Wikipedia
Percobaltate
Percobaltates are chemical compounds where the oxidation state of cobalt is +5, which is highest known oxidation state of cobalt. Its general formula is . Impure sodium percobaltate can be produced by the reaction of cobalt(II,III) oxide and sodium oxide; the oxidant is dioxygen:
$$\rm \ 4Co_3O_4 + 18Na_2O + 7O_2 \rightarrow 12Na_3CoO_4$$
The potassium salt can be synthesized similarly. Its magnetic moment has indicated the existence of cobalt(V).