Which of the following is not a condition for complex formation?

Options

A. Small atomic size
B. High nuclear charge
C. Variable oxidation states
D. Availability of vacant d orbitals

Explanation

To determine which of the following is not a condition for complex formation, let's analyze the key factors that contribute to the formation of complexes.

### Step-by-Step Solution:

1. **Understanding Complex Formation**: Complex formation involves the interaction between a metal ion and ligands, which are molecules or ions that can donate electron pairs to the metal.

2. **Small Atomic Size**: A small atomic size of the metal ion is essential because it allows for stronger interactions with the ligands. Smaller ions can approach ligands more closely, leading to stronger bonds.

3. **Higher Nuclear Charge**: A higher nuclear charge in the metal ion enhances the attraction between the metal and the ligands. This increased positive charge can stabilize the complex, making it a favorable condition for complex formation.

4. **Availability of Vacant d-Orbitals**: For transition metals, having vacant d-orbitals is crucial. Ligands donate their electron pairs to these vacant orbitals, forming coordinate bonds. Without vacant d-orbitals, complex formation cannot occur.

5. **Variable Oxidation State**: While variable oxidation states are a characteristic of transition metals and can influence the types of complexes formed, they are not a necessary condition for the formation of complexes. Complexes can form even if the metal ion does not exhibit variable oxidation states.

### Conclusion:
Based on the analysis, the correct answer is that **variable oxidation state** is not a necessary condition for complex formation.

### Final Answer:
**Variable oxidation state** is not a condition for complex formation.

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