Explanation
Chlorine has vacant π* and d–orbital of suitable energy, there is the possibility of pi backbonding and it is π –acceptor and strong σ – donor also stronger ligand than water.
Stronger the ligand, larger the value of ∆O. the increasing order of strength of ligand is,
I– < Br– < Cl– < F– <OH– < H2O < NH3 < NO2– < CN–
As CN– is the stronger ligand in [Co (CN) 6]3– , so it has highest ∆O value.
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