# Distinct Difference in the Geometries of NCCL– Anions (L = N2, CO, CS): A Balance Between π Conjugation and Steric Repulsion

## TL;DR

This paper explains why certain NCCL– anions prefer bent or linear shapes based on electron interactions and steric effects.

## Contribution

The study reveals how π conjugation and steric repulsion balance to determine the geometry of NCCL– anions.

## Key findings

- NCCNN– prefers a bent structure due to steric repulsion outweighing conjugation stability.
- NCCCO– and NCCCS– favor linear geometries due to reduced steric repulsion.
- Replacing NC with CH3 or F increases bending in RCNN– but not in RCCO– or RCCS–.

## Abstract

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. “In situ” 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.