Superhalogen Anions and Their Chemical Properties

Superhalogen anions are discrete molecular clusters whose electron affinities surpass those of conventional halogen atoms, often by several electronvolts. They typically consist of a central atom or metal centre surrounded by halogen or pseudohalogen ligands, forming a highly stable electron-rich shell. The hallmark of these species is a large vertical electron detachment energy that reflects an exceptional ability to capture and retain an extra electron. Structural variations range from mononuclear species bearing halogen ligands to polynuclear chains built upon alternately aligned heteroatoms. Bonding analyses reveal that electron density in superhalogen anions is delocalised over ligand frameworks, leading to reduced potential and kinetic energies of the excess electron. Their extraordinary oxidising power and tunable electronic structures make them attractive for redox catalysis, materials assembly and ion‐transport applications. Advances in computational chemistry, particularly density functional theory and global structure searches, have underpinned the rational design of new superhalogen motifs, informing synthetic strategies for ionic solids, superacidic media and magnetically active clusters.

Research from Nature Portfolio

Recent studies have utilised unbiased global‐optimisation algorithms combined with density functional theory to identify iron‐centred superhalogen anions with retained magnetic moments. Model clusters of the form FeL₄⁻, where L represents pseudohalogen ligands such as BO₂, NO₃ and NO₂, exhibit electron affinities that exceed those of their constituent ligands. Analysis of the extra‐electron distribution demonstrates that metal‐ligand synergy enhances vertical detachment energies while preserving the spin density on the iron centre. Thermodynamic calculations further indicate that sodium salts of these anions are stable against dissociation, suggesting viable pathways for isolating magnetically active ionic materials with superhalogen characteristics.

Superhalogen Anions and Their Chemical Properties publication trend

The graph below shows the total number of articles in superhalogen anions and their chemical properties across all publications each year (not limited to Nature Index journals).

Technical terms

Superhalogen anion: A cluster whose electron affinity exceeds that of any halogen atom.

Electron affinity: The energy released when a neutral species gains an electron.

Vertical detachment energy: The energy required to remove an electron from an anion without allowing structural relaxation.

Pseudohalogen: A ligand group (for example CN or NO₂) that mimics the electronic behaviour of halogen atoms.

Superatom: A cluster of atoms that behaves like a single atom with distinct electronic properties.

Magnetic superhalogen: A superhalogen anion in which a paramagnetic metal centre retains its spin state upon electron attachment.

References

1. Iron-based magnetic superhalogens with pseudohalogens as ligands: An unbiased structure search. Scientific Reports (2017).
2. Superhalogen Anions Supported by the Systems Comprising Alternately Aligned Boron and Nitrogen Central Atoms. Frontiers in Chemistry (2022).
3. Design and Investigation of Superatoms for Redox Applications: First-Principles Studies. Micromachines (2023).
4. Be2H3L2− (L=CH3 and F–I): Hyperhalogen anions with ultrashort beryllium-beryllium distances. Chinese Journal of Chemical Physics (2023).