Nuclearities

Nuclearities is a term used in inorganic and organometallic chemistry to denote the number of metal centers, or nuclei, within a discrete molecular entity. It is most often applied to clusters, coordination compounds, and metal-oxide assemblies. Compounds are classified as mononuclear, dinuclear, trinuclear, tetranuclear, and so on, or more generally as polynuclear when three or more metal centers are present. Nuclearity refers to the count of metal atoms bonded within the same molecule or cluster and is distinct from charge, oxidation state, or overall connectivity.

Determination of nuclearity relies primarily on structural characterization by X-ray crystallography, which directly reveals how many metal atoms are present and how they are connected. Supporting information from elemental analysis, mass spectrometry, and magnetic or spectroscopic data can corroborate the number and arrangement of metal centers, especially in cases where crystallography is challenging or ambiguous.

Nuclearity has a strong influence on electronic structure, magnetic behavior, optical properties, and catalytic activity. In catalysis, dinuclear and trinuclear sites can exhibit cooperative effects that enable multi-electron transformations not accessible to mononuclear analogs. Polynuclear clusters also serve as building blocks in materials science, including metal-organic frameworks and inorganic solids, where nuclearity affects cluster connectivity and properties such as porosity and magnetism.

Bridging ligands, including oxo, hydroxo, and carboxylate groups, commonly link metal centers to form higher nuclearity structures. The terms mononuclear, dinuclear, trinuclear, and polynuclear provide a concise framework for describing and comparing families of compounds.