Photochemical Properties of Ruthenium Nitrosyl Complexes

Ruthenium nitrosyl complexes display a rich array of photochemical behaviours rooted in the ambidentate nature of the NO ligand. Upon visible‐light irradiation, these compounds can undergo linkage isomerism, switching between N-bound (η1-NO) and O-bound (η1-ON) coordination modes, or adopt side-on (η2-NO) geometries. Such transformations create metastable states with distinct spectroscopic signatures and can be reversible on thermal relaxation. In parallel, selective photon absorption may trigger NO photorelease, offering controlled delivery of nitric oxide for therapeutic and catalytic applications. The efficiency and selectivity of these processes depend critically on the electronic properties of co-ligands, steric constraints of the coordination sphere and the rigidity of the solid-state matrix or solution environment. Experimental techniques including time-resolved spectroscopy, in situ photocrystallography and calorimetry, supported by computational potential-energy profiling, have elucidated key excited-state pathways and identified transient metal-centred states that mediate isomerisation and NO dissociation. Advances in ligand design and counter-ion selection have progressively improved conversion yields and extended operational temperatures towards ambient conditions, underpinning emerging applications in molecular switching, photoresponsive materials and targeted NO delivery systems.

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Technical terms

Linkage isomerism: Light‐induced switching of an ambidentate ligand between different coordination sites on a metal centre.

Metastable state: A non-equilibrium configuration of a molecule or crystal that persists under specified conditions before reverting thermally.

Photorelease: Controlled liberation of a ligand (e.g. NO) from a metal complex upon irradiation, enabling temporal and spatial delivery.

Photocrystallography: X-ray diffraction performed on single crystals under illumination to determine structural changes in excited or metastable states.

CASPT2: A computational method (complete active space with second‐order perturbation theory) used to calculate excited‐state energy profiles.

References

1. Heteroleptic Complexes of Ruthenium Nitrosyl with Pyridine and Bypiridine—Synthesis and Photoisomerization. Molecules (2024).
2. CASPT2 Potential Energy Curves for NO Dissociation in a Ruthenium Nitrosyl Complex. Molecules (2020).
3. Multistep Photochemical Reactions of Polypyridine-Based Ruthenium Nitrosyl Complexes in Dimethylsulfoxide. Molecules (2020).