Which one of the following complexes does not exhibit chirality?

Options

A. `[Cr(o x)_(3)]^(3)`
B. cis-`[PtCl_(2)(en)]`
C. cis-`[RhCl_(2)(NH_(3))_(4)]^(+)`
D. mer`-[Co(NO_(2))_(3)("dien")]`

Answer

To determine which of the following complexes does not exhibit chirality, we need to analyze the nature of the ligands involved and the structure of the complexes. Chirality in coordination compounds typically arises when a complex has a non-superimposable mirror image, which is often due to the presence of bidentate or polydentate ligands that can form rings with the central metal ion.

### Step-by-Step Solution:

1. **Understanding Chirality**:
- Chirality occurs when a molecule cannot be superimposed on its mirror image. This is often due to the presence of chiral centers, which can be created by certain types of ligands (like bidentate or polydentate ligands) that form rings with the central metal ion.

2. **Analyzing the Ligands**:
- Identify the types of ligands in each complex. Bidentate ligands can form rings and are more likely to create chirality, while monodentate ligands cannot.

3. **Option Analysis**:
- **Option 1**: Oxalato (a bidentate ligand) forms a chiral ring. Therefore, this complex exhibits chirality.
- **Option 2**: Platinum complex with ethylene diamine (a bidentate ligand). This also forms a chiral ring and exhibits chirality.
- **Option 3**: A cis complex with two identical monodentate ligands (e.g., Cl). Since all ligands are monodentate, they do not form a chiral ring, and thus this complex does not exhibit chirality.
- **Option 4**: A meridional form of a complex with diethylene diamine (a tridentate ligand). This can form chiral rings and thus exhibits chirality.

4. **Conclusion**:
- After analyzing all options, it is clear that the third option, which consists of monodentate ligands, does not exhibit chirality.

### Final Answer:
The complex that does not exhibit chirality is **Option 3**.