Diphosgene can be used in the synthesis of isocyanids.
Diphosgene vapour is a lung irritant and has an LC100 for rabbits at a concentration of 0.9 mg/l of air.
Diphosgene was used as a poisonous gas in artillery shells by Germany during World War I. The first recorded battlefield use was in May of 1916. Diphosgene was developed because the vapours could destroy the filters in gas masks in use at the time.
The chemical structure is:
O Cl / \\ / O = C C - Cl | | Cl Cl