# Unraveling the coordination isomerism by ligand hyperfine NMR shifts

## TL;DR

This paper shows how NMR shifts can reveal the geometry of coordination compounds by analyzing spin distribution patterns.

## Contribution

The study introduces hyperfine NMR shifts as a novel and sensitive method to detect coordination isomerism in metal complexes.

## Key findings

- Hyperfine NMR shifts distinguish equatorial and axial isomers in [Ru(acac)Cl2L2] compounds.
- Spin density transmission mechanisms differ based on ligand geometry, affecting NMR spectral fingerprints.
- The approach is generalizable to various coordination compounds, including catalysts and metalloenzymes.

## Abstract

The hyperfine (Curie) NMR shifts of ligand atoms in open-shell coordination compounds report subtle details of the spin distribution around the central metal atom. In this work, we propose hyperfine NMR shifts as simple and extremely sensitive indicators of the ligand coordination geometry. This is demonstrated for equatorial versus axial isomers of neutral octahedral [Ru(acac)Cl2L2] compounds, rationalized by two distinct mechanisms of transmission of the spin density unraveled using density-functional theory analysis. The positional interchange of the two chlorides and the two pnictogen-based ligands (L) induces modifications in the singly occupied molecular orbital composition and the related Fermi-contact hyperfine interactions of the probed atoms of the acac ligand, resulting in distinct 1H and 13C NMR spectral fingerprints. The demonstrated symmetry-driven spin-transmission mechanisms have general validity, which offers hyperfine NMR shifts as a tool to probe the geometry of various classes of coordination compounds, including transition-metal catalysts and metalloenzymes.

Paramagnetic NMR reveals how ligand positioning in transition-metal complexes governs SOMO symmetry and spin-delocalization pathways, leading to distinct Fermi-contact NMR shifts.