Periodate is an anion composed of iodine and oxygen. It is one of a number of oxyanions of iodine and is the highest in the series, with iodine existing in oxidation state +7. Unlike other perhalogenates, such as perchlorate, it can exist in two forms: metaperiodate and orthoperiodate . In this regard it is comparable to the tellurate ion from the adjacent group. It can combine with a number of counter ions to form periodates, which may also be regarded as the salts of periodic acid.
Periodates were discovered by Heinrich Gustav Magnus and C. F. Ammermüller; who first synthesised periodic acid in 1833.
Synthesis
Classically, periodate was most commonly produced in the form of sodium hydrogen periodate (). This is commercially available, but can also be produced by the oxidation of iodates with chlorine and sodium hydroxide. Or, similarly, from iodides by oxidation with bromine and sodium hydroxide:
\overset{ sodium\ iodate}{ NaIO3} + Cl2{ } + 4 NaOH -> Na3H2IO6{ } + 2NaCl{ } + H2O
NaI + 4 Br2 + 10 NaOH -> Na3H2IO6 + 8 NaBr + 4 H2O
Modern industrial scale production involves the electrochemical oxidation of iodates, on a lead dioxide () anode, with the following standard electrode potential:
H5IO6 + H+ + 2e- -> IO3- + 3 H2O
Metaperiodates are typically prepared by the dehydration of sodium hydrogen periodate with nitric acid, or by dehydrating orthoperiodic acid by heating it to 100 °C under vacuum.
Na3H2IO6 + 2 HNO3 -> NaIO4 + 2 NaNO3 + 2 H2O
H5IO6 -> HIO4 + 2 H2O
They can also be generated directly from iodates by treatment with other strong oxidizing agents such as hypochlorites: NaIO3{ } + \overset{ sodium\ hypochlorite}{ NaOCl} -> NaIO4{ } + NaCl
Forms and interconversion
Periodate can exist in a variety of forms in aqueous media, with pH being a controlling factor. Orthoperiodate has a number of acid dissociation constants.
H5IO6 <=> H4IO6- + H+
H4IO6- <=> H3IO6^2- + H+
H3IO6^2- <=> H2IO6^3- + H+
The ortho- and metaperiodate forms also exist in equilibrium.
H4IO6- <=> IO4- + 2 H2O
For this reason orthoperiodate is sometimes referred to as the dihydrate of metaperiodate, written ; however, this description is not strictly accurate as X-ray crystallography of shows 5 equivalent groups.
At extremes of pH additional species can form. Under basic conditions a dehydration reaction can take place to form the diperiodate (sometimes referred to as mesoperiodate).
2H3IO6^2- <=> H2I2O10^4- + 2 H2O
Under strongly acid conditions periodic acid can be protonated to give the orthoperiodonium cation.
H6IO6+ <=> H5IO6 + H+
Structure and bonding
In both the ortho- and metaperiodate the iodine is hypervalent, as it forms more bonds than would classically be allowed. This has been explained in terms of dative bonds, confirming the absence of double bonding in these molecules.
Exact structures vary depending on counter ions, however on average orthoperiodates adopt a slightly deformed octahedral geometry with X-ray diffraction showing I–O bond lengths of 1.89 Å. Metaperiodates adopt a distorted tetrahedral geometry with an average I–O distance of 1.78 Å.
Reactions
Cleavage reactions
Periodates can cleave carbon–carbon bonds on a variety of 1,2-difunctionalised alkanes. The most common example of this is diol cleavage, which was also the first to be discovered (Malaprade reaction). In addition to diols, periodates can cleave 1,2-hydroxy ketones, 1,2-diketones, α-keto acids, α-hydroxy acids, amino acids, 1,2-amino alcohols, and 1,2-diamines, to give aldehydes, ketones, and carboxylic acids. In the presence of strong acid catalyst, like H2SO4 or HNO3 epoxides are also converted into aldehyde or ketones or dicarbonyl compounds.
Alkenes can also be oxidised and cleaved in the Lemieux–Johnson oxidation. This uses a catalytic loading of osmium tetroxide which is regenerated in situ by the periodate. The overall process is equivalent to that of ozonolysis.