Iron II with Intermediate Spin

A remarkable number of Mossbauer studies have been published since the first spectra reported in 1966 [135], most of them performed on the p-form when not specified differently [131, 132, 136-139]. Also, high pressure has been applied [140] and thin Aims were prepared [141]. Because of the ambiguity concerning the crystalline phase, the values of the hyperfine parameters show some dispersion. The isomer shift, 5 = 0.4-0.6 mm s is found in between the t3q ical values known for high-spin iron(II) and low-spin iron(II). The quadrupole splitting is large, A q = 2.4-3.0 mm s (Table 8.3), as one might expect because of the unusual non-cubic symmetry. Applied-field measurements revealed positive F . 

Both Fe(ll)(TPP) and Fe(II)(OEP) have positive electric quadrupole splitting without significant temperature dependence which, however, cannot be satisfactorily explained within the crystal field model [117]. Spin-restricted and spin-unrestricted Xoi multiple scattering calculations revealed large asymmetry in the population of the valence orbitals and appreciable 4p contributions to the EFG [153] which then was further specified by ab initio and DFT calculations [154,155]. 

Four-coordinate, planar iron(II)-dithiolate complexes also exhibit intermediate spin. The first example described was the tetraphenylarsonium salt of the square-planar bis(benzene-l,2-dithiolate)iron(II) dianion, (AsPh4)2[Fe(II)bdt2], which showed 5 = 0.44 mm s and AEq = 1.16 mm s at 4.2 K [157]. The electronic structure of a different salt was explored in depth by DFT calculations, magnetic susceptibility, MCD measurements, far-infra red spectroscopy and applied-field Mossbauer spectroscopy [158]. 

The four-coordinate iron(II) complex of cycloheptatrienylidene is a rare example of a fully reversible singlet (5 = 0 at 6 K) to triplet (5 = 1 at 293 K) transition in the slow relaxation regime [159]. 

Konig and others published in the 1970s an impressive series of studies on six-coordinate iron(II)-bis-phenanthroline complexes [160-164] for which they inferred 5=1 from thorough magnetic susceptibility and applied-field Mossbauer measurements. Criteria for the stabilization of the triplet ground state for six-coordinate compounds with tetragonal ( 4 ) and trigonal ( 3 ) symmetry were obtained from LFT analyses [163], The molecular structures, however, were not known because the materials could not be crystallized. 

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