TABLE OF CONTENTS The closest similarity between organic and inorganic cyclic structures is displayed by the covalent rings. These include both homo- and heterocycles. Homocycles contain identical atoms; in heterocycles the constituent atoms are different. Basically, covalent inorganic rings are formed by non-metals and Main Group metals, but transition metals are not excluded. To participate in covalent ring formation an atom should be able to form at least two covalent bonds, i.e. to be at least a divalent element. This condition is satisfied by numerous elements of the Periodic Table and therefore a great variety of compositions can be expected.
In the organic rings (homo- and heterocycles) the carbon atoms are constantly tetravalent and display sp 3 (tetrahedral), sp 2 (trigonal planar) or sp (linear) hybridizations with the corresponding bonding geometries. The most common heteroatoms are oxygen, sulfur and nitrogen. Inorganic chemistry operates with all elements of the Periodic Table and consequently it can operate with a broader range of possibilities in terms of hybridizations of atomic orbitals, coordination geometries and also oxidation states. Thus, a much broader variety of structures can be expected for inorganic cyclic structures.
The inorganic rings formed through covalent bonds are the real counterparts of organic cyclic molecules and illustrate the ability of other elements to imitate carbon. Although characteristic to elements close to carbon in the Periodic Table (in particular boron, silicon, germanium, phosphorus, arsenic, antimony, sulfur and selenium, alone or in association with...
