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

Abstract

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 N2 complexes (μ-η1:η1-N2) 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 N2 complexes have traditionally been determined by comparing the experimentally observed NN distance to the bond lengths of free N2, 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-[(iPr4PONOP)MCl2]2(μ-N2) (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-N2 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-N2 complexes.

[AsCCAs]-Coordinated Rhenium Hydrides and Their Reactivities Toward Unsaturated Hydrocarbons, Heterocumulenes, and CO2

“…In view of the success of rhenium pincer and pincer-type complexes not only in hydrogenation catalysis ,,, but also in N 2 activation, we posed the question whether the employed ligands, which either comprise a central nitrogen or a central carbon in addition to two flanking phosphines, may be varied in a more substantial manner, for example, by replacing the central donor atom for a diatomic alkyne unit. In previous studies, we have shown that the alkyne backbone in 2,2′-diphosphinotolane complexes is fairly robust, at least in a number of group 5 and group 6 derivatives .…”

Syntheses and Characterizations of Hetero-Bimetallic Chromium-Dinitrogen Transition-Metal Complexes

“…The ORTEP drawings of complex 5 Cp*Cr(depe)(μ-N 2 )V(depe) Me [O, P, O], as shown in Figure 1, revealed a heterobimetallic N 2 -bridge. 49 The N−N bond distance in 5 [1.1919(19) Å], longer than that in Cr 0 −N 2 complex 4 (NN = 1.157(2) Å). This result indicates an enhancement of N 2 activation potentially due to the manifestation of the push− pull effect.…”

“…It should be noted that drawing from the recently published perspective by Hasanayn et al on how to depict Lewis structures and classify bonding in end-on bridging dinitrogen transition metal complexes, 49 complexes 5 can be classified as a BO tot π = 4 system (the total π electrons of the CrNNV core is 8). Consequently, the Cr�N bond could also be represented as a Cr�N triple bond.…”

“…The ORTEP drawings of complex 5 Cp*Cr­(depe)­(μ-N 2 )­V­(depe) Me [O, P, O], as shown in Figure , revealed a heterobimetallic N 2 -bridge . The N–N bond distance in 5 [1.1919(19) Å], longer than that in Cr 0 –N 2 complex 4 (NN = 1.157(2) Å).…”

Benchmark Study of Density Functional Theory Methods in Geometry Optimization of Transition Metal–Dinitrogen Complexes

“…Therefore, it is important to carefully select the computational protocol based on the specific system being studied. It is interesting to know that many papers have used DFT computations to explore the performance of nitrogen fixation. − This emphasizes the importance of combining experimental and computational techniques to gain a deeper understanding of chemical reactions. It also highlights the importance of selecting an appropriate methodology based on the specific system being studied, as different approaches may yield varying degrees of accuracy. − In 2021, by analyzing the structures and relative energies, Luo conducted a benchmarking study to evaluate the performance of DFT by modeling the reaction of a diiron-mediated N–N bond cleavage .…”