Answer to Question #84588 in Inorganic Chemistry for adarsh

Question

How can one decide whether the high spin or the low spin configuration of a metal complex will be more stable? Elaborate.

Answer 1

Whether a complex is high spin or low spin depends on two main factors: the crystal field splitting energy Δ and the pairing energy P. The electrons will take the path of least resistance-the path that requires the least amount of energy. If the pairing energy P is greater than Δ, then electrons will move to a higher energy orbital because it takes less energy. If the pairing energy P is less than 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 is the ligand observed is strong field or weak field.

Spectrochemical series

(weak) I⁻ < Br⁻ < S²⁻ < SCN⁻ < Cl⁻ < NO³⁻ < N³⁻ < F⁻ < OH⁻ < C₂O₄^2- ≈ 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.