Distinct Difference in the Geometries of NCCL ^– Anions (L = N ₂ , CO, CS): A Balance Between π Conjugation and Steric Repulsion

NCCL^- anions (L = N₂, CO and CS) exhibit notable geometric differences: NCCNN^- favors a bent structure, whereas NCCCO^- and NCCCS^- prefer linear configurations. Here, we investigate the origin of their geometric differences using the conventional density functional theory (DFT) and block-localized wave function (BLW) method. Computations reveal that the bent structure is preferred for all species at the BLW state in which the in-plane π_// back-donation from NCC^- to ligands is disabled. For NCCNN^-, enforcing linearity leads to a gain in the π_// electron delocalization (conjugation) stability, but it is insufficient to offset the steric penalty. Differently, NCCCO^- and NCCCS^- experience reduced steric repulsion in linear geometries, thereby favoring more linear geometries as their energy minima. "<i>In situ</i>" orbital correlation diagrams reveal an orbital swap in NCC^- with the approaching of the ligand L and confirm the carbone theory proposed by the Frenking group. Overall, the geometries (or the degree of bending) of NCCL^- anions are governed by the balance between the π_// conjugation and steric repulsion. Substituting the electron-withdrawing NC group with CH₃ or F, which provides in-plane σ-electrons or lone pairs, further bends the ∠RCL angle in RCNN^- anions, while RCCO^- and RCCS^- retain their nearly linear forms.