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For the coordination complex dichloridobis(ethane-1,2-diamine)cobalt(III), which statement about its isomers is correct?

1. It has three isomers: two are optically active and one is optically inactive
2. It has three isomers: all three are optically active
3. It has three isomers: all three are optically inactive
4. It has only one optically active isomer and two geometrical isomers

Correct answer: It has three isomers: two are optically active and one is optically inactive

Solution

[CoCl2(en)2]⁺ is an octahedral complex. There are two geometric isomers: (1) trans isomer — the two Cl ligands are trans to each other. This isomer has a plane of symmetry and is optically inactive (meso-like). (2) cis isomer — the two Cl are adjacent (cis). This isomer is chiral (no plane of symmetry), giving two enantiomers (d and l forms). Total: 3 isomers — 2 optically active (cis pair) + 1 optically inactive (trans).

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