Photochemistry of Transition Metal Carbonyl Complexes

Summary

Photochemistry of transition metal carbonyl complexes centres on the interaction between light and the metal–carbonyl bond, leading to ligand dissociation, charge redistribution and formation of reactive intermediates. These complexes, typically comprising a d‐block metal centre coordinated by carbon monoxide ligands, exhibit rich excited‐state behaviour upon ultraviolet or visible irradiation. Initial absorption often promotes an electron from a metal‐centred orbital into an anti‐bonding σ* orbital associated with a CO ligand, weakening the metal–carbon bond and triggering CO release. The ensuing photodissociation pathways can proceed via metal‐to‐ligand charge‐transfer (MLCT) states, metal‐centred (MC) states or ligand‐centred transitions, each with distinct energy landscapes and dynamics. Femtosecond spectroscopies and time‐resolved X-ray methods have revealed sub-picosecond oscillations in metal–CO distances and synchronous bursts of ligand ejection. In solution, solvent coordination and intersystem crossing events sculpt the fate of transient species, influencing ligand substitution, spin crossover and catalytic turnover. Advances in quantum‐chemical modelling complement experimental insights, enabling detailed mapping of potential‐energy surfaces, non-adiabatic transitions and solvent effects. The photochemical behaviour of metal carbonyls underpins their roles in photocatalysis, solar fuel generation and the design of light‐responsive molecular devices, underscoring global relevance in energy conversion and sustainable chemistry.

Research from Nature Portfolio

Recent studies have simulated the early excited‐state dynamics of iron pentacarbonyl in the gas phase, revealing synchronous oscillations of Fe–C bonds following a metal‐to‐ligand charge‐transfer excitation. The analyses show periodic axial CO release as the complex samples bound MLCT and dissociative MC states, akin to a “CO geyser”. This work unifies the delayed ejection of equatorial ligands with the predominant axial release mechanism, attributing selectivity to the anti‐bonding σ*(Fe–C) character in dissociative states. Semi-classical molecular dynamics coupled with non-adiabatic transition theory has provided a coherent picture of sub-picosecond photodissociation cycles and the energy redistribution that governs ligand ejection sequences.

Photochemistry of Transition Metal Carbonyl Complexes publication trend

The graph below shows the total number of articles in photochemistry of transition metal carbonyl complexes across all publications each year (not limited to Nature Index journals).

Technical terms

Metal-to-ligand charge transfer (MLCT): Excited state where an electron moves from a metal orbital to a ligand anti-bonding orbital.

Intersystem crossing (ISC): Non-radiative transition between electronic states of different spin multiplicity.

Femtosecond spectroscopy: Ultrafast optical technique resolving processes on the 10⁻¹⁵ s timescale.

σ*(Fe–C) anti-bonding orbital: Higher‐energy orbital that, when populated, weakens the metal–carbon bond.

σ-complex: Intermediate in C–H activation where an alkane σ-bond interacts with an under-coordinated metal centre.

References

  1. Photoinduced bond oscillations in ironpentacarbonyl give delayed synchronous bursts of carbonmonoxide release. Nature Communications (2022).
  2. Different Photodissociation Mechanisms in Fe(CO)5 and Cr(CO)6 Evidenced with Femtosecond Valence Photoelectron Spectroscopy and Excited-State Molecular Dynamics Simulations. The Journal of Physical Chemistry Letters (2024).
  3. Theoretical Investigation of Transient Species Following Photodissociation of Ironpentacarbonyl in Ethanol Solution. Inorganic Chemistry (2024).
  4. Photochemical Formation and Electronic Structure of an Alkane σ‑Complex from Time-Resolved Optical and X‑ray Absorption Spectroscopy. Journal of the American Chemical Society (2024).
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