The small organic molecule guanidine CN3H5 can be anionized via a facile reaction with alkali-metal hydrides or amides with the formation of metal guanidinates (MCN3H4) and their guanidine adducts. The crystal structures and thermal decomposition properties of these organic-molecule-based complex hydrides were carefully investigated. Through metallation, MCN3H4 can completely preserve carbon atoms in the system and exhibit a largely improved thermal decomposition compared to CN3H5 regarding the extent of C-N bond breaking. By pairing H+ in CN3H5 or MCN3H4 with H-from metal hydrides, the resulting composite can further reduce ammonia libration and promote an endothermic dehydrogenation. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications LLC.