# Magnetospectroscopic Studies of a Series of Fe(II) Scorpionate Complexes: Assessing the Relationship between Halide Identity and Zero-Field Splitting

## TL;DR

This study explores how different halides affect the magnetic properties of iron complexes using advanced spectroscopic and theoretical methods.

## Contribution

The paper provides a detailed electronic-structure analysis of Fe(II) scorpionate complexes with varying halides using advanced experimental and theoretical tools.

## Key findings

- Halide identity significantly influences zero-field splitting in Fe(II) complexes.
- Jahn–Teller distortions reduce symmetry in both solution and solid states.
- Zero-field splitting arises from combined Jahn–Teller and ligand field effects, not spin–orbit coupling.

## Abstract

Ferrous ions in four-coordinate
environments are common in protein
structures, synthetic catalysts, and molecular magnets. The 3d
6 configuration of high-spin Fe­(II) imparts
an S = 2 ground state, whose analysis using conventional
spectroscopic methods is often hindered by substantial zero-field
splitting (ZFS). Herein, we provide detailed electronic-structure
descriptions for [FeIIX­(Tp
tBu,Me)] (1-X; X = F, Cl, Br, I), where (Tp
tBu,Me)− is hydrotris­(3-tert-butyl-5-methyl-pyrazol-1-yl)­borate. The three pyrazolyl N-donors
of the “scorpionate” ligand facially coordinate to Fe­(II),
giving idealized C
3v symmetry with the
halide occupying the axial position. Although originally reported
by Theopold and co-workers, this series is revisited herein using
advanced experimental and theoretical tools. Ground-state transitions
were probed by high-frequency and -field electron paramagnetic resonance
(HFEPR) and far-infrared magnetic spectroscopy (FIRMS). Variable-temperature/-field
(VTVH) 57Fe Mössbauer spectroscopy, paramagnetic
susceptibility, and VTVH reduced magnetization were also utilized.
This combined approach provided complete sets of spin-Hamiltonian
parameters. Interpretation using ab initio multiconfigurational calculations
enabled quantification of halide-dependent magnetoelectronic effects.
Jahn–Teller distortions induce a descent in symmetry from C
3v to C
s in both
solution and solid state. Finally, we demonstrate that the 1-X series is ionic, with the ZFS arising from combined Jahn–Teller
and ligand field effects, rather than intrinsic spin–orbit
coupling from the halides.