What is the oxidation state of the metal ion in complex [Co(EDTA)]−?

The correct answer is Option (2) → +3

The oxidation state of the central metal is found by charge balance: the charges of all the ligands plus the oxidation state of the metal must add up to the charge on the complex.

EDTA⁴⁻ (ethylenediaminetetraacetate) is formed when ethylenediaminetetraacetic acid (H₄EDTA) loses the four \( \ce{H^{+}} \) of its –COOH groups, so the ligand carries a charge of −4. It binds through two N atoms and four O atoms, so it is a hexadentate ligand and one EDTA fills all six positions around the metal.

For [Co(EDTA)]⁻:

    \( \displaystyle x+(-4)=-1 \)

    \( \displaystyle x=+3 \)

So cobalt is in the +3 oxidation state. Co (Z = 27) is [Ar]3d⁷4s²; removing the two 4s electrons and one 3d electron gives \( \ce{Co^{3+}} \): 3d⁶.

NCERT (2026-27 edition):
Ligands – …