The magnetic property, dipole moment, plane of symmetry, colour and absorption band cann be helpful in structure elucidation of complex compounds:
Q. Which of the following complex ion is expected to absorb light in 4000 Å to 7800 Å region?

Options

A. `[Ti(en)_(3)]^(4+)`
B. `[Cr(H_(2)O)_(6)]^(3+)`
C. `[Sc(NH_(3))_(4)(H_(2)O)_(2)]^(3+)`
D. `Zn(en)_(2)(NH_(3))_(2)]^(2+)`

Answer

To determine which complex ion is expected to absorb light in the 4000 Å to 7800 Å region, we need to analyze the magnetic properties and electron configurations of the given ions. The absorption of light in this range corresponds to the visible light spectrum, which is crucial for the color exhibited by the complex.

### Step-by-Step Solution:

1. **Understanding the Absorption Range**:
- The range of 4000 Å to 7800 Å corresponds to the visible light spectrum. Complex ions that absorb light in this range will exhibit color.

**Hint**: Remember that the visible spectrum is crucial for determining the color of the complex.

2. **Identifying the Complex Ions**:
- The complex ions mentioned are Ti^4+, Cr^3+, Sc^3+, and Zn^2+. We need to analyze each of these ions to determine their electron configurations and magnetic properties.

**Hint**: Write down the electron configurations for each ion to identify unpaired electrons.

3. **Electron Configuration of Each Ion**:
- **Ti^4+**: Titanium has an atomic number of 22. Its electron configuration is [Ar] 4s^2 3d^2. After losing 4 electrons (2 from 4s and 2 from 3d), the configuration becomes [Ar], which means it has no unpaired electrons and is diamagnetic.
- **Cr^3+**: Chromium has an atomic number of 24. Its electron configuration is [Ar] 4s^2 3d^5. After losing 3 electrons (2 from 4s and 1 from 3d), the configuration becomes [Ar] 3d^3. This ion has unpaired electrons and is paramagnetic.
- **Sc^3+**: Scandium has an atomic number of 21. Its electron configuration is [Ar] 4s^2 3d^1. After losing 3 electrons (2 from 4s and 1 from 3d), the configuration becomes [Ar], which means it has no unpaired electrons and is diamagnetic.
- **Zn^2+**: Zinc has an atomic number of 30. Its electron configuration is [Ar] 4s^2 3d^10. After losing 2 electrons (from 4s), the configuration becomes [Ar] 3d^10, which means it has no unpaired electrons and is diamagnetic.

**Hint**: Identify which ions have unpaired electrons, as these are necessary for paramagnetism and color.

4. **Determining Magnetic Properties**:
- From the analysis, we find that:
- Ti^4+ is diamagnetic.
- Cr^3+ is paramagnetic (has unpaired electrons).
- Sc^3+ is diamagnetic.
- Zn^2+ is diamagnetic.

**Hint**: Only paramagnetic ions can absorb visible light and exhibit color.

5. **Conclusion**:
- Since Cr^3+ is the only ion that is paramagnetic and has unpaired electrons, it is expected to absorb light in the visible region (4000 Å to 7800 Å). Therefore, Cr^3+ is the complex ion that will absorb light in this range.

**Final Answer**: The complex ion expected to absorb light in the 4000 Å to 7800 Å region is **Cr^3+**.