are however well known, e.g. tributyltin hydride (Sn(C4H9)3H). These compound release transient tributyl tin radicals, which are rare examples of compounds of tin(III). Organotin compounds Organotin compounds, sometimes called stannanes, are chemical compounds with tin–carbon bonds. Of the tin compounds, the organic derivatives are commercially the most useful. Some organotin compounds are highly toxic and have been used as biocides. The first organotin compound to be reported was diethyltin diiodide ((C2H5)2SnI2), reported by Edward Frankland in 1849. Most organotin compounds are colorless liquids or solids that are stable to air and water. They adopt tetrahedral geometry. Tetraalkyl- and tetraaryltin compounds can be prepared using Grignard reagents: + 4 RMgBr → + 4 MgBrCl The mixed halide-alkyls, which are more common and more important commercially than the tetraorgano derivatives, are prepared by redistribution reactions: + → 2 R2 Divalent organotin compounds are uncommon, although more common than related divalent organogermanium and organosilicon compounds. The greater stabilization enjoyed by Sn(II) is attributed to the "inert pair effect". Organotin(II) compounds include both stannylenes (formula: R2Sn, as seen for singlet carbenes) and distannylenes (R4Sn2), which are roughly equivalent to alkenes. Both classes exhibit unusual reactions. Occurrence Tin is generated via the long s-process in low-to-medium mass stars (with masses of 0.6 to 10 times that of the Sun), and finally by beta decay of the heavy isotopes of indium. Tin is the 49th most abundant element in Earth's crust, representing 2 ppm compared with 75 ppm for zinc, 50 ppm for copper, and 14 ppm for lead. Tin does not occur as the native element but must be extracted from various ores. Cassiterite () is the only commercially important source of tin, although small quantities of tin are recovered from complex sulfides such as stannite, cylindrite, franckeite, canfieldite, and teallite. Minerals with tin are almost always associated with granite rock, usually at a level of 1% tin oxide content. Because of the higher specific gravity of tin dioxide, about 80% of mined tin is from secondary deposits found downstream from the primary lodes. Tin is often recovered from granules washed downstream in the past and deposited in valleys or the sea. The most economical ways of mining tin are by dredging, hydraulicking, or open pits. Most of the world's tin is produced from placer deposits, which can contain as little as 0.015% tin. About 253,000 tonnes of tin were mined in 2011, mostly in