This chapter discusses highly reduced metal–carbonyl anions. In the unusually effective reducing medium of sodium-hexamethylphosphoramide (HMPA), [Mn(CO)5]– underwent facile reduction to form a golden yellow to yellow brown solution. It is often important to use solutions rather than slurries of trianion salts to minimize the formation of side products during the reactions of these materials with electrophiles. Infrared spectra of Na3[M(CO)4] as solutions in HMPA or as mineral oil mulls of the solid are quite similar in terms of peak positions. Protonation reactions of [M(CO)4]3– and most of the known reactions of Na3[Re(CO)4] are examined. Perhaps the most persuasive chemical evidence for the existence of the monohydrides of manganese and rhenium is their high-yield conversion to corresponding dihydrides, which can be isolated as relatively stable crystalline solids. The tricesium salt was obtained as an orange–brown powder and the corresponding trirubidium compound was isolated as a golden to red–brown shiny microcrystalline substance. The materials are especially prone to violent decomposition when they are even gently scraped off of a fritted glass disk with a metal spatula. One very important and obvious limitation of this reductive labilization method is that the ligand to be expelled should not be susceptible to reduction either in a coordinated or free state.