Which of the following is a low spin (spin-paired) complex?

Options

A. `[Ni(NH_(3))_(6)]^(2+)`
B. `[Ti(H_(2)O)_(6)]^(3+)`
C. `[Cr(NH_(3))_(6)]^(3+)`
D. `[Fe(NH_(3))_(6)]^(3+)`

Answer

To determine which of the given complexes is a low spin (spin-paired) complex, we need to analyze the electronic configurations and the nature of the ligands involved in each complex.

### Step-by-Step Solution:

1. **Identify the Complexes**: The complexes given are:
- A) Ni(NH3)6^2+
- B) Ti(H2O)6^3+
- C) Cr(NH3)6^3+
- D) Fe(NH3)6^3+

2. **Analyze Each Complex**:
- **Complex A: Ni(NH3)6^2+**
- Nickel has an atomic number of 28, so its electron configuration is [Ar] 4s² 3d⁸.
- For Ni²+, the configuration becomes [Ar] 3d⁸ (removing 2 electrons from 4s).
- NH3 is a weak field ligand, which does not cause significant splitting of the d-orbitals. Thus, it will not lead to pairing of electrons.
- **Conclusion**: This is a high spin complex.

- **Complex B: Ti(H2O)6^3+**
- Titanium has an atomic number of 22, so its electron configuration is [Ar] 4s² 3d².
- For Ti³+, the configuration becomes [Ar] 3d¹ (removing 3 electrons).
- H2O is also a weak field ligand, leading to no pairing of electrons.
- **Conclusion**: This is a high spin complex.

- **Complex C: Cr(NH3)6^3+**
- Chromium has an atomic number of 24, so its electron configuration is [Ar] 4s² 3d⁴.
- For Cr³+, the configuration becomes [Ar] 3d³ (removing 3 electrons).
- NH3 is a stronger field ligand than H2O, but still does not cause pairing in this case.
- **Conclusion**: This is a high spin complex.

- **Complex D: Fe(NH3)6^3+**
- Iron has an atomic number of 26, so its electron configuration is [Ar] 4s² 3d⁶.
- For Fe³+, the configuration becomes [Ar] 3d⁵ (removing 3 electrons).
- NH3 is a stronger field ligand and can cause pairing of electrons in this case.
- The pairing energy is less than the splitting energy, leading to a low spin configuration.
- **Conclusion**: This is a low spin complex.

3. **Final Answer**: The complex that is a low spin (spin-paired) complex is **D) Fe(NH3)6^3+**.