You would have to make the sword out of the metal.
This is incorrect, syndromes do not work on weapons, or by any other form of contact with a solid material.
Creatures only contract syndromes from inhaling clouds of the material (if the syndrome has the INHALE tag), getting covered in a
liquid material (if the syndrome has the CONTACT tag), or being injected with the material (if the syndrome has the INJECTED tag, usually creatures are injected by another creature's attack, although I have heard that weapons coated with the
liquid material can also inflict injected syndromes, this is difficult to actually do because of the difficulty of coating the weapons in the first place, and I'm not sure that it even works). EDIT: I believe coated weapons
do reliably pass on CONTACT syndromes, however.
In general, it is very difficult to expose creatures to poisons in DF. Workarounds involving magma or fire are possible, but probably more trouble than they're worth. Workarounds using
bolts of the material and vaporization points are also possible, but finicky and prone to blowing up in your face in hot climates.
However, syndromes and materials work quite well when used with creature's natural attacks, so that's probably where you should start.