Which one of the following has largest number of isomers?

Options

A. `[Ru(NH_(3))_(4) Cl_(2)]^(+)`
B. `[Co(NH_(3))_(5) Cl]^(2+)`
C. `[Ir(PR_(3))_(2) H(CO)]^(2+)`
D. `[Co(en)_(2) Cl_(2)]^(+)`

Explanation

To determine which of the given coordination compounds has the largest number of isomers, we will analyze each compound one by one.

### Step 1: Analyze the first compound
**Compound A:** Ru(NH3)4Cl2

1. **Identify the type of complex:** This is a coordination complex with Ru as the central metal ion, NH3 as a monodentate ligand, and Cl as a monodentate ligand.
2. **Count the ligands:** We have 4 NH3 ligands and 2 Cl ligands.
3. **Determine the possible isomers:**
- The possible arrangements of the ligands can lead to geometric isomers (cis and trans).
- For this compound, we can have two geometric isomers: one where the two Cl ligands are adjacent (cis) and one where they are opposite (trans).
4. **Total isomers for Compound A:** 2 isomers.

### Step 2: Analyze the second compound
**Compound B:** Co(NH3)5Cl

1. **Identify the type of complex:** This is a coordination complex with Co as the central metal ion, NH3 as a monodentate ligand, and Cl as a monodentate ligand.
2. **Count the ligands:** We have 5 NH3 ligands and 1 Cl ligand.
3. **Determine the possible isomers:**
- Since there is only one Cl ligand, there can be no geometric isomers.
4. **Total isomers for Compound B:** 1 isomer.

### Step 3: Analyze the third compound
**Compound C:** Ir(PR3)O2HCO

1. **Identify the type of complex:** This is a coordination complex with Ir as the central metal ion and various ligands.
2. **Count the ligands:** The ligands are PR3 (which can be considered as monodentate), O2, and HCO.
3. **Determine the possible isomers:**
- This complex does not form valid coordination due to the presence of multiple different types of ligands and the charge.
4. **Total isomers for Compound C:** 0 isomers.

### Step 4: Analyze the fourth compound
**Compound D:** Co(en)2Cl2

1. **Identify the type of complex:** This is a coordination complex with Co as the central metal ion, en (ethylenediamine) as a bidentate ligand, and Cl as a monodentate ligand.
2. **Count the ligands:** We have 2 bidentate en ligands and 2 Cl ligands.
3. **Determine the possible isomers:**
- Geometric isomers can exist (cis and trans).
- Optical isomers can also exist due to the bidentate nature of en.
- The cis form can lead to optical isomers, while the trans form will not.
4. **Total isomers for Compound D:** 3 isomers (2 geometric + 1 optical).

### Conclusion
After analyzing all four compounds, we find:
- Compound A has 2 isomers.
- Compound B has 1 isomer.
- Compound C has 0 isomers.
- Compound D has 3 isomers.

Thus, the compound with the largest number of isomers is **Compound D**.

### Final Answer
**The correct option is D.**