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Hypervalent molecules are a class of compounds that possess a coordination number greater than four for a main group element. These molecules are also known as hypervalent compounds or hypervalence compounds. The term "hypervalent" was introduced in the 1950s to describe this phenomenon. The concept of hypervalence, however, has been discussed in the literature since the early 20th century.

The formation of hypervalent molecules is often made possible by the use of dipolar aprotic solvents and chelating ligands. These molecules typically exhibit a higher number of electron pairs than expected for a given oxidation state of the central atom. This results in an unusual electronegativity distribution around the central atom. In many cases, the additional electron pairs influenced by the central atom are delocalized into multiple orbitals.

Hypervalent compounds are found in various fields of chemistry, including organophosphorus chemistry, organoselenium chemistry, and organosulfur chemistry. They have found applications in the synthesis of pharmaceuticals, agrochemicals, and materials with unique properties. Hypervalent transformations are also used in classical and computational organic synthesis. They can be involved in acid-base, electrophilic aromatic substitution, nucleophilic aromatic substitution reactions, pericyclic reactions, and Friedel Crafts reactions.

Examples of hypervalent compounds include phosphoranes and phosphine oxides, which are phosphorus(V) compounds that exhibit hypervalent character due to the presence of an expanded octet of phosphorus. They also include selenium and sulfur(VI) compounds such as phenyl selenenyl fluoride and phenyl sulfenyl fluoride.