Iridium compounds

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Oxidation states[a]
−3 [Ir(CO)
3]3−
−1 [Ir(CO)3(PPh3)]1−
0 Ir4(CO)12
+1 [IrCl(CO)(PPh3)2]
+2 Ir(C5H5)2
+3 IrCl3
+4 IrO2
+5 Ir4F20
+6 IrF
6
+7 [Ir(O2)O2]+
+8 IrO4
+9 [IrO4]+[1]
See also: Category:Iridium compounds

Iridium compounds are compounds containing the element

iridium(VIII) oxide (IrO4) was generated under matrix isolation conditions at 6 K in argon.[4] The highest oxidation state (+9), which is also the highest recorded for any element, is found in gaseous [IrO4]+.[1]

Oxides

Only one

rutile structure, featuring six coordinate iridium and three coordinate oxygen.[5] It adopts the fluorite structure.[2] A sesquioxide, Ir
2O
3, has been described as a blue-black powder, which is oxidized to IrO
2 by HNO
3.[6] The corresponding disulfides, diselenides, sesquisulfides, and sesquiselenides are known, as well as IrS
3.[2]

Another oxide, iridium tetroxide, is also known, with iridium in the +8 oxiation state.[7] This compound was formed by photochemical rearrangement of [(η1-O2)IrO2] in solid argon at a temperature of 6 K (−267.15 °C; −448.87 °F). At higher temperatures, the oxide is unstable.[8] The detection of the iridium tetroxide cation IrO+
4 by infrared photodissociation spectroscopy with formal oxidation state +9 has been reported, the highest currently known of any element, though the +10 oxidation state has been theorized for platinum, but not confirmed.[9][10]

Halides

Binary trihalides, IrX
3 are known for all of the halogens.

tetrafluoride, pentafluoride and hexafluoride are known.[2] Iridium hexafluoride, IrF
6, is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to IrF
4,.[2] Iridium pentafluoride is also a strong oxidant, but it is a tetramer, Ir
4F
20, formed by four corner-sharing octahedra.[2]

Complexes

iridium trichloride
, a common salt of iridium.

The coordination complexes of iridium are extensive.

Iridium in its complexes is always

low-spin. Ir(III) and Ir(IV) generally form octahedral complexes.[2] Polyhydride complexes are known for the +5 and +3 oxidation states.[11] One example is IrH5(PiPr3)2.[12] The ternary hydride Mg
6Ir
2H
11 is believed to contain both the IrH4−
5 and the 18-electron IrH5−
4 anion.[13]

Iridium also oxyanions with oxidation states +4 and +5. K
2IrO
3 and KIrO
3 can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures. Such solids are not soluble in conventional solvents.[14]

As for many elements, the chlorides are key complexes. Hexachloroiridic(IV) acid, H
2IrCl
6, and its ammonium salt are the most common iridium compounds from an industrial and preparative perspectives.[15] They are intermediates in the purification of iridium and used as precursors for most other iridium compounds, as well as in the preparation of anode coatings. The IrCl2−
6 ion has an intense dark brown color, and can be readily reduced to the lighter-colored IrCl3−
6 and vice versa.[15] Iridium trichloride, IrCl
3, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C,[15] or in hydrated form by dissolving Ir
2O
3 in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds.[2] Another compound used as a starting material is ammonium hexachloroiridate(III), (NH
4)
3IrCl
6.

In the presence of air, iridium metal dissolves in molten alkali-metal cyanides to produce the Ir(CN)3−
6 (hexacyanoiridate) ion.

Oxyanions

α-Li2IrO3 (scale bar 0.3 mm)[16] (left) β-Li2IrO3 (scale bar 0.2 mm)[16] (right)
See also: Lithium iridate

Iridium forms oxyanions in the +4 oxidation state. It forms compounds such as

antiferromagnetic upon cooling to 15 K.[16] Lithium iridate is a potential electrode material for the lithium-ion battery.[17] This application is hindered by the high costs of Ir, as compared to the cheaper Li2MnO3 alternative.[18]

Organoiridium chemistry

Cyclooctadiene iridium chloride dimer is a common complex of Ir(I).
Main article: Organoiridium chemistry

Organoiridium compounds contain iridium–

ammonium hexachloroiridate
. These salts are reduced upon treatment with CO, hydrogen, and alkenes. Illustrative is the carbonylation of the trichloride:

IrCl3(H2O)x + 3 CO → [Ir(CO)2Cl2]− + CO2 + 2 H+ + Cl− + (x-1) H2O
Skeletal formula presentation of a chemical transformation. The initial compounds have a C5H5 ring on their top and an iridium atom in the center, which is bonded to two hydrogen atoms and a P-PH3 group or to two C-O groups. Reaction with alkane under UV light alters those groups.
Oxidative addition to hydrocarbons in organoiridium chemistry[21][22]

Many organoiridium(III) compounds are generated from pentamethylcyclopentadienyl iridium dichloride dimer. Many of derivatives feature kinetically inert cyclometalated ligands.[23] Related half-sandwich complexes were central in the development of C-H activation.[24][25]

Iridium complexes played a pivotal role in the development of carbon–hydrogen bond activation (C–H activation), which promises to allow functionalization of hydrocarbons, which are traditionally regarded as unreactive.[26]

See also

Notes

  1. ^ Most common oxidation states of iridium are in bold. The right column lists one representative compound for each oxidation state.

References

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