Iodanes are chemical compounds containing hypervalent iodine. These iodine compounds are hypervalent because the iodine atom in it contains more than the 8 electrons in the valence shell required for the octet rule. When iodine is complexed with a monodentate electronegative ligand such as chlorine, iodine compounds occur with a +3 oxidation number as iodine(III) or λ3-iodanes or as a +5 oxidation number as iodine(V) or λ5-iodanes. Iodine itself contains 7 valence electrons and in a λ3-iodane three more are donated by the ligands making it a decet structure. λ5-iodanes are dodecet molecules. In an ordinary iodine compound such as iodobenzene the number of valence electrons is eight as expected. In order to get from iodine to a hypervalent iodine compound it gets oxidized with removal of first 3 electrons and then 5 electrons. The ligands in turn contribute electrons pairs and form coordinate covalent bonds by adding a total of 6 or 10 electrons back to iodine. In the L-I-N notation L stands for the number of electrons donated by ligands and N the number of ligands.
The concept of hypervalent iodine was developed by J.J. Musher in 1969. In order to accommodate the excess of electrons in hypervalent compounds the 3-center-4-electron bond was introduced in analogy with the 3-center-2-electron bond observed in electron deficient compounds. One such bond exists in iodine(III) compounds and two such bonds reside in iodine(V) compounds.
The first hypervalent iodine compound, (dichloroiodo)benzene () was prepared in 1886 by the German chemist Conrad Willgerodt by passing chlorine gas through iodobenzene in a cooled solution of chloroform.