Molecular clusters are complexes of atoms. The term is most often applied to metal atom complexes involving between 2 and 12 metal atoms.

Atomic and molecular clusters provide means to study large scale molecules on a smaller scale. In some cases they represent small analogues for the large scale reactions (e.g., heterogeneous catalysis at metal surfaces). In addition, complex phenomena such as crystal formation and protein folding can be better analyzed in systems with smaller numbers of particles.

In metallic clusters, the metal atoms are either directly bonded through metal-metal interactions or are bridged by appropriate ligands. Cluster properties are influenced by the delocalization of electrons (electrons not associated with a particular atom) within the complex. In delocalized clusters, electrons have the ability to respond collectively.

A variety of geometries are observed for clusters. Simple binuclear and trinuclear complexes form linear and trigonal arrangements but a tetranuclear complex can occur as a tetrahedron (i.e. white phosphorus or P₄) or a cubane structure (i.e. Fe₄S₄). Cluster compounds with five to eight vertices are also possible and exhibit a variety of geometries. In most clusters (e.g., metal and semiconductor clusters) the geometric and electronic shell structure interact to determine the overall patterns of molecular shape and stability.

Perhaps the richest and certainly the most extensively studied cluster compounds are the carbonyl complexes. A large variety of structures, particularly with second and third row transition metal complexes, have been identified. These include clusters with more than one type of metal atom present. In addition, substitution of the neutral carbonyl ligand for phosphines, amines, or other neutral species significantly extends the number of possible compounds.