The oxidation number of cobalt in K3[Co(NO2)6] is:

The correct answer is Option (2) → +3

The oxidation number of the central metal is the charge it would carry if all the ligands were removed along with their shared electron pairs. It follows from a charge balance over the whole compound.

• Charges of the parts: each K is +1; each \( \ce{NO2^-} \) is −1; the compound is neutral.

• Charge of the complex ion: three \( \ce{K^+} \) balance it, so it is \( \ce{[Co(NO2)6]^3-} \).

• Charge balance: 3(+1) + x + 6(−1) = 0, so x = +3.

NCERT (2026-27 edition):
(g) Oxidation number of central atom – “The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. The oxidation number is represented by a Roman numeral in parenthesis following the name of the coordination entity. For example, oxidation number of copper in [Cu(CN)₄]^3– is +1 and it is written as Cu(I).”