Crystal field theory can be used to figure out the electron arrangement in a complex. The electrons can go into either a high spin or low spin arrangement depending on the magnitude of the crystal field splitting energy D.

High spin: small D	Low spin: large D

The size of D is determined by the ligands that surround the metal ion. Chemists have derived what is known as the spectrochemical series to list approximately how strong a given ligand is: an abbreviated version is shown below

CN^- > NO₂^- > en > NH₃ > NCS^- > H₂0 > F^- > Cl^-

CN^- is a strong field ligand: it makes the splitting energy D large and thus complex with cyanide ions tend to have low spin configuration. F^- and Cl^- are weak field ligands: D is small in complexes with these ligands and these complexes tend to be high spin.

Example: Which of the following complexes is most likely to be high spin?

• [Co(H₂O)₆]⁺³
• [Co(CN^-)₆]^-3
• [Co(NH₃)₆]⁺³
• [Co(en)₃]⁺³

Solution: Compare the ligands on the spectrochemical series. Since we want a high spin complex, we want a weak field ligand. The weakest field ligand in the above is H₂O, so the water complex is most likely to be high spin. (The cyanide complex is least likely.)

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