I'm assuming there is nothing in the halogen compound except the halogen and the metal?
Thallium, polonium, mercury, and lead are the metals I know have been used in poisonings. I can't find a sufficiently heavy halide for any of them, however. Cadmium and lead might be added to the list, although I don't know specifically of intentional poisonings with either of these, only accidental ones. Does this jog anybodies thought processes?