Organoselenium compounds (or seleno-organic) are chemical compounds containing carbon-to-selenium chemical bonds. Organoselenium chemistry is the corresponding science exploring their properties and reactivity. Selenium belongs with oxygen and sulfur to the group 16 elements or chalcogens, and similarities in chemistry are to be expected. Organoselenium compounds are found at trace levels in ambient waters, soils and sediments. Selenium can exist with oxidation state −2, +2, +4, +6. Se(II) is the dominant form in organoselenium chemistry. Down the group 16 column, the bond strength becomes increasingly weaker (234 kJ/mol for the C−Se bond and 272 kJ/mol for the C−S bond) and the bond lengths longer (C−Se 198 pm, C−S 181 pm and C−O 141 pm). Selenium compounds are more nucleophilic than the corresponding sulfur compounds and also more acidic. The pKa values of XH2 are 16 for oxygen, 7 for sulfur and 3.8 for selenium. In contrast to sulfoxides, the corresponding selenoxides are unstable in the presence of β-protons and this property is utilized in many organic reactions of selenium, notably in selenoxide oxidations and in selenoxide eliminations. The first organoselenium compound to be isolated was diethyl selenide in 1836. Structural classification of organoselenium compounds Selenols (RSeH) are the selenium equivalents of alcohols and thiols. These compounds are relatively unstable and generally have an unpleasant smell. Benzeneselenol (also called selenaphenol or PhSeH) is more acidic (pKa 5.9) than thiophenol (pKa 6.5) and also oxidizes more readily to the diselenide. Selenaphenol is prepared by reduction of diphenyldiselenide. Diselenides (R−Se−Se−R) are the selenium equivalents of peroxides and disulfides. They are useful shelf-stable precursors to more reactive organoselenium reagents such as selenols and selanyl halides. Best known in organic chemistry is diphenyldiselenide, prepared from phenylmagnesium bromide and selenium followed by oxidation of the product PhSeMgBr. Selanyl halides (R−Se−Cl, R−Se−Br) are prepared by halogenation of diselenides. Bromination of diphenyldiselenide gives phenylselanyl bromide (PhSeBr). These compounds are sources of "PhSe+". Selenides (R−Se−R), also called selenoethers, are the selenium equivalents of ethers and sulfides. One example is dimethylselenide ((CH3)2Se). These are the most prevalent organoselenium compounds. Symmetrical selenides are usually prepared by alkylation of alkali metal selenide salts, e.g. sodium selenide. Unsymmetrical selenides are prepared by alkylation of selenoates. These compounds typically react as nucleophiles, e.g. with alkyl halides (R'−X) to give selenonium salts R'RRSe+X−. Divalent selenium can also interact with soft heteroatoms to form hypervalent selenium centers. They also react in some circumstances as electrophiles, e.g. with