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Question:

The correct increasing order of trans-effect of the following species is:

Br⁻>CN⁻>NH₃>C₆H₅⁻

CN⁻>C₆H₅⁻>Br⁻>NH₃

CN⁻>Br⁻>C₆H₅>NH₃

NH₃>CN⁻>Br⁻>C₆H₅⁻

Solution:

Trans effect can be defined as the effect of a ligand towards substitution on other ligand which is trans to it (weak)F⁻<HO⁻<H₂O<NH₃<py<Cl⁻<Br⁻<I⁻<SCN⁻<NO₂⁻<SC(NH₂)₂<Ph⁻<SO₃⁻<PR₃<AsR₃<SR₂<H₃C⁻<H⁻<NO<CO<NC⁻<C₂H₄(strong).Trans effect is nothing but those ligands which are trans directing. That means, which prefer a trans structure. Mostly in Square planar complexes.Hence option D is correct.