Select correct statement :

Options

A. Geometrical isomers of complexes may differ in dipole moment and visible / UV spectra
B. Complexes of the type `[Ma_3b_3]` can also have facial (fac) and meridional (mer) isomer
C. No optical isomer exists for the complex trans - `[Co(en)_2Cl_2]^(+)`
D. All are correct

Answer

To solve the question, we need to analyze the statements given regarding geometrical isomers and their properties. Let's break it down step by step.

### Step-by-Step Solution:

1. **Understanding Geometrical Isomers:**
- Geometrical isomers occur when ligands in a coordination complex can be arranged differently in space. For example, in an octahedral complex, ligands can be positioned either adjacent to each other (cis) or opposite each other (trans).

**Hint:** Geometrical isomers are determined by the spatial arrangement of ligands around the central metal atom.

2. **Dipole Moment and Spectra:**
- The first statement claims that geometrical isomers may differ in dipole moment and UV-visible spectra. This is true because the arrangement of ligands affects the distribution of charge and the electronic transitions that can occur, leading to differences in dipole moments and absorption spectra.

**Hint:** Consider how the arrangement of polar ligands affects the overall charge distribution in the molecule.

3. **Facial and Meridional Isomers:**
- The second statement discusses complexes of the type Me3B3, which can exhibit facial and meridional isomers. This is also correct. In octahedral complexes with three identical ligands and three different ones, the arrangement can lead to two distinct isomeric forms: facial (where the identical ligands occupy one face of the octahedron) and meridional (where they are arranged in a plane that cuts through the center).

**Hint:** Visualize the octahedral geometry to understand how ligands can occupy different positions.

4. **Optical Activity:**
- The discussion about optical activity indicates that certain arrangements of ligands can lead to optically inactive compounds due to the presence of a plane of symmetry. If a complex has a plane of symmetry, it cannot rotate plane-polarized light, making it optically inactive.

**Hint:** Remember that optical activity is related to the symmetry of the molecule; a lack of symmetry typically leads to optical activity.

5. **Conclusion:**
- After analyzing the statements, we find that both the first and second statements are correct. Therefore, the correct answer is that all the statements provided are true.

### Final Answer:
All statements are correct.