Metallacarboxylic acids mainly arise by the attack of hydroxide on electrophilic metal carbonyl complexes.  An illustrative synthesis is the reaction of a cationic iron carbonyl with a stoichiometric amount of base:
:[(CH)(CO)FeCO]BF + NaOH → [(CH)(CO)FeCOH + NaBF
When applied to simple metal carbonyls, this kind of conversion is sometimes called the Hieber base reaction. Decarboxylation of the resulting anion gives the anionic hydride complex.  This conversion is illustrated by the synthesis of [[Iron tetracarbonyl hydride|[HFe(CO)]]] from iron pentacarbonyl.
:Fe(CO) + NaOH → NaFe(CO)COH
:NaFe(CO)COH → NaHFe(CO) + CO