Identify a lowest field strength ligand from following.

Step-by-step solution

The spectrochemical series establishes ligand strength through crystal field splitting magnitude . Standard ordering of increasing strength: I ⁻ Among the options, I ⁻ initiates the series with minimal splitting, S ²⁻ exhibits moderate strength, while en and CO represent progressively stronger field ligands due to -acceptor capability and chelation. I ⁻ yields the weakest crystal field.