The tendency of 3d-metal ions to form stable complexes is due to their

Options

A. Variable oxidation state
B. Strong electronegative nature
C. High charge/size ratio and vacant d-orbitals
D. Very low ionization energies

Explanation

To answer the question regarding the tendency of 3d-metal ions to form stable complexes, we can break down the reasoning into several key points:

### Step-by-Step Solution:

1. **Understanding 3d-Metal Ions**:
- 3d-metal ions are transition metals that have partially filled d-orbitals. These include elements like Scandium (Sc), Titanium (Ti), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), and Zinc (Zn).

2. **Variable Oxidation States**:
- Transition metals are known for their ability to exhibit multiple oxidation states. This property allows them to form a variety of complexes with different ligands, enhancing their ability to stabilize different coordination environments.

3. **High Charge-to-Size Ratio**:
- 3d-metal ions typically have a high charge-to-size ratio. This means that as the charge of the ion increases, the size decreases, leading to a stronger electrostatic attraction between the metal ion and the ligands. This strong interaction helps in stabilizing the complex.

4. **Vacant d-Orbitals**:
- The presence of vacant d-orbitals in 3d-metal ions allows for the overlap with the orbitals of the ligands. This overlap facilitates the formation of coordinate bonds, which are crucial for the stability of the complexes.

5. **Low Ionization Energy**:
- Transition metals generally have relatively low ionization energies compared to other elements. This characteristic means that they can lose electrons more easily, which is beneficial for forming complexes.

6. **Conclusion**:
- The tendency of 3d-metal ions to form stable complexes is primarily due to their high charge-to-size ratio and the presence of vacant d-orbitals. These factors contribute to strong metallic bonding and the ability to stabilize various coordination complexes.

### Final Answer:
The tendency of 3d-metal ions to form stable complexes is due to their high charge-to-size ratio and vacant d-orbitals.