What is the oxidation number of the metal of \( [\text{Co}(\text{H}_2\text{O})(\text{CN})(\text{ox})_2]^{2-} \) coordination entity?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → +3
The oxidation number of the central metal is found from a charge balance: the oxidation number of the metal plus the charges of all the ligands equals the charge on the coordination entity. Here the ligands are:
\( \ce{H2O} \) (aqua): neutral, charge 0.
\( \ce{CN^-} \) (cyanido): charge −1.
ox, oxalate \( \ce{C2O4^2-} \): charge −2 each, and there are two of them.
With the oxidation number of cobalt as x and the entity charge −2:
\( \displaystyle x + 0 + (-1) + 2(-2) = -2 \)
\( \displaystyle x - 5 = -2 \Rightarrow x = +3 \)
So cobalt is in the +3 oxidation state.