The oxidation number of cobalt in the complex: (i) `K[Co(CO)_(4)]`
(ii) `[Co(C_(2)O_(4))_(3)]^(-3)`

Explanation

To find the oxidation number of cobalt in the given complexes, we will follow a systematic approach for each complex.

### (i) For the complex \( K[Co(CO)_{4}] \):

1. **Identify the components**:
- The complex consists of potassium ion \( K^{+} \) and the complex ion \( [Co(CO)_{4}]^{-} \).
- The carbonyl group \( CO \) is a neutral ligand, meaning it does not contribute any charge.

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