Many CF-containing metal complexes have been prepared, and some are useful for trifluoromethylation. The most obvious reagent is CFLi, which can be generated by lithium-iodide exchange.  This compound is however unstable even at low temperatures.  It degrades to lithium fluoride and difluorocarbene.  Trifluoromethyl copper(I) reagents are more useful.  These reagents are generated in situ by reaction of CFI with copper powder in polar solvents.   Hg(CF), prepared by decarboxylation of the trifluoroacetate, has proven useful for the trifluoromethylation of other metals, although for low-temperature reactions it may prove useful to transmetallate to bis(trifluoromethyl)cadmium.