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In crystals composed of larger molecules, or macromolecular crystals, their structure contains large voids (cage-like cavities) that can enclose other smaller molecules (known as guests) within them. The resulting type of unique mixture is called an inclusion complex, and it is also often referred to as a \"cage\" inclusion complex, a \"cage\" adduct, or an encapsulated compound. The term \"complex\" originates from Latin and means \"grid,\" with most being non-stoichiometric. These two substances do not actually form a compound; they are held together only by weak forces such as hydrogen bonds or van der Waals forces. There are many types of inclusion complexes; common ones include gas hydrates, substituted ammonium salt hydrates, Hofmann-type inclusion complexes, phenol inclusion complexes, quinone alcohol inclusion complexes, and urea adducts. It is known that water can form hydrates with a clathrate structure together with certain gases, such as some noble gases, chlorine gas, bromine vapor, propane, and hydrogen sulfide. In these hydrates, water molecules are linked by hydrogen bonds to form tetrahedral and dodecahedral structures that create a cage-like arrangement; the oxygen atoms are connected to four adjacent hydrogen atoms at the corners of a tetrahedron through hydrogen bonds. Substances with a G-type clathrate structure can have the composition M(CN)4·M′(NH3)2·2H2O, where both M and M′ are metals with an oxidation state of II. Known values for M include Ni, Pd, and Pt, while possible values for M′ include Mn, Fe, Co, Ni, Cu, Zn, and Cd. Such as the clathrate with G=C6H6 and M=M′=Ni(Ⅱ). When benzene is added to an ammonium solution of nickel cyanide, pale purple crystals precipitate. The vapor pressure of C6H6 cannot be detected at room temperature, but benzene separates out at around 120°C; this is how the complex Ni(CN)4·Ni(NH3)2·2C6H6 is formed. This inclusion complex is a plate-like macromolecule formed by the combination of planar Ni(C)4 complexes and hexagonal Ni(N)6 complexes, with benzene inserted into the gaps between their layers. Once molecules such as C6H6 and C6H5NH2 enter, larger benzene substitutes cannot enter. Therefore, for any type of inclusion complex, there are certain limitations on the size and shape of the molecules that can be included within it. Taking advantage of this property, inclusion complexes are often used in chemical separation processes such as seawater desalination and petroleum refining. Urea and aliphatic compounds can form adducts of the type CnH2n+2·xCO(NH2)2, which are used in the separation and purification of hydrocarbons. The molecules enclosed within the inclusion complexes are chemically isolated, which provides a useful reagent for the study of isolated single molecules.