In solids with three-dimensional framework structures, connecting and linking elements are combined by means of directed bonds. Metal ions or metal clusters with coordination numbers > 2 are possible as connectors, in organic framework structures also organic building blocks with a defined spatial arrangement of functional groups. Linkers can be simple inorganic moieties (oxygen in zeolites or aluminophosphates, CN in cyanometallates), but also organic groups of different size and functionality (in zeolitic imidazolates, metal-organic and organic framework structures). The diversity of possible network structures is considerably increased, if both the connectors and linkers are capable of forming more than two linkages. Not only the network topologies are characteristic of framework structures, but also the channel systems resulting from the interconnection of open cavities.
