The reactions of phosgene, a World War I-style chemical weapon, with two transition metals (TMs) are examined by complete active space self consistent field and multireference single and double excitation configuration interaction theory. We clarify the reason why TM ions are effective at decomposing phosgene yet do not react catalytically. In particular, we find that phosgene reacts much more exothermically with Zn2+. an ion present in protective mask filters, than with the isoelectronic neutral TM case of Ni. Zn ions react by three barrierless, extremely exothermic routes, with oxidative cleavage of C-Cl bonds as the most favored. By contrast, we find the isoelectronic Ni atoms undergo only C-Cl bond insertion. The predicted reaction energetics can be nicely rationalized within a valence bond view of oxidation states and organometallic bonding. These energetics then are used to give a chemical reason why saturation of the charcoal filters occurs.