Which of the following statements is correct about stability of the complexes of lanthanoids?

Options

A. Stability of complexes increases as the size of lanthanoid decreases.
B. Stability of complexes decreases as the size of lanthanoid decreases.
C. Lanthanoids do not form complexes.
D. All the complezes of lanthanoids have same stability.

Answer

To determine which statement is correct about the stability of the complexes of lanthanoids, we need to analyze each option given in the question based on our understanding of lanthanoid chemistry.

### Step-by-Step Solution:

1. **Understanding Lanthanoid Contraction**:
- As we move from Lanthanum (La) to Lutetium (Lu) in the lanthanoid series, there is a phenomenon known as lanthanoid contraction. This results in a decrease in the ionic radii of the lanthanoid ions.

**Hint**: Remember that lanthanoid contraction is due to the increase in effective nuclear charge (Z effective) as protons are added to the nucleus without a significant increase in shielding.

2. **Effect of Ionic Size on Complex Stability**:
- The stability of complexes is influenced by the size of the metal ion. Generally, as the size of the cation decreases, the covalent character of the complexes increases. According to Fajans' rules, smaller cations tend to polarize the electron cloud of the ligands more effectively, leading to stronger interactions and increased stability.

**Hint**: Recall that smaller ions can attract ligands more strongly due to their higher charge density.

3. **Analyzing the Options**:
- **Option 1**: "Stability of complex increases as the size of lanthanoid decreases."
- This is correct because as we move from La to Lu, the size decreases and thus the stability of the complexes increases.

- **Option 2**: "Stability of complexes decreases as the size of lanthanoid decreases."
- This is incorrect because it contradicts the first option.

- **Option 3**: "Lanthanoids do not form complexes."
- This is incorrect as lanthanoids do form complexes due to their ability to coordinate with ligands.

- **Option 4**: "All the complexes of lanthanoids have the same stability."
- This is also incorrect because the stability varies due to the differences in ionic sizes and the resulting covalent character.

4. **Conclusion**:
- The correct statement about the stability of the complexes of lanthanoids is **Option 1**: "Stability of complex increases as the size of lanthanoid decreases."

### Final Answer:
**The correct statement is: Stability of complex increases as the size of lanthanoid decreases.**