Iodine oxide
Class of chemical compounds
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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.[1]
Molecular compounds
| Molecular formula | I2O | IO[3] | IO2 | I2O4 | I4O9 | I2O5 | I2O6 | I2O7 |
|---|---|---|---|---|---|---|---|---|
| Name | Diiodine oxide | Iodine monoxide | Iodine dioxide | Diiodine tetroxide (Iodine tetroxide) | Tetraiodine nonoxide | Iodine pentoxide (Diiodine pentoxide) | Diiodine hexaoxide | Diiodine heptoxide |
| Structure | I2O | IO | IO2 | O2IOIO | I(OIO2)3 | O(IO2)2 | (I4O12)n[4][5][6] | polymeric[4] |
| Molecular model |  |  |  |  | ||||
| CAS registry | 39319-71-6 | 14696-98-1 | 13494-92-3 | 1024652-24-1 | 66523-94-2 | 12029-98-0 | 65355-99-9 | |
| Appearance | red solutions in CH2Cl2[7] | dilute gas; condenses to I2O4[8] | yellow solid | dark yellow solid | white crystalline solid | yellow solid | unstable yellow solid | |
| Oxidation state | +1 | +2 | +4 | +3 and +5 | +3 and +5 | +5 | +5 and +7 | +7 |
| Melting point | not isolable | not isolable | not isolable | decomp. 100 °C | decomp. 75 °C[9] | decomp. 300–350 °C | decomp. 179–197 °C[4] | decomp. <60 °C[4] |
| Solubility in water | decomp. to HIO3 + I2 | decomp. to HIO3 + I2 | 187 g/100 mL (hydrolyzes into HIO3) |
Diiodine monoxide has largely been the subject of theoretical study,[10] but there is some evidence that it may be prepared in a similar manner to dichlorine monoxide, via a reaction between HgO and I2.[11] The compound appears to be highly unstable but can react with alkenes to give halogenated products.[12]
As of 2019, diiodine monoxide has finally been synthesized by comproportionation of suspensions of HIO3 and elemental iodine in concentrated sulfuric acid. The compound was extracted using dichloromethane and its presence was confirmed by spectroscopy.[7]
Radical iodine oxide (IO) and iodine dioxide (IO2), collectively referred to as IOx, and also iodine tetroxide (I2O4) 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.[8][13] Despite the small quantities produced (typically below ppt) they are thought to be powerful ozone depletion agents.[14][15]
Diiodine pentoxide (I2O5) is the anhydride of iodic acid and the only stable anhydride of all the halogen oxoacids. Unlike other halogen oxides, it can be synthesized by thermal dehydration of iodic acid and is highly resilient to thermal decomposition. Iodine pentoxide adopts multiple polymorphs that can be synthesized by varying pressures.[6]
Tetraiodine nonoxide (I4O9) has been prepared by the gas-phase reaction of I2 with O3 but has not been extensively studied.[16]
Diiodine hexaoxide (I2O6), 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.[4] It can be synthesized in hot concentrated sulfuric acid, starting either with pure H5IO6 or a mix of H5IO6 and HIO3[5][4] It adopts a polymeric structure consisting of IV and IVII centers.
Diiodine heptoxide (I2O7) has been reported as a yellow solid that slowly decomposes into oxygen at room temperature with rapid decomposition beginning at 60 °C, converting it into I2O6.[4]
Iodate anions
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 [I2O9]4-.[17]
The conjugate acids of the iodine oxides are:
| Iodine oxidation state | +1 | +3 | +5 | +7 |
|---|---|---|---|---|
| Name | Hypoiodous acid | Iodous acid | Iodic acid | Periodic acid |
| Formula | HIO | HIO2 | HIO3 | HIO4 or H5IO6 |
The periodates include two main variants: metaperiodate IO−
4 and orthoperiodate IO5−
6.