alkynes yield the corresponding acid or ester, internal alkynes yield diketones, and alkenes give epoxides. MTO also catalyses the conversion of aldehydes and diazoalkanes into an alkene. Nonahydridorhenate A distinctive derivative of rhenium is nonahydridorhenate, originally thought to be the rhenide anion, Re−, but actually containing the anion in which the oxidation state of rhenium is +7. Occurrence Rhenium is one of the rarest elements in Earth's crust with an average concentration of 1 ppb; other sources quote the number of 0.5 ppb making it the 77th most abundant element in Earth's crust. Rhenium is probably not found free in nature (its possible natural occurrence is uncertain), but occurs in amounts up to 0.2% in the mineral molybdenite (which is primarily molybdenum disulfide), the major commercial source, although single molybdenite samples with up to 1.88% have been found. Chile has the world's largest rhenium reserves, part of the copper ore deposits, and was the leading producer as of 2005. It was only recently that the first rhenium mineral was found and described (in 1994), a rhenium sulfide mineral (ReS2) condensing from a fumarole on Kudriavy volcano, Iturup island, in the Kuril Islands. Kudriavy discharges up to 20–60 kg rhenium per year mostly in the form of rhenium disulfide. Named rheniite, this rare mineral commands high prices among collectors. Production Approximately 80% of rhenium is extracted from porphyry molybdenum deposits. Some ores contain 0.001% to 0.2% rhenium. Roasting the ore volatilizes rhenium oxides. Rhenium(VII) oxide and perrhenic acid readily dissolve in water; they are leached from flue dusts and gasses and extracted by precipitating with potassium or ammonium chloride as the perrhenate salts, and purified by recrystallization. Total world production is between 40 and 50 tons/year; the main producers are in Chile, the United States, Peru, and Poland. Recycling of used Pt-Re catalyst and special alloys allow the recovery of another 10 tons per year. Prices for the metal rose rapidly in early 2008, from $1000–$2000 per kg in 2003–2006 to over $10,000 in February 2008. The metal form is prepared by reducing ammonium perrhenate with hydrogen at high temperatures: 2 NH4ReO4 + 7 H2 → 2 Re + 8 H2O + 2 NH3 There are technologies for the associated extraction of rhenium from productive solutions of underground leaching of uranium ores. Applications Rhenium is added to high-temperature superalloys that are used to make jet engine parts, using 70% of the