Spin-state crossover

This chapter will concentrate on the electronic spin-state crossover observed in the iron and cobalt complexes formed with the HB(pz)3 and HC(pz)3 ligands and their various methyl derivatives. In the majority of cases, the spin-state crossover occurs in the solid state and, as a consequence, solid state studies will be covered first, followed by the more limited studies... 

The relaxation fits of the Mossbauer spectra of [Fe(HB(pz)3)2] yield [30] the temperature dependence of both the population of the iron(II) high-spin and low-spin states and the relaxation rate between these two states. The resulting population of the high-spin state has a striking resemblance to that of the magnetic moment shown in Fig. 1 and these populations provide clear support both for the spin-state crossover and for the difference in populations upon heating and cooling. 

The long-range cooperative nature of the electronic spin-state crossover in [Fe(HB(pz)3)2] and the accompanying crystallographic phase transition is... 

The electronic spin-state crossover in [Fe(HB(pz)3)2] has also been observed in the fine structure of its fC-edge x-ray absorption spectrum [38]. The changes in the x-ray absorption spectra of [Fe(HB(pz)3)2] are especially apparent between 293 and 450 K at ca. 25 eV, as is shown in Fig. 5. The 293 K x-ray absorption spectral profile observed in Fig. 5 for [Fe(HB(pz)3)2] has been reproduced [39] by a multiple photoelectron scattering calculation, a calculation that indicated that up to 33 atoms at distances of up to 4.19 A are involved in the scattering. As expected, the extended x-ray absorption fine structure reveals [38] no change in the average low-spin iron(II)-nitro-gen bond distance of 1.97 A in [Fe(HB(pz)3)2] upon cooling from 295 to 77 K. 

As was mentioned above, the [Fe(HB(3,5-(CH3)2pz)3)2] complex represents a classic example [27, 28] of an iron(II) spin-state crossover that may be induced in a high-spin complex upon cooling. The room temperature crystal structure of this complex [26] reveals a structure rather similar to that of [Fe(HB(pz)3)2], but with a substantially longer average iron-nitrogen bond... 

The temperature dependence of the isomer shift and quadrupole splitting for the high-spin and low-spin iron(II) states in [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2 and details of the fits and their temperature dependence may be found elsewhere [46]. The extent of the spin-state crossover is shown in Fig. 18, a figure which clearly indicates that the spin-state crossover in [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2 stops at 50 percent. In contrast it should be noted that, in the structurally very similar [Fe(HC(3,5-(CH3)2pz)3)2]l2 complex, [49] the spin-state crossover is 100 percent complete at 4.2 K. 

Fig. 20. Changes in orbital occupancy accompanying spin-state crossovers in manganocene.

Interest toward spin-state crossover in iron(II) complexes is increasing because of potential technological applications,198199 such as intelligent contrast agents for biomedical imaging, as temperature/pressure threshold indicators, and as optical elements... 

V-methylpyridyl species (Scheme 1.10). This unusual effect, which did not appear in the corresponding reactions of the Mn(IV) and Mn(III) states, was ascribed to the low spin, d electronic configuration of oxoMn(V). Thus, a spin-state crossover is required during reduction with the promotion of an electron from d to d. ... 

The extent of new and insightful knowledge regarding metal complex photophysics that can now be derived from a diverse variety of time-resolved pump-probe spectroscopic techniques is illustrated by recent examples in the field of spin-state crossover complexes. This is especially so in the solution state/ but also in the solid, crystalline state straddling several time domains, from the steady-state to femtoseconds. Examples are discussed in Section 4 below on Molecular bi-stability in solution and the solid state . First however we look at recent examples where Raman spectroscopy in both steady-state and time-resolved modes has been applied to the investigation of metal-centred species of bioinorganic and catalytic interest. 

Reedijk and co-workers have studied the iron(II) spin-state crossover in [Fe(teec)6]X2, where X is BF4, CIO4, or PFg, and teec is the monodentate l-(2-chloroethyl)tetrazole ligand. Depending upon the anion and the rate of precipitation, these complexes exhibit differing, partial or complete, spin-state crossover behavior with gradual or sharp, one- or two-step, transitions both with and without hysteresis. 

In contrast to iron(II) spin-state crossover complexes which usually show a cooperative behavior, iron(III) complexes exhibit a gradual, noncooperative, spin-state transition. There has been extensive Mossbauer spectral studies of these transitions, including ambient and high-pressure studies of the iron(III) trisdithiocarbamate complexes, the first iron complexes which were reported " in 1931 to undergo a spin-state transition. 

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