In Cobalt, why isn't the 4 oxidation state stable?

Cobalt is [math]\mathrm{[Ar]4s^23d^7}[/math]. Now if it is [math]\mathrm{Co^{3+}}[/math], then the two outer 4s electrons are gone, and the spin state looks like [math]\mathrm{\uparrow\downarrow\uparrow\uparrow\uparrow\uparrow}[/math]. read more

Thus 2+ is the most common valency state, 3+ is rarer but still common, and 4+ exists too, but only in the presence of extremely strong oxidizers. For example, [math]\mathrm{Ba_2CoO_4}[/math] has an inverse spinel structure where the oxidation state of cobalt is 4. read more

The cobalt species can show a variety of oxidation states, ranging from at least -1 to +4. Of these states, Co(II) - known as colbatous - and Co(III) - known as cobaltic - are the most commonly found in everyday life. read more

The chemistry of the transition metal cobalt (most common oxidation states +2 and +3) is dominated by the stability of the cobalt(II) ion which forms a wide variety of stable complexes with most ligands such as water, ammonia, chloride ion etc. read more