Unusual Electronic Structures of an Electron Transfer Series of [Cr(μ-η 1  : η 1 -N 2 )Cr] 0/1+/2 .

In dinitrogen (N 2 ) fixation chemistry, bimetallic end-on bridging N 2 complexes M(μ-η 1  : η 1 -N 2 )M can split N 2 into terminal nitrides and hence attract great attention. To date, only 4d and 5d transition complexes, but none of 3d counterparts, could realize such a transformation. Likewise, complexes {[Cp*Cr(dmpe)] 2 (μ-N 2 )} 0/1+/2+ (1-3) are incapable to cleave N 2 , in contrast to their Mo congeners. Remarkably, cross this series the N-N bond length of the N 2 ligand and the N-N stretching frequency exhibit unprecedented nonmonotonic variations, and complexes 1 and 2 in both solid and solution states display rare thermally activated ligand-mediated two-center spin transitions, distinct from discrete dinuclear spin crossovers. In-depth analyses using wave function based ab initio calculations reveal that the Cr-N 2 -Cr bonding in complexes 1-3 is distinguished by strong multireference character and cannot be described by solely one electron configuration or Lewis structure, and that all intriguing spectroscopic observations originate in their sophisticate multireference electronic structures. More critical is that such multireference bonding of complexes 1-3 is at least a key factor that contributes to their kinetic inertness toward N 2 splitting. The mechanistic understanding is then used to rationalize the disparate reactivity of related 3d M(μ-η 1  : η 1 -N 2 )M complexes compared to their 4d and 5d analogs.