Tetrakis(acetonitrile)copper(I) hexafluorophosphate is a salt with the formula [Cu(CH<sub>3</sub>CN)<sub>4</sub>]PF<sub>6</sub>. It is a colourless solid that is used in the synthesis of other copper complexes. The cation [Cu(CH<sub>3</sub>CN)<sub>4</sub>]<sup>+</sup> is a well-known example of a transition metal nitrile complex.
As confirmed by X-ray crystallographic studies, the copper(I) ion is coordinated to four almost linear acetonitrile ligands in a nearly ideal tetrahedral geometry. Similar complexes with other anions including the perchlorate, tetrafluoroborate, and nitrate are known.
The cation was first reported in 1923 with a nitrate anion as a byproduct of the reduction of silver nitrate with a suspension of copper powder in acetonitrile. [Cu(CH<sub>3</sub>CN)<sub>4</sub>]PF<sub>6</sub> is generally produced by the addition of HPF<sub>6</sub> to a suspension of copper(I) oxide in acetonitrile:
The reaction is highly exothermic, and may bring the solution to a boil. Upon crystallization, the resulting microcrystals should be white, though a blue tinge is common, indicating the presence of Cu<sup>2+</sup> impurities.
The acetonitrile ligands protect the Cu<sup>+</sup> ion from oxidation to Cu<sup>2+</sup>, but are rather poorly bound: with other counterions, the complex forms di- and tri-acetonitrilo complexes and is also a useful source of unbound Cu(I).
Water-immiscible organic nitriles have been shown to selectively extract Cu(I) from aqueous chloride solutions. Through this method, copper can be separated from a mixture of other metals. Dilution of acetonitrile solutions with water induces disproportionation: