The previously unknown F5Se-N= C= O is prepared from Xe (OSeF5) 2 and HCN; F5Te-N= C= O can be prepared similarly but also by more straightforward routes. 2 Compared with that of F5S-N= C= O, the isocyanate reactivity is reduced in F, Te-N= C= O and even more so in F5Se-N= C= O.