Bonding situation in dimeric group 15 complexes [(NHC)₂(E₂)] (E = N-Bi)

Quantum chemical calculations using density functional theory at the BP86 level in conjunction with triple-zeta polarized basis sets have been carried out for the title compounds. The nature of the bonding between the diatomic fragment and the NHC ligands is investigated with an energy decomposition analysis. The chemical bonds in the [(NHC^Me)₂(E₂)] complexes can be discussed in terms of donor-acceptor interactions which consist of two NHC^Me→E₂←NHC^Me donor components and two weaker components of the NHC^Me←E₂→NHC^Me π backdonation. The out-of-phase (+)=(-) contribution of the σ donation is always stronger than the in-phase (+)=(+) contribution. The electronic reference state of N₂ in the dinitrogen complex [(NHC^Me)₂(N₂)] is the highly excited 1¹Γ_g state which explains the anti-periplanar arrangement of the ligands. The gauche arrangement of the ligands in the heavier homologues [(NHC^Me)₂(E₂)] (E = P-Bi) may be discussed using either the excited 1¹Γ_g state or the X¹∑⁺_g ground state of E₂ as reference states for the donor-acceptor bonds. The EDA-NOCV calculations suggest that the latter bonding model is better suited for the complexes where E = As-Bi while the phosphorus complex is a borderline case.