Iridium is a chemical element; it has the symbol Ir and atomic number 77. This very hard, brittle, silvery-white transition metal of the platinum group is considered the second-densest naturally occurring metal (after osmium), with a density of as defined by experimental X-ray crystallography. 191Ir and 193Ir are the only two naturally occurring isotopes of iridium, as well as the only stable isotopes; the latter is the more abundant. It is one of the most corrosion-resistant metals, even at temperatures as high as .
Iridium was discovered in 1803 in the acid-insoluble residues of platinum ores by the British chemist Smithson Tennant. The name iridium, derived from the Greek word iris (rainbow), refers to the various colors of its compounds. Iridium is one of the rarest elements in Earth's crust, with an estimated annual production of only in 2023.
The dominant uses of iridium and its alloys are in high-performance spark plugs, crucibles for recrystallization of semiconductors at high temperatures, and electrodes for the production of chlorine in the chloralkali process. Important compounds of iridium are chlorides and iodides in industrial catalysis. Iridium is a component of some OLEDs.
Iridium is found in meteorites in much higher abundance than in the Earth's crust. For this reason, the unusually high abundance of iridium in the clay layer at the Cretaceous–Paleogene boundary gave rise to the Alvarez hypothesis that the impact of a massive extraterrestrial object caused the extinction of non-avian dinosaurs and many other species 66 million years ago, now known to be produced by the impact that formed the Chicxulub crater. Similarly, an iridium anomaly in core samples from the Pacific Ocean suggested the Eltanin impact of about 2.5 million years ago.
Characteristics Physical properties
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 . It has the 10th highest boiling point among all elements and becomes a superconductor at temperatures below .
Iridium's modulus of elasticity is the second-highest among the metals, surpassed only 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 for iridium and for osmium.
Iridium is extremely brittle, to the point of being hard to weld because the heat-affected zone cracks, but it can be made more ductile by addition of small quantities of titanium and zirconium (0.2 % of each apparently works well).
The Vickers hardness of pure platinum is 56 HV, whereas an alloy of 50 % platinum and iridium can reach over 500 HV.
Chemical properties Iridium is the most corrosion-resistant metal known. It is not attacked by acids, including aqua regia, but it can be dissolved in concentrated hydrochloric acid in the presence of sodium perchlorate. In the presence of oxygen, it reacts with cyanide salts. Traditional oxidants also react, including the halogens and oxygen at higher temperatures. Iridium also reacts directly with sulfur at atmospheric pressure to yield iridium disulfide.
Isotopes
Iridium has two naturally occurring stable isotopes, 191Ir and 193Ir, with natural abundances of 37.3 % and 62.7 %, respectively. There are also 40 known radioisotopes, ranging in mass number from 164 to 205. 192Ir, which falls between the two stable isotopes, is the most stable radioisotope, has a half-life of 73.82 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. Synthetic isotopes heavier than 193 decay by β− decay, and 192Ir has both, but β+ decay (95.24 %) dominates over electron capture (4.76 %).
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 ground-state radioisotopes.
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.
Chemistry
Oxidation states −3 −2 −1 0 +1 +2 +3 +4 +5 +6 +7 +8 +9
Oxidation states Iridium forms compounds in oxidation states between −3 and +9, but the most common oxidation states are +1, +2, +3, and +4. Well-characterized compounds containing iridium in the +6 oxidation state include and the oxides and . Iridium(VIII) oxide () was generated under matrix isolation conditions at 6 K in argon. The highest oxidation state (+9), which is also the highest recorded for any element, is found in gaseous .
Binary compounds Iridium forms trihydride at very high pressures. Only one binary oxide is well-characterized: iridium dioxide, . It is a blue-black solid that adopts the fluorite structure. A sesquioxide, , has been described as a blue-black powder, which is oxidized to by . Other iridium chalcogenides include the sulfides , and , and the selenides and .
