Which of the following complexes will exhibit maximum attraction to an applied magnetic field ?

Options

A. `[Ni(H_2O)_6]^(2+)`
B. `[Co(H_2O)_6]^(2+)`
C. `[Co(en)_3]^(3+)`
D. `[Zn(H_2O)_6]^(2+)`

Answer

To determine which complex exhibits maximum attraction to an applied magnetic field, we need to analyze the number of unpaired electrons in each complex. The more unpaired electrons present, the greater the attraction to the magnetic field.

### Step-by-Step Solution:

1. **Identify the Oxidation States**:
- For the first complex (Ni with H2O): Nickel is in the +2 oxidation state.
- For the second complex (Co with H2O): Cobalt is in the +2 oxidation state.
- For the third complex (Co with en): Cobalt is in the +3 oxidation state.
- For the fourth complex (Zn with H2O): Zinc is in the +2 oxidation state.

2. **Determine the Electronic Configurations**:
- Nickel (Ni) in +2 state: The ground state electronic configuration is 3d^8 4s^2. For Ni^2+, it becomes 3d^8.
- Cobalt (Co) in +2 state: The ground state electronic configuration is 3d^7 4s^2. For Co^2+, it becomes 3d^7.
- Cobalt (Co) in +3 state: For Co^3+, it becomes 3d^6.
- Zinc (Zn) in +2 state: The ground state electronic configuration is 3d^10 4s^2. For Zn^2+, it becomes 3d^10.

3. **Analyze the Ligands**:
- H2O is a weak field ligand, which means it will lead to high spin complexes.
- Ethylene diamine (en) is a strong field ligand, which means it will lead to low spin complexes.

4. **Count the Unpaired Electrons**:
- For Ni^2+ with H2O (3d^8):
- Configuration: ↑↓ ↑↓ ↑↓ ↑ ↑ (2 paired, 2 unpaired)
- Unpaired Electrons = 2
- For Co^2+ with H2O (3d^7):
- Configuration: ↑↓ ↑↓ ↑↓ ↑ ↑ ↑ (3 paired, 4 unpaired)
- Unpaired Electrons = 3
- For Co^3+ with en (3d^6):
- Configuration: ↑↓ ↑↓ ↑↓ ↑ ↑ (3 paired, 1 unpaired)
- Unpaired Electrons = 1
- For Zn^2+ with H2O (3d^10):
- Configuration: ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ (all paired)
- Unpaired Electrons = 0

5. **Conclusion**:
- The complex with the highest number of unpaired electrons is Co^2+ with H2O, which has 3 unpaired electrons. Therefore, this complex will exhibit the maximum attraction to an applied magnetic field.

### Final Answer:
The complex that will exhibit maximum attraction to an applied magnetic field is Co^2+ with H2O.