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RESEARCH PRODUCT

Spin Transition in Iron Compounds

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First-row transition metal complexes with d4 up to d7 electron configuration in octahedral ligand fields (and d8 electron configuration in six-coordinate complexes of lower symmetry) may undergo temperature dependent high spin (HS) ⇌ low spin (LS) transition, provided the ligand field strength (Δ), including low symmetry contributions, becomes comparable in magnitude with the mean spin pairing energy (P). At a critical field strength Δcrit = P, the energy levels of the two spin states cross. This is illustrated in Fig. 1, where a simplified Tanabe-Sugano type energy level diagram for d6 systems (e.g. Fe2+, Co3+) is shown as an example. For weak ligand fields, Δ Δcrit, the LS state1A1g(Oh) becomes more stable. If the difference ∣Δ-P∣, or, in terms of thermodynamics, the difference in Gibbs free energy ΔG=G(HS)-G(LS), is on the order of thermal energy kT, both spin states HS and LS may be populated in thermal equilibrium.