Abstract
Reaction of the side‐on end‐on dinitrogen complex [{(NPN)Ta}~2~(μ‐H)~2~(μ‐η^1^:η^2^‐N~2~)] (1; in which NPN=(PhNSiMe~2~CH~2~)~2~PPh), with the Lewis acids XR~3~ results in the adducts [{(NPN)Ta}~2~(μ‐H)~2~(μ‐η^1^:η^2^‐NNXR~3~)], XR~3~=GaMe~3~ (2), AlMe~3~ (3), and B(C~6~F~5~)~3~ (4). The solid‐state molecular structures of 2, 3, and 4 demonstrate that the NN bond length increases relative to those found in 1 by 0.036, 0.043, and 0.073 Å, respectively. In solution complexes 2–4 are fluxional as evidenced by variable‐temperature ^1^H NMR spectroscopy. The ^15^N{^1^H} NMR spectra of 2–4 are reported; furthermore, their vibrational properties and electronic structures are evaluated. The vibrational structures are found to be closely related to that of the parent complex 1. Detailed spectroscopic analysis on 2–4 leads to the identification of the theoretically expected six normal modes of the Ta~2~N~2~ core. On the basis of experimental frequencies and the QCB‐NCA procedure, the force constants are determined. Importantly, the NN force constant decreases from 2.430 mdyn Å^−1^ in 1 to 1.876 (2), 1.729 (3), and 1.515 mdyn Å^−1^ (4), in line with the sequence of NN bond lengths determined crystallographically. DFT calculations on a generic model of the Lewis acid adducts 2–4 reveal that the major donor interaction between the terminal nitrogen atom and the Lewis acid is mediated by a σ/π hybrid molecular orbital of N~2~, corresponding to a σ bond. Charge analysis performed for the adducts indicates that the negative charge on the terminal nitrogen atom of the dinitrogen ligand increases with respect to 1. The lengthening of the NN bond observed for the Lewis adducts is therefore explained by the fact that charge donation from the complex fragment into the π* orbitals of dinitrogen is increased, while electron density from the NN bonding orbitals p~σ~ and π~h~ is withdrawn due to the σ interaction with the Lewis acid.