A member of the platinum group metals, iridium is white, resembling platinum, but with a slight yellowish cast. Because of its hardness, brittleness, and very high melting point, solid iridium is difficult to machine, form, or work; thus powder metallurgy is commonly employed instead. It is the only metal to maintain good mechanical properties in air at temperatures above 1,600 °C (2,910 °F). It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below 0.14 K.
Iridium's modulus of elasticity is the second-highest among the metals, only being surpassed by osmium. This, together with a high shear modulus and a very low figure for Poisson's ratio (the relationship of longitudinal to lateral strain), indicate the high degree of stiffness and resistance to deformation that have rendered its fabrication into useful components a matter of great difficulty. Despite these limitations and iridium's high cost, a number of applications have developed where mechanical strength is an essential factor in some of the extremely severe conditions encountered in modern technology.
The measured density of iridium is only slightly lower (by about 0.12%) than that of osmium, the densest metal known. Some ambiguity occurred regarding which of the two elements was denser, due to the small size of the difference in density and difficulties in measuring it accurately, but, with increased accuracy in factors used for calculating density X-ray crystallographic data yielded densities of 22.56 g/cm3 for iridium and 22.59 g/cm3 for osmium.
Iridium is the most corrosion-resistant metal known: it is not attacked by almost any acid, aqua regia, molten metals, or silicates at high temperatures. It can, however, be attacked by some molten salts, such as sodium cyanide and potassium cyanide, as well as oxygen and the halogens (particularly fluorine) at higher temperatures. Iridium also reacts directly with sulfur at atmospheric pressure to yield iridium disulfide.
|Oxidation states[note 1]
Iridium forms compounds in oxidation states between −3 and +9; the most common oxidation states are +3 and +4. Well-characterized examples of the high +6 oxidation state are rare, but include 6 and two mixed oxides Sr
6 and Sr
6. In addition, it was reported in 2009 that iridium(VIII) oxide (IrO
4) was prepared under matrix isolation conditions (6 K in Ar) by UV irradiation of an iridium-peroxo complex. This species, however, is not expected to be stable as a bulk solid at higher temperatures. The highest oxidation state (+9), which is also the highest recorded for any element, is only known in one cation, IrO+
4; it is only known as gas-phase species and is not known to form any salts.
Iridium dioxide, IrO
2, a blue black solid, is the only well-characterized oxide of iridium. A sesquioxide, Ir
3, has been described as a blue-black powder which is oxidized to IrO
2 by HNO
3. The corresponding disulfides, diselenides, sesquisulfides, and sesquiselenides are known, and IrS
3 has also been reported. Iridium also forms iridates with oxidation states +4 and +5, such as K
3 and KIrO
3, which can be prepared from the reaction of potassium oxide or potassium superoxide with iridium at high temperatures.
Although no binary hydrides of iridium, Ir
y are known, complexes are known that contain IrH4−
5 and IrH3−
6, where iridium has the +1 and +3 oxidation states, respectively. The ternary hydride Mg
11 is believed to contain both the IrH4−
5 and the 18-electron IrH5−
No monohalides or dihalides are known, whereas trihalides, IrX
3, are known for all of the halogens. For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known. Iridium hexafluoride, IrF
6, is a volatile and highly reactive yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to 4, a crystalline solid, by iridium black. Iridium pentafluoride has similar properties but it is actually a tetramer, Ir
20, formed by four corner-sharing octahedra. Iridium metal dissolves in molten alkali-metal cyanides to produce the Ir(CN)3+
6 (hexacyanoiridate) ion.
Hexachloroiridic(IV) acid, H
6, and its ammonium salt are the most important iridium compounds from an industrial perspective. They are involved 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. Iridium trichloride, IrCl
3, which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C, or in hydrated form by dissolving Ir
3 in hydrochloric acid, is often used as a starting material for the synthesis of other Ir(III) compounds. Another compound used as a starting material is ammonium hexachloroiridate(III), (NH
6. Iridium(III) complexes are diamagnetic (low-spin) and generally have an octahedral molecular geometry.
Organoiridium compounds contain iridium–carbon bonds where the metal is usually in lower oxidation states. For example, oxidation state zero is found in tetrairidium dodecacarbonyl, Ir
12, which is the most common and stable binary carbonyl of iridium. In this compound, each of the iridium atoms is bonded to the other three, forming a tetrahedral cluster. Some organometallic Ir(I) compounds are notable enough to be named after their discoverers. One is Vaska's complex, IrCl(CO)[P(C
2, which has the unusual property of binding to the dioxygen molecule, O
2. Another one is Crabtree's catalyst, a homogeneous catalyst for hydrogenation reactions. These compounds are both square planar, d8 complexes, with a total of 16 valence electrons, which accounts for their reactivity.
An iridium-based organic LED material has been documented, and found to be much brighter than DPA or PPV, so could be the basis for flexible OLED lighting in the future.
Iridium has two naturally occurring, stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3% and 62.7%, respectively. At least 37 radioisotopes have also been synthesized, ranging in mass number from 164 to 202. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, with a half-life of 73.827 days, and finds application in brachytherapy and in industrial radiography, particularly for nondestructive testing of welds in steel in the oil and gas industries; iridium-192 sources have been involved in a number of radiological accidents. Three other isotopes have half-lives of at least a day—188Ir, 189Ir, and 190Ir. Isotopes with masses below 191 decay by some combination of β+ decay, α decay, and (rare) proton emission, with the exception of 189Ir, which decays by electron capture. Synthetic isotopes heavier than 191 decay by β− decay, although 192Ir also has a minor electron capture decay path. All known isotopes of iridium were discovered between 1934 and 2008, with the most recent discoveries being 200–202Ir.
At least 32 metastable isomers have been characterized, ranging in mass number from 164 to 197. The most stable of these is 192m2Ir, which decays by isomeric transition with a half-life of 241 years, making it more stable than any of iridium's synthetic isotopes in their ground states. The least stable isomer is 190m3Ir with a half-life of only 2 µs. The isotope 191Ir was the first one of any element to be shown to present a Mössbauer effect. This renders it useful for Mössbauer spectroscopy for research in physics, chemistry, biochemistry, metallurgy, and mineralogy.