Spin Crossover Phenomena in Fe II Complexes

Although STs have been observed for the first (3d) transition series and also partially for the second transition series, by far the greatest number has been reported for iron(II). The earlier developments in iron(II) ST systems were reviewed by Konig [1], Goodwin [2] and Giitlich [3]. Since then, numerous reviews have appeared on specific aspects of the phenomenon [4,5 and references therein]. 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq 1 mm s 3 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole 

At room temperature, the spectmm shows only a HS quadrupole doublet characteristic of the HS T2) state. On lowering the temperature, a new quadrupole doublet (inner two lines with AEq 0.5 mm s characteristic of the LS state) develops. The thermally induced ST occurs abruptly near 178 K (from [7]) 

Giitlich et al. [4, 6] have studied SCO in solid [Fe(2-pic)3]Cl2-EtOH (2-pic = 2-picolylamine), particularly the influence of dilution with Zn and Co, the nature of noncoordinated anions and crystal solvent, the HID and isotope effect, 

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Giitlich, Garcia and Goodwin (2000) Spin crossover phenomena in Fe(II) complexes [246]. 

Jahn-Teller effects are rarely adopted in theoretical treatments of spin-crossover phenomena, except Kambara s model for Fe(II) complexes [24] and Bersuker s for Fe(III) complexes [19,25]. In the Kambara theory, abrupt spin-crossover transitions are given an FO => LS character, while gradual transitions are of the HS LS type. However, experimental evidences of cooperative Jahn-Teller transitions (FO HS) have not yet been reported for Fe(II) spin-crossover complexes. Kambara s theory ignores the spin-orbit interaction and appears to overestimate the Jahn-Teller coupling, producing an unphysical (for Fe(II) complexes) FO => HS transition. 

Photoinduced spin-related phenomena are a particularly important field of the solid-state photophysics, because fast spin switching is a prospective basis for applications in the field of spintronics. An illustrative example is the production of the metastable state of the iron propyltetrazole (ptz) complex [Fe(ptz)6](BF4)2 by laser light-induced excited spin-state trapping (LIESST) and the determination of the resulting structure by steady-state X-ray photodiffraction [68]. In another example, steady-state X-ray photodiffraction at cryogenic temperatures was successfully utilized to study photoinduced phase transition due to spin crossover in the tris(a-picolylamine)iron(II) complex [69]. The phase transition is accompanied by... 

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