Walter Hieber prepared the first metal carbonyl hydride in 1931 by the so-called Hieber base reaction of metal carbonyls. In this reaction a hydroxide ion reacts with the carbon monoxide ligand of a metal carbonyl such as iron pentacarbonyl in a nucleophilic attack to form a metallacarboxylic acid.  This intermedia releases of carbon dioxide in a second step, giving the iron tetracarbonyl hydride anion. The synthesis of cobalt tetracarbonyl hydride (HCo(CO)) proceeds in the same way.
:Fe(CO)  +  NaOH  →   Na[Fe(CO)COH]
:Na[Fe(CO)COH]   →   Na[HFe(CO)]  +  CO
A further synthetic route is the reaction of the metal carbonyl with hydrogen.  The protonation of metal carbonyl anions, e.g. [Co(CO)], leads also to the formation of metal carbonyl hydrides.