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 which complex exists as a pair of enantiomers, we need to analyze the given complexes for chirality. Enantiomers are non-superimposable mirror images of each other, which typically occurs in chiral molecules that lack an internal plane of symmetry.

### Step-by-Step Solution:

1. **Identify the Complexes**: We have four complexes to analyze:
- (A) Trans diethylene diamine dichloridocobalt cation
- (B) Cis tetramine dichloridocobalt
- (C) Square planar complex with four substituents
- (D) Tris ethylene diamine cobalt

2. **Analyze Complex (A)**:
- The trans diethylene diamine dichloridocobalt complex has a symmetrical arrangement of ligands.
- Since it has a plane of symmetry, it cannot exist as a pair of enantiomers.

3. **Analyze Complex (B)**:
- The cis tetramine dichloridocobalt complex also has a symmetrical arrangement of ligands.
- Similar to complex (A), it has a plane of symmetry and cannot exist as a pair of enantiomers.

4. **Analyze Complex (C)**:
- The square planar complex with four substituents is also symmetrical.
- Planar complexes typically do not show optical isomerism due to their symmetry.

5. **Analyze Complex (D)**:
- The tris ethylene diamine cobalt complex is a chiral molecule.
- It has three ethylene diamine ligands, which can create non-superimposable mirror images.
- This complex does not have a plane of symmetry and can exist as a pair of enantiomers.

6. **Conclusion**:
- The complex that exists as a pair of enantiomers is the tris ethylene diamine cobalt complex (option D).

### Final Answer:
The complex that exists as a pair of enantiomers is the tris ethylene diamine cobalt complex.