Hypervalent iodine

Periodinanes 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 (C₆H₅Cl₂I) was prepared in 1886 by the German chemist Conrad Willgerodt by passing chlorine gas through iodobenzene in a cooled solution of chloroform.

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