The species that can have a trans-isomer is: (en = ethane -1,2-diamine, ox = oxalate)

Options

A. `[Pt(en)Cl_(2)]`
B. `[Pt(en)_(2)Cl_(2)]^(2+)`
C. `[Zn(en)Cl_(2)]`
D. `[Cr(en)_(2)("ox")]^(+)`

Explanation

To determine which species can have a trans-isomer, we need to analyze the coordination complexes formed by the given ligands (ethane-1,2-diamine (en) and oxalate (ox)) with different metal ions.

### Step-by-Step Solution:

1. **Identify the Ligands and Their Properties:**
- Ethane-1,2-diamine (en) is a bidentate ligand, meaning it can form two bonds with a metal center.
- Oxalate (ox) is also a bidentate ligand.

2. **Analyze Each Option:**
- **Option 1: [Pt(en)Cl2]**
- Platinum (Pt) can have a coordination number of 4, leading to square planar geometry.
- In a square planar arrangement, the two en ligands will occupy adjacent positions, and the two Cl ligands will also occupy adjacent positions. This arrangement does not allow for trans isomerism.

- **Option 2: [Pt(en)2Cl2]**
- Here, platinum has a coordination number of 6, leading to an octahedral geometry.
- The two en ligands can be placed opposite each other, allowing the two Cl ligands to also be opposite each other. This arrangement permits the formation of a trans isomer.

- **Option 3: [Zn(en)Cl2]**
- Zinc typically has a coordination number of 4, leading to tetrahedral geometry.
- In a tetrahedral arrangement, all ligands are equivalent and cannot exhibit trans isomerism.

- **Option 4: [Cr(en)2(ox)]**
- Chromium in this case has a coordination number of 6, leading to an octahedral geometry.
- The two en ligands can be placed adjacent to each other, and the oxalate can occupy the remaining positions. This arrangement does not allow for trans isomerism.

3. **Conclusion:**
- The only complex that can exhibit trans isomerism is **Option 2: [Pt(en)2Cl2]**.

### Final Answer:
The species that can have a trans-isomer is **Option 2: [Pt(en)2Cl2]**.