The complex that exists as a pair of enantiomers is

Options

A. trans `-[Co(H_(2)NCH_(2)CH_(2)NH_(2))_(2)Cl_(2)]^(+)`
B. `cis - [Co(NH_(3))_(4)Cl_(2)]^(+)`
C. `[Pt(PPh_(3))(Cl)(Br)(CN)]^(-)`
D. `[Co(H_(2)NCH_(2)CH_(2)NH_(2))_(3)]^(3+)`

Answer

To determine the complex that exists as a pair of enantiomers, we need to follow these steps:

### Step 1: Understand Enantiomers
Enantiomers are pairs of molecules that are non-superimposable mirror images of each other. This means that if you were to place one molecule over its mirror image, they would not align perfectly.

**Hint:** Remember that enantiomers must lack a plane of symmetry.

### Step 2: Identify the Complexes
We need to analyze the given options to see which complex can form enantiomers. The options provided are:
1. Trans [CoCl2(en)]
2. Cis [CoCl2(en)]
3. Square planar [Pt(PPh3)(Cl)(Br)(CN)]
4. Octahedral [Co(en)3]3+

**Hint:** Look for complexes that have chiral centers or lack a plane of symmetry.

### Step 3: Analyze Each Option
1. **Trans [CoCl2(en)]**: In this complex, the two chloride ligands are positioned opposite each other (180 degrees apart). This arrangement allows for a plane of symmetry, making it optically inactive and therefore not capable of forming enantiomers.

**Hint:** Check if the ligands are arranged in a way that creates symmetry.

2. **Cis [CoCl2(en)]**: In this complex, the two chloride ligands are adjacent to each other. This arrangement does not allow for a plane of symmetry, resulting in the potential for non-superimposable mirror images, thus forming enantiomers.

**Hint:** Look for arrangements where ligands are not symmetrically placed.

3. **Square planar [Pt(PPh3)(Cl)(Br)(CN)]**: This complex is square planar and has a symmetrical arrangement of ligands. It also possesses a plane of symmetry, making it optically inactive and not capable of forming enantiomers.

**Hint:** Visualize the structure to see if it can be divided into two identical halves.

4. **Octahedral [Co(en)3]3+**: This complex consists of three bidentate ethylenediamine (en) ligands. The arrangement of these ligands can lead to non-superimposable mirror images, indicating that it can form enantiomers.

**Hint:** Consider the geometry of the complex and the nature of the ligands involved.

### Step 4: Conclusion
After analyzing all the options, the complex that exists as a pair of enantiomers is **Cis [CoCl2(en)]** and **Octahedral [Co(en)3]3+**. However, since the question asks for a single complex, the best answer is **Cis [CoCl2(en)]** as it is the most straightforward example of a complex that can form enantiomers.

**Final Answer:** The complex that exists as a pair of enantiomers is **Cis [CoCl2(en)]**.