Iodine oxides are chemical compounds of oxygen and iodine. Iodine has at least three stable oxides which are isolable in bulk, diiodine tetroxide, iodine pentoxide, and diiodine hexaoxide, but a number of other oxides are formed in trace quantities or have been hypothesized to exist. The most significant of these is iodine pentoxide, which is distinguished by being the most thermodynamically and kinetically stable of all halogen oxides. The chemistry of these compounds is complicated, with only a few having been well characterized. Many have been detected in the atmosphere and are believed to be particularly important in the marine boundary layer.
Diiodine monoxide has largely been the subject of theoretical study, but there is some evidence that it may be prepared in a similar manner to dichlorine monoxide, via a reaction between HgO and I<sub>2</sub>. The compound appears to be highly unstable but can react with alkenes to give halogenated products.
Radical iodine oxide (IO) and iodine dioxide (IO<sub>2</sub>), collectively referred to as IO<sub></sub>, and also iodine tetroxide (I<sub>2</sub>O<sub>4</sub>) all possess significant and interconnected atmospheric chemistry. They are formed, in very small quantities, in the marine boundary layer by the photochemical reaction of ozone with diiodomethane, produced by macroalga such as seaweed, or through the oxidation of molecular iodine, produced by the reaction of gaseous ozone and iodide present at the seasurface. Despite the small quantities produced (typically below ppt) they are thought to be powerful ozone depletion agents.
Diiodine pentoxide (I<sub>2</sub>O<sub>5</sub>) is the anhydride of iodic acid and the only stable anhydride of out of all halogen oxoacids. Unlike other halogen oxides, it can be synthesized by thermal dehydration of iodic acid. Iodine pentoxide adopts multiple polymorphs that can be synthesized by varying pressures.
Tetraiodine nonoxide (I<sub>4</sub>O<sub>9</sub>) has been prepared by the gas-phase reaction of I<sub>2</sub> with O<sub>3</sub> but has not been extensively studied.
Diiodine hexaoxide (I<sub>2</sub>O<sub>6</sub>), also known as iodine trioxide or diiodine(V,VII) oxide, is a hygroscopic yellow solid. It has been isolated in bulk and has been studied as a mix with aluminium for destroying chemical and biological agents. It can be synthesized in hot concentrated sulfuric acid, starting either with pure H<sub>5</sub>IO<sub>6</sub> or a mix of H<sub>5</sub>IO<sub>6</sub> and HIO<sub>3</sub> It adopts a polymeric structure consisting of I<sup>V</sup> and I<sup>VII</sup> centers.
Diiodine heptoxide (I<sub>2</sub>O<sub>7</sub>) has been reported as a yellow solid that slowly decomposes into oxygen at room temperature with rapid decomposition beginning at 60 ðC, converting it into I<sub>2</sub>O<sub>6</sub>.
Iodine oxides also form negatively charged anions, which (associated with complementary cations) are components of acids or salts. These include the iodates and periodates, which can form multinuclear structures such as [I<sub>2</sub>O<sub>9</sub>]<sup>4-</sup>.
The conjugate acids of the iodine oxides are:
The periodates include two main variants: metaperiodate and orthoperiodate .