Transition spin states

The discontinuous spin-state transitions in complexes generally seem to be first- 

Spin-state transitions in metal oxides are by and large limited to those containing 

Chenavas Joubert (1971) have reported a synthesis of CojOj under high pressure. The high-pressure phase seemed to consist of only low-spin Co ions which on heating transformed to the high-spin state with a positive volume change of 6.7%. The results are suggestive of a first-order transition, but it is not entirely certain that a pure phase of C02O3 was indeed prepared. 

Demazeau et al. (1982) reported that Ni is in the low-spin t g state in LaSrNi04, but in the high-spin tig e state in LaBaNi04. A magnetic susceptibility study of LaSr, Ba Ni04 has shown the eg electrons in this system to be always in extended states forming a band with increase in x, the band-width decreases, accom- 

Spiering et al. (1982) have developed a model where the high-spin and low-spin states of the complex are treated as hard spheres of volume and respectively and the crystal is taken as an isotropic elastic medium characterized by bulk modulus and Poisson constant. The complex is regarded as an inelastic inclusion embedded in spherical volume V. The decrease in the elastic self-energy of the incompressible sphere in an expanding crystal leads to a deviation of the high-spin fraction from the Boltzmann population. Pressure and temperature effects on spin-state transitions in Fe(II) complexes have been explained based on such models (Usha et al., 1985). 

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The nature of spin state transitions in solid complexes of iron(II) and the interpretation of some associated phenomena. E. Konig, G. Ritter and S. K. Kulshreshtha, Chem. Rev., 1985, 85, 219 (91). 

Raimondi M, Cooper DL (1999) Ab Initio Modern Valence Bond Theory. 203 105-120 Rao CNR, Seikh MM, Narayana C (2004) Spin-State Transition in LaCoOj and Related Materials. 234 1-21... 

Rates of Spin-State Transition for Solid Metal Complexes Based on the Line Shape... 

Most complexes showing spin-state transitions are in fact of low symmetry. In order to describe their electronic structure it is convenient to employ term symbols appropriate to cubic symmetry and this practice will be followed below. The most common transition-metal ions for which spin-state transitions have been observed are Fe " (3d ), Fe " (3d ) and Co (3d ), a minor role being played by Co " (3d ), Mn " (3d ), as well as Cr " and Mn " (3d ). The relevant ground states for an octahedral disposition of the ligands are LS Ui,(t ,) and HS r2,(t ,e ) for iron(II), LS and HS Ai,(t, e ) for... 

So far, we have considered the phenomenon of spin-state transition employing both the properties of the macroscopic complexes as well as those of the isolated molecules. However, with respect to the nature of the energy variation at the transition, these two situations should be clearly distinguished. Thus, if... 

Fig. 1. Schematic one-dimensional cross section through the Gibbs free energy surface G(R) of a spin-state transition system along the totally symmetric stretching coordinate. The situation for three characteristic temperatures is shown (B = barrier height, ZPE = zero-point energy, 28 = asymmetry parameter, J = electronic coupling parameter, AG° = Gh — GJ...

Spin-state transitions have been studied by the application of numerous physical techniques such as the measurement of magnetic susceptibility, optical and vibrational spectroscopy, the Fe-Mbssbauer effect, EPR, NMR, and EXAFS spectroscopy, the measurement of heat capacity, and others. Most of these studies have been adequately reviewed. The somewhat older surveys [3, 19] cover the complete field of spin-state transitions. Several more recent review articles [20, 21, 22, 23, 24, 25] have been devoted exclusively to spin-state transitions in compounds of iron(II). Two reviews [26, 27] have considered inter alia the available theoretical models of spin-state transitions. Of particular interest is the determination of the X-ray crystal structures of spin transition compounds at two or more temperatures thus approaching the structures of the pure HS and LS electronic isomers. A recent survey [6] concentrates particularly on these studies. 

This report has been written in order to demonstrate the nature of spin-state transitions and to review the studies of dynamical properties of spin transition compounds, both in solution and in the solid state. Spin-state transitions are usually rapid and thus relaxation methods for the microsecond and nanosecond range have been applied. The first application of relaxation techniques to the spin equilibrium of an iron(II) complex involved Raman laser temperature-jump measurements in 1973 [28]. The more accurate ultrasonic relaxation method was first applied in 1978 [29]. These studies dealt exclusively with the spin-state dynamics in solution and were recently reviewed by Beattie [30]. A recent addition to the study of spin-state transitions both in solution and the... 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

Since AG° = AH° — TAS° where AH° and AS° are the enthalpy difference and the entropy difference, respectively, associated with the spin-state transition, Eq. (13) may be expressed as ... 

