A

Iridium

---

MELTING POINT: 2,454°C
BOILING POINT: 3,590°C
DENSITY: 22.6 g/cm³ (at 25°C)
MOST COMMON IONS: Ir³⁺

Iridium was discovered by English chemist Smithson Tenant. It was named after the Greek goddess Iris (a goddess related to the rainbow) because of the variety of color in its compounds. A very rare element (0.0001 ppm of Earth's crust), iridium is found in the naturally occurring alloys osmiridium (approximately 50% iridium) and iridiosmium (approximately 70% iridium) in Alaska and South Africa. It is obtained by igniting ammonium chloroiridate, (NH₄)₃IrCl₆, in hydrogen atmosphere. Its ground state electronic configuration is [Xe]4p¹⁴5d⁷6s².

Iridium is the densest of all elements and its metal is lustrous, silvery, and very hard. The metal has the face-centered cubic crystal structure. Iridium has two stable isotopes: ¹⁹¹Ir (37.3%) and ¹⁹³Ir (62.7%).

IRIDIUM COMPLEXES WITH EXAMPLES
Oxidation States	Examples
(−I)	K[Ir(PF3)4]
(0)	[Ir(NH3)5Br]Br (yellow)
(I)	Ir(CO)Cl(PPh3)2
(II)	Ir(NO)Br3(PPh3)2 (µB = 1.34 BM)
(III)	K3[IrCl6] (olive-green) diamagnetic
(IV)	K2[IrF6] (red)
(V)	K[IrF6]

Iridium is extremely inert to acids, but reacts slowly with oxygen and halogens at high temperatures. It forms the fluoride compounds IrF₆, (IrF₅)₄, and IrF₄. There is some doubt of the existence of other halides with the composition IrX₄. The most stable halides are the trihalides. Compounds that have the composition [IrX₆]²⁻ are strongly colored.

Ir₂O₃ · nH₂O (a brown solid) is obtained via the addition of alkali to IrCl₆²⁻ in the presence of H₂O₂. This hydrated oxide is oxidized in air to IrO₂ · nH₂O. IrO₂ (black), with rutile structure, is the product of the reaction of Ir with O₂ at approximately 1,100°C (2,012°F); it dissociates at higher temperatures (>1,100°C, or >2,012°F).

Iridium may form complexes in its several oxidation states. Table 1 contains some examples of these complexes.

Iridium oxide, IrO₂, is used in the fabrication of thin films for stable electrochromic materials and as an electrode material. Iridium metal is used in the manufacture of fountain pen points, airplane spark plugs, and hypodermic needles.

SEE ALSO INORGANIC CHEMISTRY.

Lea B. Zinner

Bibliography

Allred, A. L. (1961). Journal of Inorganic Nuclear Chemistry 17:215.

Cotton, F. Albert, and Wilkinson, Geoffrey (1972). Advanced Inorganic Chemistry: A Comprehensive Text, 3rd edition. New York: Interscience.

Greenwood, Norman N., and Earnshaw, A. (1984). Chemistry of the Elements. Oxford, U.K.: Pergamon.

Livingstone, Stanley E. (1973). "The Platinum Metals." In Comprehensive Inorganic Chemistry, Vol. 3, edited by J. C. Bailar Jr., et al. Oxford, U.K.: Pergamon.