Iron III with Intermediate Spin

In a simple crystal-field description, a spin-quartet ground state (S = 3/2) of iron(III) is energetically stabilized over the spin-sextet (S = 5/2) when the energy separation 

The single-electron crystal-field picture may be a crude over-simplification because Coulomb repulsion and SOC can lead to a situation where the ground term is a composite mixture of = 1/2, 3/2, and 5/2 states that does not derive from a single configuration [71, 72] the corresponding physical description is obtained from proper quantum chemical calculations [73-75]. 

Intermediate spin occurs mainly for square- and rhombic-pyramidal five-coordinate iron complexes and also for planar four-coordinate and for asymmetric six-coordinate 

Bis(dithiooxalato)ferric complexes, [Fe (dto)2X] with dto = S2C2O2 and X = P, Br , CP, respectively (compounds 5-7), have been studied by magnetic susceptibility, EPR and applied-field Mossbauer spectroscopy, [67]. The isomer shifts at 4.2 K are in the range 0.25-0.30 mm s and the quadrupole splittings are larger than -1-3.25 mm s (Table 8.1). 

Coordination compounds of dianionic dithiolene (S2C2 R2) and benzene-1,2-dithiolene (bdt = (S2C6H4) and their derivatives have been studied since the 1960s by Mossbauer spectroscopy [87] and other techniques. Nevertheless, many aspects of their electronic structure remained uncertain for a long time. The five-coordinate ferric complexes with two equatorial dithiolene ligands exhibit intermediate spin and show the Mossbauer parameters = 0.25-0.38 mm s and A q = 1.6-3.2 mm s For example, [Fe° mnt)2/ y] with two mnt ligands (=S2C2(CN)2) and an 

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