Binary trihalides, , are known for all of the halogens. For oxidation states +4 and above, only the tetrafluoride, pentafluoride and hexafluoride are known. Iridium hexafluoride, , is a volatile yellow solid, composed of octahedral molecules. It decomposes in water and is reduced to . Iridium pentafluoride is also a strong oxidant, but it is a tetramer, , formed by four corner-sharing octahedra.
Complexes
Iridium has extensive coordination chemistry.
Iridium in its complexes is always low-spin. Ir(III) and Ir(IV) generally form octahedral complexes. Polyhydride complexes are known for the +5 and +3 oxidation states. One example is (iPr = isopropyl). The ternary hydride is believed to contain both the and the 18-electron anion.
Iridium also forms oxyanions with oxidation states +4 and +5. and 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.
Just like many elements, iridium forms important chloride complexes. Hexachloroiridic (IV) acid, , and its ammonium salt are common iridium compounds from both industrial and preparative perspectives. 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 ion has an intense dark brown color, and can be readily reduced to the lighter-colored and vice versa. Iridium trichloride, , which can be obtained in anhydrous form from direct oxidation of iridium powder by chlorine at 650 °C, or in hydrated form by dissolving 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 potassium hexachloroiridate(III), .
Organoiridium chemistry
Organoiridium compounds contain iridium–carbon bonds. Early studies identified the very stable tetrairidium dodecacarbonyl, . In this compound, each of the iridium atoms is bonded to the other three, forming a tetrahedral cluster. The discovery of Vaska's complex () opened the door for oxidative addition reactions, a process fundamental to useful reactions. For example, Crabtree's catalyst, a homogeneous catalyst for hydrogenation reactions.
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.
History Platinum group
The discovery of iridium is intertwined with that of platinum and the other metals of the platinum group. The first European reference to platinum appears in 1557 in the writings of the Italian humanist Julius Caesar Scaliger as a description of an unknown noble metal found between Darién and Mexico, "which no fire nor any Spanish artifice has yet been able to liquefy". From their first encounters with platinum, the Spanish generally saw the metal as a kind of impurity in gold, and it was treated as such. It was often simply thrown away, and there was an official decree forbidding the adulteration of gold with platinum impurities.
In 1735, Antonio de Ulloa and Jorge Juan y Santacilia saw Native Americans mining platinum while the Spaniards were travelling through Colombia and Peru for eight years. Ulloa and Juan found mines with the whitish metal nuggets and took them home to Spain. Ulloa returned to Spain and established the first mineralogy lab in Spain and was the first to systematically study platinum, which was in 1748. His historical account of the expedition included a description of platinum as being neither separable nor calcinable. Ulloa also anticipated the discovery of platinum mines. After publishing the report in 1748, Ulloa did not continue to investigate the new metal. In 1758, he was sent to superintend mercury mining operations in Huancavelica.
In 1741, Charles Wood, a British metallurgist, found various samples of Colombian platinum in Jamaica, which he sent to William Brownrigg for further investigation.
In 1750, after studying the platinum sent to him by Wood, Brownrigg presented a detailed account of the metal to the Royal Society, stating that he had seen no mention of it in any previous accounts of known minerals. Brownrigg also made note of platinum's extremely high melting point and refractory metal-like behaviour toward borax. Other chemists across Europe soon began studying platinum, including Andreas Sigismund Marggraf, Torbern Bergman, Jöns Jakob Berzelius, William Lewis, and Pierre Macquer. In 1752, Henrik Scheffer published a detailed scientific description of the metal, which he referred to as "white gold", including an account of how he succeeded in fusing platinum ore with the aid of arsenic. Scheffer described platinum as being less pliable than gold, but with similar resistance to corrosion.
Discovery Chemists who studied platinum dissolved it in aqua regia (a mixture of hydrochloric and nitric acids) to create soluble salts. They always observed a small amount of a dark, insoluble residue. Joseph Louis Proust thought that the residue was graphite. The French chemists Victor Collet-Descotils, Antoine François, comte de Fourcroy, and Louis Nicolas Vauquelin also observed the black residue in 1803, but did not obtain enough for further experiments.