potassium perruthenate, KRuO4, with ruthenium in the +7 oxidation state. Potassium perruthenate can also be produced by oxidising potassium ruthenate, K2RuO4, with chlorine gas. The perruthenate ion is unstable and is reduced by water to form the orange ruthenate. Potassium ruthenate may be synthesized by reacting ruthenium metal with molten potassium hydroxide and potassium nitrate. Some mixed oxides are also known, such as MIIRuIVO3, Na3RuVO4, NaRuO, and MLnRuO. Halides and oxyhalides The highest known ruthenium halide is the hexafluoride, a dark brown solid that melts at 54 °C. It hydrolyzes violently upon contact with water and easily disproportionates to form a mixture of lower ruthenium fluorides, releasing fluorine gas. Ruthenium pentafluoride is a tetrameric dark green solid that is also readily hydrolyzed, melting at 86.5 °C. The yellow ruthenium tetrafluoride is probably also polymeric and can be formed by reducing the pentafluoride with iodine. Among the binary compounds of ruthenium, these high oxidation states are known only in the oxides and fluorides. Ruthenium trichloride is a well-known compound, existing in a black α-form and a dark brown β-form: the trihydrate is red. Of the known trihalides, trifluoride is dark brown and decomposes above 650 °C, tribromide is dark-brown and decomposes above 400 °C, and triiodide is black. Of the dihalides, difluoride is not known, dichloride is brown, dibromide is black, and diiodide is blue. The only known oxyhalide is the pale green ruthenium(VI) oxyfluoride, RuOF4. Coordination and organometallic complexes Ruthenium forms a variety of coordination complexes. Examples are the many pentaammine derivatives [Ru(NH3)5L]n+ that often exist for both Ru(II) and Ru(III). Derivatives of bipyridine and terpyridine are numerous, best known being the luminescent tris(bipyridine)ruthenium(II) chloride. Ruthenium forms a wide range compounds with carbon-ruthenium bonds. Grubbs' catalyst is used for alkene metathesis. Ruthenocene is analogous to ferrocene structurally, but exhibits distinctive redox properties. The colorless liquid ruthenium pentacarbonyl converts in the absence of CO pressure to the dark red solid triruthenium dodecacarbonyl. Ruthenium trichloride reacts with carbon monoxide to give many derivatives including RuHCl(CO)(PPh3)3 and Ru(CO)2(PPh3)3 (Roper's complex). Heating solutions of ruthenium trichloride in alcohols with triphenylphosphine gives tris(triphenylphosphine)ruthenium dichloride (RuCl2(PPh3)3), which converts to the hydride complex chlorohydridotris(triphenylphosphine)ruthenium(II) (RuHCl(PPh3)3). History Though naturally occurring platinum alloys containing all six platinum-group metals were used for a long time by pre-Columbian Americans and known as a material to European chemists from the mid-16th century, not until the mid-18th century was platinum identified as a