Consider the complex ions, trans-[Co(en)2Cl2]+ (A) and cis-[Co(en)2Cl2]+ (B). The correct statement regarding them is:

🧠 Cis vs Trans Chirality of $[\mathrm{Co(en)_2 Cl_2}]^+$

(A) trans-$[\mathrm{Co(en)_2 Cl_2}]^+$: 2 Cl axial (180° apart), 2 en in equatorial plane → has horizontal mirror plane → achiral.

(B) cis-$[\mathrm{Co(en)_2 Cl_2}]^+$: 2 Cl cis (90° apart), 2 en wrapped around → only $C_2$ axis, no mirror plane → chiral (Δ/Λ enantiomers).

🗺️ Trans Symmetry

In trans, the molecule has $C_{2h}$ symmetry: 2-fold axis through Cl–Co–Cl, plus a horizontal mirror plane (containing the two en ligands). The mirror plane makes it superimposable on its mirror image → not optically active.

🗺️ Cis Symmetry

In cis, the two Cl are adjacent. The two en ligands wrap such that their N atoms occupy the four remaining positions. The molecule has only a $C_2$ axis (through the midpoint of the Cl-Cl edge and the midpoint of the opposite en-en edge). No mirror planes → chiral.

⚡ General Rule for $[M(AA)_2 X_2]$

| Form | Chiral? | |---|---| | trans | ✗ | | cis | ✓ (Δ, Λ) |

⚠️ Same Rule Applies to Other M(AA)₂X₂

$[\mathrm{Co(en)_2 Cl_2}]^+$, $[\mathrm{Cr(en)_2 Cl_2}]^+$, $[\mathrm{CrCl_2(ox)_2}]^{3-}$, $[\mathrm{PtCl_2(en)_2}]^{2+}$ (Pt(IV)) — all follow this pattern.

$\boxed{\text{Answer: (4) (A) cannot be optically active, but (B) can be optically active}}$