Two main factors determine whether a complex is high spin or low spin: the crystal field splitting energy Δ and the pairing energy P.
If P > Δ, then electrons will move to a higher energy orbital because it takes less energy.
If P < Δ, then the electrons will pair up rather than moving singly to a higher energy orbital.
To predict the value of Δ we should determine from the spectrochemical series if the ligand observed is strong field or weak field.
(weak) I− < Br− < S2− < SCN− < Cl− < NO3− < N3− < F− < OH− < C2O42- ≈ H2O <
NCS− < CH3CN < py < NH3 < en < bipy < phen < NO2− < PPh3 < CN− ≈ CO (strong)
If ligand is strong field, then P<Δo and we should expect a low spin complex to be formed.
If ligand is weak field, then P>Δo and we should expect a high spin complex to be formed.