ligandfree

Ligandfree is a term used in chemistry to describe a species, typically a metal center, that is not coordinated to any ligands or stabilizing molecules. In coordination chemistry and catalysis, ligand-free states can occur as transient intermediates or as deliberately prepared systems such as bare metal ions, bare metal centers on supports, or ligand-free nanoparticles. The term emphasizes the absence of donors such as phosphines, amines, halides, or other coordinating ligands that would normally occupy coordination sites.

Ligand-free species are usually highly reactive and short-lived because they possess open coordination sites and a tendency to bind whatever is in their surroundings. In solution, many metal ions exist as solvated forms rather than truly ligand-free species; truly ligand-free centers require carefully controlled conditions to prevent rapid binding, hydrolysis, or aggregation. In solid-state or colloidal systems, surfaces or cores can be considered ligand-free if no covalent or strongly coordinating ligands remain attached, though stabilizing interactions with supports or solvents may still influence behavior.

Applications and occurrence of ligand-free centers are most common in studies of catalysis and materials science. They are of particular interest where the activity of an unblocked metal surface or site can be exploited, such as in certain nanoparticle catalysts and supported metal catalysts. However, their practical use is often limited by stability concerns, including aggregation, oxidation, and uncontrolled reactivity, which can complicate reproducibility and selectivity.

See also: coordination chemistry, catalysis, nanoparticles, surface science.