3 (Ar = 3,5-C6H3Me2), does not undergo cleavage of the dinitrogen bridge in contrast to the analogous MoIIIMoIII complex which, although having a less activated N–N bond, undergoes spontaneous dinitrogen cleavage at room temperature. The calculations reveal that although the overall reaction is exothermic for both systems, the actual cleavage step is endothermic by 144 kJ mol−1 for the MoIIINbIII complex whereas the MoIIIMoIII system is exothermic by 94 kJ mol−1. The reluctance of the MoIIINbIII system to undergo N2 cleavage is attributed to its d3d2 metal configuration which is one electron short of the d3d3 configuration necessary to reductively cleave the dinitrogen bridge. This is confirmed by additional calculations on the related d3d3 MoIIINbII and NbIINbII systems for which the cleavage step is calculated to be substantially exothermic, accounting for why in the presence of the reductant KC8, the [Ar(tBu)N]3Mo- (µ-N2)Nb[N(iPr)Ar]3 complex was observed to undergo spontaneous cleavage of the dinitrogen bridge. On the basis of these results, it can be concluded that the level of activation of the N–N bond does not necessarily correlate with the ease of cleavage of the dinitrogen bridge.
Christian, G., & Stranger, R. (2004). Activation and cleavage of dinitrogen by three-coordinate metal complexes involving Mo(iii) and Nb(ii/iii). Dalton Transactions, (16), 2492–2495. doi:10.1039/B407515C