# Bonding Trends in Pyridine-2-thiolato Complexes of Tetravalent Actinides

## TL;DR

This study explores the bonding behavior of tetravalent actinides with a specific ligand, revealing increasing covalency in bonds from thorium to plutonium.

## Contribution

The first structurally characterized neptunium complex with (N,S)-donor ligands is reported, along with bonding trends across actinides.

## Key findings

- Covalent contributions in An–S bonds increase from Th to Pu, with Pu–S showing >34% covalency.
- Sulfur donors show higher covalent contributions than nitrogen or oxygen donors in actinide complexes.
- Magnetic behavior varies significantly at low temperatures depending on complex composition.

## Abstract

Complexes of tetravalent
actinides (An: Th, U, Np, and Pu) with
the bidentate (N,S)-donor ligand pyridine-2-thiolate
(2-PyS, PyS–) were synthesized in 1:4 or 1:5 ratios.
This includes the first structurally characterized Np complex with
(N,S)-donor ligands, filling a notable gap in the
An coordination chemistry. An improved synthetic approach with PyS–SiMe3 enabled efficient formation of the 1:4 complexes in THF as
a coordinating solvent. The compounds were comprehensively characterized
in solution and in the solid phase, supported by quantum chemical
calculations. Experimental and theoretical results show matching trends
in the binding behavior of AnIV. The covalent bond contributions
in An–N and An–S bonding increase along the series of
An from Th to Pu. The bonds to the soft sulfur donors consistently
have the highest covalent contributions with a remarkably high percentage
for Pu–S (IQA analysis: >34%) over the harder N donors or
An–O
bonds of coordinating THF. High-resolution X-ray absorption and infrared
spectroscopy indicate similar electronic and structural properties
across AnIV complexes, while SQUID magnetometry uncovered
significant differences in magnetic behavior at low temperatures depending
on the complex compositions. This work advances the understanding
of An-ligand bonding, emphasizing covalency and electronic structures,
and expands fundamental insights into An chemistry.