Calculate the oxidation number of cobalt in K[Co(CO)_4]

The correct answer is Option (1) → -1

The oxidation number of the central metal follows from a charge balance: the oxidation number of the metal plus the charges of all the ligands equals the charge on the complex ion. Neutral ligands such as CO, \( \ce{NH3} \) and \( \ce{H2O} \) contribute zero.

In \( \ce{K[Co(CO)4]} \), potassium is present as \( \ce{K+} \), so the complex ion carries a charge of −1:

    \( \displaystyle \ce{K[Co(CO)4] -> K+ + [Co(CO)4]^-} \)

Let the oxidation number of cobalt be x. The four carbonyl ligands are neutral:

    \( \displaystyle x + 4(0) = -1 \)

    \( \displaystyle x = -1 \)

So the oxidation number of cobalt is −1. A negative oxidation state is possible here because carbonyl is a π-acceptor ligand: it takes electron density back from the metal and so stabilises metals in low and even negative oxidation states.