Which of the following is more stable ?
(1) `K_(4)[Fe(CN)_(6)] or K_(3)[Fe(CN)_(6)]`
(2) `[Co(NH_(3))_(6)]^(3+) or [Co(en)_(2)(NH_(3))_(2)]^(3+)`
(3) `[NiCl_(4)]^(2-) or [Ni(CN)_(4)]^(2-)`

Explanation

To determine the stability of the given coordination compounds, we will analyze each pair based on the nature of the metal ion, the charge on the metal, and the ligands involved.

### Step-by-Step Solution:

1. **Comparing K₄[Fe(CN)₆] and K₃[Fe(CN)₆]:**
- In K₄[Fe(CN)₆], the oxidation state of iron (Fe) is +2.
- In K₃[Fe(CN)₆], the oxidation state of iron (Fe) is +3.
- A higher oxidation state (+3) leads to greater splitting of the d-orbitals due to increased effective nuclear charge, which stabilizes the complex.
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