Therefore, this fairly simple catalytic system is amendable to computational investigations to further elucidate the mechanism, to identify critical intermediates, and to better characterize the key C-N bond formation step. To this end, DFT calculations were performed on the B3LYP 49,50 /def2-TZVP level. Coordination of azide 1a on Fe(HMDS)2 forming anti-i2 was calculated to be 1.0 kcal/mol uphill (Scheme 1). Notably, conformational isomerization of anti-i2 to gauche-i2 led to stabilization of the complex by 1.7 kcal/mol, making azide coordination 0.6 kcal/mol downhill compared to Fe(HMDS)2. This small energy difference is in line with the observed equilibrium of azide coordination in 1 H NMR and FTIR spectroscopies (vide supra). Loss of dinitrogen to form the corresponding nitrene complex gauche-i3 has an energy barrier of 26.6 kcal/mol (gauche-TS-i2/i3), yet only 22.6 kcal/mol from the anticonformer (anti-TS-i2/i3). Therefore, loss of dinitrogen and formation of the corresponding nitrene complex is predicted to occur through anti-i2/i3, as the activation energy is 2.4 kcal/mol lower. Formation of anti-i3 was calculated to be 18.8 kcal/mol downhill from the azide complex i2, with further stabilization of 2.2 kcal/mol after isomerization to the gauche conformation of the nitrene complex (gauche-i3). The pentet spin state (S = 2) was determined to be the most stable configuration in comparison to the triplet (S = 1, DE = +3.6 kcal/mol), septet (S = 3, DE = +6.7 kcal/mol), or the pentet broken symmetry solution with two anti-ferromagnetically coupled unpaired electrons (DE = +3.1 kcal/mol; Table ). Interestingly, the pentet spin state resulted in a computed expectation value <S 2 > of 6.48; significantly higher than the ideal 6.00 for a pentet spin-system. On top of that, the spin density plot displayed both α-and β-spin on the nitrene nitrogen. This indicates multireference character, which cannot be computed by DFT and more sophisticated methods such as complete active space self-consistent field (CASSCF) calculations are required to elucidate the exact spin-state (vide infra). The next step in the mechanism, based on literature examples, would be a hydrogen atom abstraction (HAA) to form a carbon-centered radical. In order to find the transition state of this transformation a relaxed surface scan was performed by decreasing the distance of the benzylic hydrogen and the nitrene. Throughout the whole scan, the carbon-centered radical was not formed and during the HAA, the cyclized product was directly formed with just one energy barrier. Accordingly, the HAA and the formation of the C-N bond proceed in one single concerted step with an energy barrier of 11.2 kcal/mol yielding the coordinated cyclized product (i4), which is 46.8 kcal/mol downhill from the nitrene complex. The corresponding transition state (TS-i3/i4) consists of the hydrogen atom bonded to both the carbon and the nitrogen center with a distance of 1.273 Å and 1.318 Å respectively, and a C … N internuclear distance of 2.537 Å (Fig. ). To close the catalytic cycle, either 1b has to dissociate from i4 to regenerate Fe(HMDS)2, or alternatively, the coordinated amine is substituted by the azide substrate through an associative or an interchange mechanism. The dissociative mechanism to form Fe(HMDS)2 was computed to be 8.1 kcal/mol uphill for the dissociation of the amine, followed by 1.0 kcal/mol uphill for the azide coordination to form i2. Performing a geometry optimization for the associative mechanism, resulted in decoordination of the azide, making the associative mechanism unlikely. We were unable to compute any transition state for the three possible mechanisms, despite multiple attempts using relaxed surface scan and nudged elastic band (NEB) calculations. Finding transition states with computational methods of reactions where entropy is a major contributor such as associations and dissociations is a major challenge, as only electronic energies are scanned on the energy surface. Due to this, we cannot distinguish the energy pathway of the dissociative and interchange mechanism by DFT, as they both lead to the same product without going through higher-energy computationally characterized intermediates. However, several experimental facts are incompatible with a dissociative mechanism and hence strongly support an interchange process: i) crystallization of the iron-coordinated product 4 (Fig. ) identifies this complex as the resting state, ii) the reaction is first-order in azide while in a dissociative substitution, it should be zero-order in incoming ligand unless Fe(HMDS)2 were the resting state, and iii) stoichiometric experiments using Fe(HMDS)2 and half an equivalent of 1a did not show any exchange of pyrrolidine between 4 and Fe(HMDS)2. Notably, the overall calculated reaction profile is in excellent agreement also with the pre-equilibrium between 4 and 2, which renders the overall rate zero-order in iron. Furthermore, nitrene formation through loss of dinitrogen as rate-limiting process features a computed ~24 kcal mol -1 energy barrier, which is just about accessible even at room temperature, rationalizing the slow kinetics.