Table 2. Hysteresis effects associated with abrupt spin-state transitions...

However, the calculation has taken account only of the change of the unit cell volume of the compound, AKhl = Fhs Fls which occurs due to the spin-state transition, the individual HS and LS complexes being treated as incompressible spheres. On the basis of detailed X-ray structure investigations [6], the crystal experiences not only a change in size but also a change in shape in the course of the HS LS transition. Every lattice vector x(T) may be expressed according to [70] ... 

Of considerable interest is the pressure dependence of a spin-state transition which will be considered in Sect. 8 in some detail. It is assumed that the pressure... 

Various other interactions have been considered as the driving force for spin-state transitions such as the Jahn-Teller coupling between the d electrons and a local distortion [73], the coupling between the metal ion and an intramolecular distortion [74, 75, 76] or the coupling between the d electrons and the lattice strain [77, 78]. At present, based on the available experimental evidence, the contribution of these interactions cannot be definitely assessed. Moreover, all these models are mathematically rather ambitious and do not show the intuitively simple structure inherent in the effect of a variation of molecular volume considered here. Their discussion has to be deferred to a more specialized study. 

Relaxation of Thermally Quenched or Optically Excited High-Spin States Resulting from the Spin-State Transition in Solid Metal Complexes... 

Alternatively, optical excitation of the LS state in a solid metal complex which is involved in a thermally driven spin-state transition may result in the formation of a trapped HS state, the study of the kinetics of the HS -> LS relaxation being then possible [134]. This process is initiated, e.g., in an iron(II) complex, by irradiation into the spin-allowed absorption band at... 

Fig. 16. Schematic potential energy diagram of the low-lying ligand field states for d spin-state transition compounds. Full lines indicate the mechanism of forward and reverse LIESST effect. According to Ref. [135]...

Fe(ptz)6] (BF4)2 and [Zni Fej.(ptz)6] (BF4,)2. The solid iron(II) complex of the unidentate ligand ptz = 1-propyltetrazole shows a reasonably sharp spin-state transition at about 130 K [112]. Optical excitation into the spin-allowed -> absorption band produces via LIESST the HS T2 state which remains trapped at temperatures below 50 K. Relaxation of the metastable J2 state to the LS state has been studied by following the changes of... 

The description of states participating in a spin-state transition as electronic isomers with discrete nuclear configurations, in particular different metal-ligand distances, requires that separate electronic and vibrational spectra of the two spin states exist. Indeed, a superposition of the individual vibrational spectra of the two states is in general observed, the relative contribution of the states being a function of temperature [41, 139, 140, 141, 142]. This observation sets a lower limit for the spin-state lifetime longer than the nuclear vibrational period, i.e.,... 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

Fig. 19. Calculated lineshapes for various values of for the spin-state transition between LS and HS states. The parameter values employed in the calculation are, AEq = 0.50 mm s 5 = 8 f = 0,...

The nitrosyl iron complex Fe( J-mph) NO shows a spin-state transition between the S = f and S = ground states. The structure of the complex is characterized by ... 

Fig. 24. Arrhenius plots of InilciJ ( ) and Inlfc J (A) versus l/T where 1c,l and Icli are the rate constants for the spin-state transitions S = 3/2 - S = 1/2, and S = 1/2 - S = 3/2, respectively. For clarity of presentation, the data points have been shifted in temperature by one degree up for ln(fc,L) and by one degree down for In(lcLi). According to Ref. [164]...

Fe(mim)2(salacen)]PFe. The iron(III) complex of the tetradentate ligand salacenH2 = ethylene(lV-acetylacetonylideneimine) (IV -tx-methylsalicylideneimine) shows, between 78 and 326 K, a gradual spin-state transition in... 

Fe(2-pic)3l(PF6)2. The iron(II) complex of the bidentate ligand 2-pic = 2-amino-methylpyridine shows a gradual spin-state transition in the... 

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