Ligand-Metal Coordination Complexes and Electron Transfer Mechanisms

Ligand-metal coordination complexes constitute a central theme in inorganic and materials chemistry, uniting metal centres with organic or inorganic ligands through coordinate covalent bonds. The resulting architectures exhibit a rich variety of electronic structures, from simple mononuclear assemblies to polynuclear networks bridged by redox-active ligands. Electron transfer within and between these complexes underpins processes as diverse as catalytic activation of small molecules, charge transport in molecular electronics and light-driven energy conversion. Two principal pathways govern these transfers: outer-sphere mechanisms, in which electron tunnelling occurs without breaking metal–ligand bonds, and inner-sphere mechanisms, in which a bridging ligand or direct metal–metal contact mediates the transfer. Redox-active (“non-innocent”) ligands can themselves participate in charge delocalisation, giving rise to mixed-valent species and intervalence charge-transfer bands observable by spectroelectrochemistry. Control of ligand field strength, donor/acceptor character and bridging geometry allows fine-tuning of redox potentials, electronic coupling and spin states. Advances in computational methods and time-resolved spectroscopy have deepened understanding of transient states and enabled the design of complexes with tailored kinetic and thermodynamic profiles. The interplay between coordination geometry, ligand electronics and solvent or solid-state environment continues to open new frontiers in catalysis, molecular electronics and bioinorganic modelling.

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Ligand-Metal Coordination Complexes and Electron Transfer Mechanisms publication trend

The graph below shows the total number of articles in ligand-metal coordination complexes and electron transfer mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Coordination complex: A chemical species comprising a central metal atom or ion bonded to surrounding molecules or ions (ligands) via coordinate covalent bonds.

Inner-sphere electron transfer: A redox process in which the electron moves through a shared ligand or direct metal–metal contact between two metal centres.

Outer-sphere electron transfer: A redox process in which the electron moves between metal centres without changes to the coordination sphere or ligand connectivity.

Mixed valency: A state in which chemically identical metal centres exist in different oxidation states, often leading to delocalised electronic states.

Intervalence charge transfer (IVCT): An optical absorption arising from electron transfer between two metal centres of a mixed-valent complex.

Non-innocent ligand: A ligand that can undergo reversible redox changes, participating directly in electron transfer and affecting the overall oxidation state of the complex.

References

1. The Indigo Isomer Epindolidione as a Redox‐Active Bridging Ligand for Diruthenium Complexes. Chemistry - A European Journal (2021).
2. Intramolecular Charge Transfer in Ruthenium Complexes [Ru(acac)2(ciq)] with Ambidentate Camphoriminoquinone (ciq) Ligands. Zeitschrift für anorganische und allgemeine Chemie (2021).
3. Mixed Valency vs Radical Bridge Formulation in Symmetrically and Asymmetrically Ligated Diruthenium Complexes. European Journal of Inorganic Chemistry (2022).