Manganese is an essential element, and one of moderate toxicity. In its highest oxidation state [Mn(VII)] as permanganate, MnO—, it is a strong oxidizing agent. Its more stable states are Mn(IV), which is encountered in natural systems chiefly as insoluble MnO2, or as compounds of Mn(II). The chemistry of these states resembles that of Fe(II) and Fe(III); Mn(II) salts are soluble in water, but the dioxide is insoluble, although often producing a colloidal or gelatinous and highly absorbent precipitate. Mn(II) is encountered only under reducing conditions in natural waters, as is Fe(II). Sources of manganese are industrial pollution and acid mine drainage. Long-term inhalation of MnO2 produces neurological problems. An organometallic manganese compound, methylcyclopentadienyltricarbonylmanganese, has been used for some years in Canada as an alternative to tetraalkyllead as an antiknock agent in gasoline and is more toxic than inorganic manganese compounds. As mentioned in Section 6.7.4, attempts by Canada to prohibit its use were prevented by legal action on the grounds that the prohibition violated the North American Free Trade Agreement (NAFTA).

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