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

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 ν_as 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 ν_as 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)¹Γ_g singlet state and the terminal N₂ fragments in the ¹Σ_g⁺ 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 ⁵Σ_u⁺ 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 (⁴Σ_u⁺) state and the terminal (L)₂⁻ ligand as symmetry-adapted quartet. In contrast to the common bonding model for N₆ using Lewis structures N⁻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.