Redox-Active Ligand-Mediated Processes in Transition Metal Complexes

Redox-active ligands have transformed our understanding of coordination chemistry by engaging directly in electron-transfer events alongside the metal centre. Unlike traditional spectator ligands, these architectures can undergo reversible oxidation or reduction, enabling metal–ligand cooperativity that enriches electronic structure and reactivity. Such systems support valence tautomerism, where electron density shifts between metal and ligand, offering pathways to modulate catalytic turnover, magnetic behaviour and photophysical response. In catalytic contexts, ligand-centred redox steps can lower activation barriers for bond formation or cleavage, while in materials science the ability to store and release electrons underpins charge-storage devices and switchable molecular assemblies. Central to these advances are versatile coordination motifs—from tridentate pincer frameworks to non-innocent catecholates and iminophenolates—that dictate orbital overlap, electronic delocalisation and proton-coupled electron transfer. By tailoring ligand substituents and coordination geometry, researchers achieve fine control over redox potentials, stabilise open-shell intermediates and enable multi-electron processes in small-molecule activation, energy conversion and electrosynthesis.

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Redox-Active Ligand-Mediated Processes in Transition Metal Complexes publication trend

The graph below shows the total number of articles in redox-active ligand-mediated processes in transition metal complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Redox-active ligand: A ligand capable of reversible electron uptake or release, participating directly in redox chemistry rather than acting solely as a spectator.

Redox non-innocent ligand: A ligand whose oxidation state cannot be assigned independently of the metal, often leading to delocalised electronic structures and metal–ligand cooperativity.

Secondary coordination sphere: The network of non-bonded interactions, hydrogen bonds or pendant groups beyond the primary metal–ligand bonds that influence reactivity and stabilisation of intermediates.

Electrocatalysis: The acceleration of redox transformations at an electrode surface by a catalyst, enabling efficient charge transfer and energy conversion.

Pincer ligand: A rigid, typically tridentate ligand framework that binds meridionally to a metal centre, providing robustness and precise control over electronic properties.

References

1. Cobalt(III) complex with a redox non-innocent bis (o-aminophenol). Inorganic Chemistry Communications (2024).
2. Redox Chemistry of Pt(II) Complex with Non-Innocent NHC Bis(Phenolate) Pincer Ligand: Electrochemical, Spectroscopic, and Computational Aspects. Catalysts (2023).
3. Ni II and Cu II complexes of a salen ligand bearing ferrocenes in its secondary coordination sphere. RSC Advances (2023).