Spin crossover transitions

A series of NFS spectra of the spin-crossover complex [Fe(tpa)(NCS)2] were recorded over a wide temperature range [45]. A selection of spectra around the spin-crossover transition temperature is shown in Fig. 9.13. At 133 K, the regular quantum-beat structure reflects the quadrupole splitting from the pure high-spin (HS) phase, and the envelope of the spectrum represents the dynamical beating with a minimum around 200 ns. Below the transition, at 83 K, the QBs appear with lower frequency because of smaller AEq of the low-spin (LS) phase. Here the minima of... 

Fig. 3. Temperature dependence of Raman spectra of [Mn(taa)]. Arrows indicate peaks showing remarkable intensity changes at around spin-crossover transition Tc = 48 K.

Both spin-crossover transitions (HS < LS, FO LS) are first order accompanied by definite jumps of populations, while the cooperative Jahn-Teller transition (HS FO) is weak first-order (very close to a second-order transition). It suggests a possibility of observation of hidden cooperative Jahn-Teller transition (the broken line in Fig. 7) between the metastable HS and FO phases, if the HS phase could be supercooled enough below the spin-crossover transition temperature Tc by a rapid cooling. 

The three adjustable parameters are determined, A/kB = 90 K, Jo/kB = -36 K, and J /kB = 125 K, so as to reproduce the spin-crossover transition temperature Tc = 48 K, the virtual Jahn-Teller transition temperature rJT = 6 = 26 K, and the effective LS-HS gap in the LS phase Acff/kB = 340 K. (Note Aeff is approximated by A + 2Jx in this mean-field model.) This choice of model parameters gives a phase sequence from the LS to HS with increasing temperature, corresponding to the arrow path in Fig. 7. Temperature dependence of thermodynamic quantities (Fig. 8) is calculated along the path indicated by the arrow in Fig. 7, where the discontinuities arising from the first-order spin-crossover transition are recognized Ap0 = 0.99, AH = 0.64 kJ mol-1, and AS = 13.3 J K-1 mol-1 These theoretical... 

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. 

Spin-crossover phase transition of a manganese(IU) complex [Mn(taa)] was studied by variable-temperature laser Raman spectroscopy and it was found that the vibrational contribution in the transition entropy is not dominant in contrast to the cases of ordinary iron spin-crossover systems. The discovery of a dynamic disorder in the HS phase by means of dielectric measurements provided an alternative entropy source to explain the thermally induced spin-crossover transition. This dynamic disorder was attributed to the reorienting distortion dipoles accompanying the E e Jahn-Teller effect in HS manganese(III) ions. 

Figure 9.12 shows the crystal structure of [Fe(pyrazine) Pt(CN)4 ] [13]. This complex shows a thermally induced spin-crossover transition (Tct = 284 K, T l = 308 K) with a thermal hysteresis of 24 K, which was observed by means of magnetic susceptibility measurement and Raman spectroscopy. The spin-crossover transition has been confirmed by Fe Mossbauer spectroscopy [13]. The Mossbauer spectrum at 300 K in the cooling mode consists of a single doublet with quadrupole splitting (QS) of 1. 159(5) mm s and isomer shift (IS) of 1.047(3) mm s whose values are typical of the HS state ( T2g, S = 2) of Fe(ll). At 80 K, a new doublet with quadrupole splitting of 0.306(4) mm s and isomer shift of 0.439(2) mm s whose values are typical of the LS state ( A g, 5 = 0) of Fe(ll). The photoinduced spin conversion between the LS and HS states around room temperature has been confirmed by means of Raman spectroscopy within the thermal hysteresis loop of spin-crossover transition, which is shown in Fig. 9.13 [13]. In this complex, the frequency of...

J. C. Green, J. N. Harvey, R. Poll. Theoretical investigation of the spin crossover transition states of the addition of methane to a series of Group 6 metallocenes using minimum energy crossing points. /. Chem. Soc., Dalton Trans., (2002) 1861-1866. 

Probably the most important experimental work which confirms the cooperative nature of the spin crossover transition is the heat capacity measurements on Fe(phenanthroline)2(NCS)2 and Fe(phenanthroline)2-(NCSe)2 by Sorai Seki (1974). The variation in the molar heat capacity of the latter compound with temperature is shown in Figure 3.27. Not only does the heat capacity show a sharp peak at the transition temperature (see Table 3.9), but there is also a change in the heat capacity, for the two different spin states (see Figure 3.27) at the transition. These authors propose that the total heat capacity of each spin state is made up of contributions from lattice vibrations, intramolecular vibrations, and electron thermal excitation. From this they have determined the changes in enthalpy and entropy associated with the change in spin state at the crossover. The results of these calculations are given in Table 3.9 and they... 

Editor (with H.A. Goodwin) of the series of Spin Crossover Transition Metal Compounds, 3 volumes (Nr. 233, 234, 235) in Topics in Current Chemistry (Springer), 2004... 

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