F-Block Chemistry

F-block chemistry encompasses the lanthanide and actinide series, whose frontier 4f and 5f electrons confer distinctive electronic structures, bonding modes and reactivity profiles. Lanthanides commonly adopt the +3 oxidation state and engage in largely ionic coordination chemistry, underpinning key applications in optical materials, permanent magnets and homogeneous catalysis. Actinides, by contrast, access multiple oxidation states and exhibit pronounced covalency through relativistic 5f–ligand orbital overlap, giving rise to linear actinyl units (AnO₂ⁿ⁺) and redox-active species central to nuclear fuel cycles, waste remediation and advanced functional materials. The interplay between electrostatic forces and covalent interactions in f-element complexes governs redox potentials, spectroscopic signatures and magnetic behaviour, and has inspired fields as diverse as single-molecule magnetism and small-molecule activation. Recent progress in ligand design, high-precision spectroscopy and theoretical methods has illuminated the extent of f-orbital participation and enabled the stabilisation of unprecedented oxidation states and bonding paradigms across the series.

Research from Nature Portfolio

New combined computational and spectroscopic studies have identified a fourth bonding component in terminal actinide oxo and imido complexes, demonstrating that the inverse trans influence, in concert with “pushing-from-below” 6p orbital donation, generates multi-centre or charge-shift bonds. High-resolution X-ray absorption and resonant inelastic X-ray scattering have directly visualised 5f orbital contributions in uranium and neptunium complexes, confirming significant f-valence covalency. Complementing this, a series of uranium complexes supported by a tripodal tris(amido)arene ligand has been shown to reversibly access oxidation states II–VI within a single ligand framework. Arene interactions function alternately as δ-acceptors or π-donors, stabilising both low- and high-valent uranium centres and enabling controlled small-molecule activation.

Research from all publishers

Density functional theory investigations of AnCl₃ compounds (Th–No) have revealed a non-monotonic trend in actinide–chloride covalency across the series, highlighting energy-degeneracy-driven bonding in later actinides and clarifying how QTAIM delocalisation indices capture both overlap- and degeneracy-based covalent contributions. In hybrid uranyl tetrahalide materials, integrated DFT+thermodynamics benchmarks correlated packing efficiency, cation protonation enthalpy and hydrogen-bond energy with formation enthalpies and Raman/IR red-shifts of U≡O stretching modes, establishing hydrogen-bond networks as a tunable design parameter. Solid-state [UO₂Cl₄(H₂O)]²⁻ complexes with piperazinium counter-cations have demonstrated that organic cations and coordinated water significantly strengthen the U=O bond, blue-shifting ν₁ and ν₃ stretches and enhancing thermochemical stability via augmented electrostatic interactions.

F-Block Chemistry publication trend

The graph below shows the total number of articles in f-block chemistry across all publications each year (not limited to Nature Index journals).

Technical terms

F-block element: A metal from the lanthanide or actinide series with valence electrons in f orbitals.

Actinyl: A linear dioxo cation (AnO₂ⁿ⁺) formed by early actinides, featuring strong An=O bonds.

Covalency: Electron sharing between metal and ligand orbitals, quantified by orbital overlap and energy degeneracy.

Inverse trans influence: A bonding phenomenon in which two strong donor ligands in trans positions mutually reinforce covalent interactions.

Pushing-from-below: Enhancement of actinide covalency through donation from inner-shell 6p orbitals into valence 5f orbitals.

Quantum Theory of Atoms in Molecules (QTAIM): A computational framework analysing electron density topology to quantify bond critical points and delocalisation indices.

Density Functional Theory (DFT): A quantum-mechanical method for electronic-structure calculations, balancing accuracy and efficiency for heavy-element systems.

References

1. Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding. Nature Communications (2023).
2. The role of the 5f valence orbitals of early actinides in chemical bonding. Nature Communications (2017).
3. Accessing five oxidation states of uranium in a retained ligand framework. Nature Communications (2023).
4. Covalency in AnCl 3 (An = Th–No). Dalton Transactions (2021).
5. Guiding Principles for the Rational Design of Hybrid Materials: Use of DFT Methodology for Evaluating Non‐Covalent Interactions in a Uranyl Tetrahalide Model System. Angewandte Chemie International Edition (2023).
6. Synthesis, Characterization, and Density Functional Theory Investigation of the Solid-State [UO2Cl4(H2O)]2– Complex. Inorganic Chemistry (2023).