Sodium tetracarbonylcobaltate is the organocobalt compound with the formula NaCo(CO)<sub>4</sub>. It is a common derivative of the tetracarbonylcobaltate anion, [Co(CO)<sub>4</sub>]<sup>-</sup>, although several other alkali metal salts are known. The tetracarbonylcobaltate anion is the conjugate base of cobalt tetracarbonyl hydride, HCo(CO)<sub>4</sub>. According to X-ray crystallography, the Co(CO)<sub>4</sub><sup>-</sup> center is nearly tetrahedral (isoelectronic with Ni(CO)<sub>4</sub>).
Conventionally, this compound is prepared by reduction of dicobalt octacarbonyl with sodium amalgam:
It reacts with mercuric cyanide to give a HgCo<sub>2</sub> derivative: