Which of the following pairs of compounds are more stable?

Options

A. `K_(2)[NiCl_(6)],[Pt(CN)_(4)]^(2-)`
B. `NiCl_(4),PtCl_(4)`
C. `[Ni(CN)_(4)]^(2-), K_(2)[PtCl_(6)]`
D. `PtCl_(2), NiCl_(2)`

Answer

To determine which pair of compounds is more stable, we need to analyze the oxidation states of nickel (Ni) and platinum (Pt) in the given compounds, as well as their ionization energies and thermodynamic stability.

### Step-by-step Solution:

1. **Identify the Oxidation States**:
- In the given options, we have nickel in +2 and +4 oxidation states, and platinum in +2 and +4 oxidation states.
- We need to compare the stability of Ni²⁺ and Ni⁴⁺ with Pt²⁺ and Pt⁴⁺.

2. **Understand Ionization Energy Trends**:
- Ionization energy generally decreases down a group in the periodic table.
- Since nickel is above platinum in the periodic table, nickel will have a higher ionization energy than platinum.

3. **Thermodynamic Stability**:
- Higher ionization energy typically correlates with greater thermodynamic stability for the corresponding cation.
- Therefore, Ni²⁺ (which has a higher ionization energy) will be more stable than Pt²⁺.

4. **Comparison of Oxidation States**:
- For the +4 oxidation states, we compare Ni⁴⁺ and Pt⁴⁺.
- Platinum in the +4 oxidation state is known to be more stable than nickel in the +4 oxidation state due to the inert pair effect and the overall stability of the platinum complex.

5. **Conclusion**:
- Based on the above analysis, Ni²⁺ is more stable than Pt²⁺, while Pt⁴⁺ is more stable than Ni⁴⁺.
- Therefore, the pair of compounds that is more stable is the one with Ni²⁺ and Pt⁴⁺.

### Final Answer:
The most stable pair of compounds is found in **Option 3**.

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