A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element.
Electronic configuration of Cr is 4s1 3d5. In the complex [Cr(NH3)6]3+ Cr is in the +3 oxidation state i.e., d3 configuration. Also, NH3 is a weak field ligand that does not cause the pairing of the electrons in the 3d orbital.
CN^- is a strong field ligand.
Answer. The spectrochemical series is based on the interaction of metal with ligand. Greater the interaction of the ligand with metal, greater will be the splitting of d-orbitals and thus strong pairing will takes place.
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