Interaction of benzo-1,3-thiazoline-2-thione and related ligands with copper( II ) salts and the single-crystal X-ray structure of µ-[1,2-bis-(diphenylphosphino)ethane]-bis{(benzo-1,3-thiazole-2-thiolato-S)[1,2-bis(diphenylphosphinoethane]copper( I )}