# Dinitrogen complexes N2L2 (L = N2, CO, CS, NO+, CN−)

## TL;DR

This paper uses quantum chemical methods to study the structure, stability, and bonding of N2L2 complexes with various ligands, revealing insights into their dissociation and vibrational properties.

## Contribution

The study provides new insights into the thermodynamic stability and bonding mechanisms of N2L2 complexes using advanced quantum chemical calculations.

## Key findings

- N2L2 complexes with L = N2, CO, NO+, CN− are thermodynamically unstable, while N2(CS)2 is slightly stable.
- The dissociation of N2L2 into N2 + 2L is energetically more favorable than into 2 NL.
- Vibrational frequencies show distinct shifts for different ligands, aiding in the identification of these complexes.

## Abstract

Quantum chemical calculations using ab initio methods and density functional theory have been carried out on the equilibrium structures and the vibrational spectra of the (valence) isoelectronic compounds N2L2 (L = N2, CO, CS, NO+, CN−). The molecules have a trans-periplanar arrangement of the L2 ligands at the N2 unit. The complexes with L = N2, CO, NO+, CN− are predicted as thermodynamically unstable for dissociation into N2 + 2L with ΔG298 value lying in between −257 kcal mol−1 (L = NO+) and −73 kcal mol−1 (L = CO), but the adduct N2(CS)2 is calculated as slightly stable with ΔG298 = 4 kcal mol−1. The homolytic dissociation reaction into two fragments N2L2 → 2 NL is energetically less favorable than the heterolytic fragmentation N2L2 → N2 + 2 L, which proceeds synchronously but asymmetrically. The activation barriers for the fragmentation reaction N2L2 → N2 + 2L have values between ΔG≠(298 K) = 17 kcal mol−1 for L = N2 and ΔG≠(298 K) = 84 kcal mol−1 for L = CS. The calculated vibrational frequencies suggest that the molecules N2L2 can be identified by the IR active antisymmetric stretching mode νas of the ligands L, which is blue shifted for L = CO (Δ = 55 cm−1) and L = NO+ (Δ = 118 cm−1) but it is red shifted for L = CS (Δ = −242 cm−1) and L = CN− (Δ = −133 cm−1) relative to the νas mode of L = N2. The analysis of the bonding situation reveals that there is a total charge donation L→(1Γ-N2)←L in all complexes, ranging between 1.38 e (L = CN−) and 0.56 e (L = N2), except in the dication with L = NO+, where a small backdonation in reverse direction L←(1Γ-N2)→L with 0.10 e is calculated. EDA-NOCV calculations of N6 show that the best description of the bonding situation is given in terms of dative interactions N2→(1Γ-N2)←N2 between central N2 in the excited (1)1Γg singlet state and the terminal N2 fragments in the 1Σg+ electronic ground state. In contrast, the best description of the complexes with L = CO, CS, NO+ is calculated for the interactions between the central N2 in the 5Σu+ quintet state and the terminal ligands in the symmetry-adapted (L)2 quintet state. For N2L2 with L = CN−, it is found that the bonding is best described for the interaction between N2− in the electronic quartet (4Σu+) state and the terminal (L)2− ligand as symmetry-adapted quartet. In contrast to the common bonding model for N6 using Lewis structures N−

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N+N–NN+=N−, the donor–acceptor model N2→(N2)←N2 explains that the lowest activation barrier is found for the concerted cleavage of the two formal double bonds, leading to the experimentally observed dissociation into 3 N2.

Quantum chemical calculations using ab initio methods and density functional theory have been carried out on the equilibrium structures, energies and vibrational spectra of the (valence) isoelectronic compounds N2L2 (L = N2, CO, CS, NO+, CN−).