Iridium is an extremely dense, hard, and corrosion-resistant platinum-group transition metal. It has one of the highest melting points and retains luster at high temperatures.
The electron configuration of iridium is [Xe] 4f14 5d7 6s2. The 5d electrons contribute to its metallic bonding and high chemical stability.
Iridium has a density of about 22.56 g/cm³, nearly equal to that of osmium. This high density results from its large atomic mass and tightly packed atoms in a face-centered cubic (fcc) crystal structure.
Iridium exhibits a wide range of oxidation states, from −3 to +9, though +3 and +4 are the most stable and common. For example, IrCl3 contains Ir3+, while IrO2 contains Ir4+. The +9 state occurs in the compound IrO4, one of the highest oxidation states known in chemistry.
Iridium is one of the most chemically inert elements. It resists attack by acids, alkalis, and most oxidizing agents, even at high temperatures. It only reacts with halogens like chlorine or fluorine at elevated temperatures to form halides such as IrCl3 or IrF6.
Iridium has several specialized applications:
Iridium maintains its mechanical strength and luster at very high temperatures (melting point ~2446 °C). It is used in crucibles for crystal growth and as a heat-resistant material in spacecraft, including satellite thrusters and deep-space probes.
Iridium is extremely rare in Earth’s crust but more abundant in meteorites and the Earth’s mantle. It occurs with other platinum-group metals in ores like osmiridium and iridosmine. The high concentration of iridium in the boundary layer between the Cretaceous and Paleogene periods supports the asteroid impact theory for dinosaur extinction.
Iridium is stable in air at room temperature but can form oxides and halides at high temperatures. For example:
\(\mathrm{Ir(s) + O_2(g) \rightarrow IrO_2(s)}\)
\(\mathrm{2\,Ir(s) + 3\,Cl_2(g) \rightarrow 2\,IrCl_3(s)}\)
IrO2 is a black solid used as a catalyst and electrode material in electrochemical cells.
Iridium-192 (Ir-192) is a radioactive isotope used in brachytherapy (internal radiation therapy) for treating cancer and in industrial radiography to inspect welds and metal structures. It emits gamma rays during its decay process:
\(\mathrm{^{192}_{77}Ir \rightarrow ^{192}_{78}Pt + \beta^- + \gamma}\)
Metallic iridium is non-toxic and inert, but finely divided iridium dust can irritate the respiratory tract. Radioactive isotopes like Ir-192 are hazardous due to radiation exposure and must be handled with appropriate shielding and safety protocols.
Iridium complexes are used as catalysts in hydrogenation reactions. For example, the Wilkinson’s catalyst (RhCl(PPh3)3) has an iridium analog that catalyzes the addition of hydrogen to alkenes:
\(\mathrm{RCH=CH_2 + H_2 \xrightarrow{Ir\text{-}catalyst} RCH_2CH_3}\)