Exohedral

Exohedral is an adjective used in chemistry and crystallography to describe a species in which a substituent, atom, or cluster lies outside a polyhedral cage, as opposed to being inside the cage (endohedral). The term derives from the Greek exo, outside, and hedron, a seat or face.

In boron hydride and carborane chemistry, exohedral refers to substituents attached to the exterior of a closo- or polyhedral boron cluster, while endohedral refers to atoms or groups located inside the cage cavity. This distinction helps classify clusters by substitution pattern and anticipated reactivity, solubility, and orbital interactions.

In fullerene chemistry, exohedral adducts are compounds formed when reagents add to the exterior surface of the fullerene cage (for example, on C60), in contrast to endohedral complexes where atoms or clusters reside inside the cage.

In metal cluster chemistry, exohedral descriptions apply to ligands or fragments bound to the outer surface of a metal cluster, as opposed to endohedral species that include atoms or fragments within the internal void of the cluster.

The exohedral/endohedral distinction provides a framework for understanding structural motifs, synthetic strategies, and spectroscopic properties across several subfields of inorganic and materials chemistry. It is commonly used alongside related terms such as carboranes, boranes, fullerenes, and metalloclusters. See also endohedral, fullerene, borane chemistry, and carborane.