Considering the strength of the ligand, the highest excitation energy will be observed in:
Stronger ligand → higher splitting energy.
[Co(Hâ‚‚O)₆]³âº
[Co(NH₃)₆]³âº
[Co(CN)₆]³â»
[CoCl₆]³â»
CN⻠is a strong field ligand → large splitting → high excitation energy.