Question

The oxidation number of Co in [Co(NH$_3$)$_5$(ONO)]SO$_4$ is :

• A. +3
• B. +2
• C. +4
• D. +6

Hint:

Neutral ligands such as NH$_3$, H$_2$O and CO contribute zero charge while calculating oxidation state.

Answer 1

The Correct Option is A

Step 1: Determine complex ion charge from counter ion.
$SO_4^{2-}$ is outside the coordination sphere with charge $-2$. To form a neutral compound, the complex cation must carry charge $+2$.
Step 2: Assign ligand charges.
Five $NH_3$ ligands are neutral (charge = 0 each). One $ONO^-$ (nitrito) has charge $-1$. Total ligand charge = $-1$.
Step 3: Calculate oxidation state of Co.
$x + (-1) = +2$, so $x = +3$.
\[ \boxed{+3} \]