Clusters can be found in all classes of chemical compounds.  According to the commonly accepted definition, a cluster consists minimally of a triangular set of atoms that are directly bonded to each other.  But metal-metal bonded dimetallic complexes are highly relevant to the area.  Clusters occur in "pure" inorganic systems, organometallic chemistry, main group chemistry, and bioinorganic chemistry.  The distinction between very large clusters and bulk solids is increasingly blurred.  This interface is the chemical basis of nanoscience or nanotechnology and specifically arise from the study of quantum size effects in cadmium selenide clusters. Thus, large clusters can be described as an array of bound atoms intermediate in character between a molecule and a solid.
* Examples: Fe(CO), BH, [[Molybdenum(II) chloride|[MoCl]]], 4Fe-4S