Iron pentacarbonyl has been found to react with ammonia under high pressure of carbon monoxide at temperatures higher than 150°C, to result in a stable powder usually having the practical composition Fe2(CN)4(CO)(NH3)4. In a particular case of the reaction the composition tends to increase in (CO) but decrease in (NH3), till it attains the composition Fe2(CN)4(CO)2(NH3)3. The crude powder is insoluble in most of solvents, and can not therefore be further purified by recrystallisation, while a potassium salt of the complex anion has been crytallized from its caustic potash solution, in yellow fine prismatic crystals, which have appeared to have the composition K2(Fe(CN)4(CO)(NH3))·1.5H2O. considering this composition in conjunction with that of the crude powder, the powder seems to consist of a complex anion (Fe(CN)4(CO)(NH3))2− and a complex cation (Fe(NH3)3)2+. Besides this compound having a different chemical composition from those found by J. A. Müller and thereafter by W. Hieber et al., its manufacturing procedure is characterized by the use of iron pentacarbonyl and ammonia as the starting materials. The product obtained by the reaction of iron pentacarbonyl and ammonia under high pressure of carbon monoxide has been found to accelerate markedly the formation of form-amide from carbon monoxide and ammonia. The product comprises powder and brown liquor, and further the liquor consists of formamide, ammonium carbamate, urea and a minute quantity of an iron salt, among which urea seems most effective for the acceleration.