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

🧠 Trans Possible Only When Type Allows It

Trans-isomerism in octahedral/sq pl complexes requires:

• Two non-bidentate ligands of one kind that can sit at 180° apart, AND
• Geometry that allows positioning them across the metal.

Bidentate ligands (en, ox) cannot span trans positions (their two donors are constrained at ~80° due to bite angle).

🗺️ Evaluate Each

| Complex | Geometry | Trans possible? | |---|---|---| | $[\mathrm{Cr(en)_2(ox)}]^+$ | oct with 3 bidentates | No — all 6 positions are filled by chelate donors; no monodentate to trans-arrange | | $[\mathrm{Pt(en)Cl_2}]$ | sq pl Pt(II); en bidentate forces Cl cis | ✗ (en occupies 2 adjacent corners, Cl must be cis) | | $[\mathrm{Zn(en)Cl_2}]$ | tetrahedral Zn(II) (d¹⁰) | ✗ (Td has no cis/trans) | | $[\mathrm{Pt(en)_2 Cl_2}]^{2+}$ | Pt(IV) octahedral, 2 en + 2 Cl | ✓ trans possible (2 en occupy four equatorial positions, 2 Cl can be axial = trans) |

🗺️ Why $[\mathrm{Pt(en)_2 Cl_2}]^{2+}$ Allows Trans

Pt(IV) is d⁶ octahedral. With 2 en (each spanning two cis-positions in the equatorial plane) and 2 Cl, the Cl ligands occupy the two axial positions, 180° apart → trans.

The cis isomer also exists: one Cl axial + one Cl equatorial (with en wrapping diagonally) — chiral (Δ/Λ).

⚡ Bidentate Ligand Bite Angle

Standard bite angles:

• en: ~85°
• ox: ~80°
• bipy: ~78°

All ~90° or less → bidentate spans cis positions only, never trans.

⚠️ Beware Geometry Confusion

$[\mathrm{Zn(en)Cl_2}]$ is tetrahedral (Zn d¹⁰ has no CFSE preference, weak ligands → Td). No cis/trans in Td.

$\boxed{\text{Answer: (4) } [\mathrm{Pt(en)_2 Cl_2}]^{2+}}$