Synthesis, structure, and magnetic properties of fluoride-bridged copper(II) dimers. Crystal and molecular structures of bis(.mu.-fluoro)bis[tris(3,4,5-trimethylpyrazole-N2)copper(II)] bis(tetrafluoroborate) and bis(.mu.-fluoro)bis[(5-methylpyrazole-N2)bis(3,5-dimethylpyrazole-N2)copper(II)] bis(tetrafluoroborate)