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Hypervalent compounds are chemical species that contain one or more main group elements (groups 13-18) in oxidation states that exceed their normal valency. These compounds are of significant interest in chemistry due to their unique bonding and reactivity. The term "hypervalent" was first used by N.V. Sidgwick in 1940 to describe compounds where the central atom exceeds the octet rule, which states that atoms tend to form compounds in which their valence shells contain eight electrons.

The most well-known hypervalent compounds are those of phosphorus, sulfur, and silicon. For example, phosphorus pentachloride (PCl5) is a hypervalent compound where phosphorus is in the +5 oxidation state, exceeding its normal valency of +3. Similarly, sulfur hexafluoride (SF6) is a hypervalent compound with sulfur in the +6 oxidation state. These compounds often exhibit unusual geometries and bonding, such as trigonal bipyramidal or octahedral arrangements, which are not typical of main group elements.

Hypervalent compounds are typically prepared by the reaction of the main group element with highly electronegative ligands, such as halogens or oxygen. The bonding in these compounds is often described as dative or coordinate covalent, where the central atom donates electron density to the ligands. This results in a more covalent character compared to traditional ionic compounds.

The study of hypervalent compounds has led to the development of new synthetic methods and has provided insights into the nature of chemical bonding. These compounds have applications in various fields, including catalysis, materials science, and medicine. For instance, hypervalent iodine compounds are used as oxidizing agents in organic synthesis, and hypervalent silicon compounds are used in the preparation of polymers and ceramics.