Copper(II) tetrafluoroborate
Copper tetrafluoroborate or cupric tetrafluoroborate is any inorganic compound with the formula Cux(BF4)2. As usually encountered, it is assumed to be the hexahydrate, but this salt can be partially dehydrated to the tetrahydrate. Regardless, these compounds are aquo complexes of copper in its +2 oxidation state, with two weakly coordinating tetrafluoroborate anions.
The compound is used in organic synthesis, e.g. as a Lewis acid for Diels Alder reactions, for cyclopropanation of alkenes with diazo reagents, and as a Lewis Acid in Meinwald Rearrangement reactions on Epoxides. In the former two applications, the copper is reduced to a copper catalyst. The compound is also used for copper electroplating in fluoroborate-based plating baths.