Copper(II) tetrafluoroborate

Copper(II) tetrafluoroborate is any inorganic compound with the formula Cu(H₂O)_x((BF₄)₂. As usually encountered, it is assumed to be the hexahydrate (x = 6), 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 and for cyclopropanation of alkenes with diazo reagents. In both applications, the copper(II) is reduced to a copper(I) catalyst.