The coordination number (CN) and oxidation state (OS) of Co in \( [Co(C_2O_4)_3]^{3-} \) is ...........
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → CN 6 and OS +3 respectively
The coordination number of a metal ion is the number of ligand donor atoms bonded to it, not the number of ligands. The oxidation state is the charge the metal would carry if all the ligands were removed with their electron pairs.
The oxalate ion, \( \ce{C2O4^2-} \), is a didentate ligand: it binds through two oxygen atoms. Three oxalate ions therefore give:
\( \displaystyle \text{CN} = 3 \times 2 = 6 \)
For the oxidation state, let the oxidation state of Co be x; each oxalate carries −2 and the complex ion carries −3:
\( \displaystyle x + 3(-2) = -3 \)
\( \displaystyle x = +3 \)
So Co has coordination number 6 and oxidation state +3.
Why the other options are incorrect
Option (1) CN 3 and OS +3 respectively — Incorrect. The oxidation state is right, but 3 counts the oxalate ions instead of the donor atoms.
Option (3) CN 6 and OS +6 respectively — Incorrect. The coordination number is right, but +6 ignores the −3 charge of the complex ion; it would make the complex neutral.
Option (4) CN 3 and OS +6 respectively — Incorrect. Both values are wrong: the ligands are counted instead of the donor atoms, and the charge of the complex ion is left out.