Which of the following pair of compounds is a pair of enantiomers

Options

A. `[Co(NH_(3))_(4)Cl_(2)]^(+)`
B. `[Cr(en)_(3)]^(3+)`
C. `[Co(P(C_(2)H_(5))_(3))_(2)ClBr]`
D. trans- `[Co(en)_(2)Cl_(2)]^(+)`

Explanation

To determine which pair of compounds is a pair of enantiomers, we need to understand what enantiomers are. Enantiomers are a type of stereoisomer that are non-superimposable mirror images of each other. This typically occurs in chiral molecules, which have no plane of symmetry.

### Step-by-Step Solution:

1. **Identify the Options**:
We need to analyze the provided options to identify which pair of compounds could potentially be enantiomers.

2. **Analyze Option 1**:
The first option is a coordination compound with the formula Co(NH3)2Cl2. This compound can exist in cis and trans forms but does not have chiral centers, hence it cannot form enantiomers.

3. **Analyze Option 2**:
The second option is [Cr(en)3]3+, where "en" stands for ethylenediamine, a bidentate ligand. This compound can exist as two non-superimposable mirror images (enantiomers) because it has chiral centers. Therefore, this option is a candidate for enantiomers.

4. **Analyze Option 3**:
The third option is another coordination compound that can also exist in cis and trans forms. Similar to option 1, it does not have chiral centers and thus cannot be enantiomers.

5. **Analyze Option 4**:
The fourth option is trans-Co(en)2Cl2. This compound has a plane of symmetry due to its trans configuration, which means it cannot have enantiomers.

6. **Conclusion**:
After analyzing all options, the only pair of compounds that can be classified as enantiomers is from option 2, [Cr(en)3]3+.

### Final Answer:
The pair of compounds that is a pair of enantiomers is option 2: [Cr(en)3]3+.