METAL:IRON should work just as well as INORGANIC:IRON - the game considers METAL an acceptable substitute when specifying material.
Did you make these changes to the raws before making a new world? Adding reactions requires a world regen.
Correct me if I'm wrong, but I think that's a leftover from an early version when metals and minerals were different objects. When the two were merged into "inorganics," STONE and METAL were simply re-pointed to the same place as INORGANIC, so they're literally the same thing. That's why dwarves in strange moods ask for "rock bars" - the pointer for METAL:NONE was moved to the newer "inorganic" object class and "rock" is the default name for inorganic materials, while "metal" was the default name for the old "metal" objects.
I'm also guessing that's why Toady hasn't bothered to fix it - for such a minor issue, it's a hell of a lot more complicated than just changing a string somewhere. If he changed "rock" to "metal," you'd get dwarves asking for "metal blocks," and if he changed it to "inorganic," both would sound awkward. In order to fix it, he'd probably need to make the mood look for the IS_METAL token within inorganic materials for bars, and it really just isn't important enough to warrant the time needed to do that when there are so many other things to do.