Lewis Structures and the Bonding Classification of End-on Bridging Dinitrogen Transition Metal Complexes.

The activation of dinitrogen by coordination to transition metal ions is a widely used and promising approach to the utilization of Earth's most abundant nitrogen source for chemical synthesis. End-on bridging N 2 complexes (μ-η 1 :η 1 -N 2 ) are key species in nitrogen fixation chemistry, but a lack of consensus on the seemingly simple task of assigning a Lewis structure for such complexes has prevented application of valence electron counting and other tools for understanding and predicting reactivity trends. The Lewis structures of bridging N 2 complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N 2 , diazene, and hydrazine. We introduce an alternative approach here and argue that the Lewis structure should be assigned based on the total π-bond order in the MNNM core (number of π-bonds), which derives from the character (bonding or antibonding) and occupancy of the delocalized π-symmetry molecular orbitals (π-MOs) in MNNM. To illustrate this approach, the complexes cis,cis -[( iPr4 PONOP)MCl 2 ] 2 (μ-N 2 ) (M = W, Re, and Os) are examined in detail. Each complex is shown to have a different number of nitrogen-nitrogen and metal-nitrogen π-bonds, indicated as, respectively: W≡N-N≡W, Re═N═N═Re, and Os-N≡N-Os. It follows that each of these Lewis structures represents a distinct class of complexes (diazanyl, diazenyl, and dinitrogen, respectively), in which the μ-N 2 ligand has a different electron donor number (total of 8e - , 6e - , or 4e - , respectively). We show how this classification can greatly aid in understanding and predicting the properties and reactivity patterns of μ-N 2 complexes.