Spin forms

Figure 5 shows the spin density distribution of the low-spin form of C54. It is interesting to note that also here the spin density is almost entirely localized on the cartx>n atom that would carry the dangling bond in a simple valence orbital picture. 

The energy calculations show that the low spin state of C54 is indeed about 1.3 eV lower in energy than the high spin state, at least for the molecular geometries used here, whereas for C24 nearly identical energies for the two states are found. Inclusion of electron correlation would favor the low spin form further, possibly... 

Figure 2. Spin-density populations for the high-spin form (S=6) of Co...

Figure 3. Net atomic and overlap populations for the low-spin form of C54.

Figure 5. Spin density distribution in the low-spin form of C...

In solution, [Co(terpy)2]2+ is also in a high-spin/low-spin equilibrium. Ultrasonic absorption measurements determined the spin equilibrium relaxation time in both water and MeOH solution to be less than 2 ns.249 Electron-donating functional groups such as methoxyl appended to the terpy ring result in a shift towards the high-spin form of the complex,250 as does replacement of one pyridyl ring with a pyrazole.251... 

The high-spin/low-spin interconverison in a Ni11 complex of the cyclam derivative (639) bearing a luminescent naphthalene substituent has been used as a fluorescent molecular thermometer.161 The Ni11 tends to quench fluorescence of the proximate naphthalene subunit, but the two spin states exert a different influence on the emission properties. Emission is temperature dependent, since the high spin —> low spin conversion is endothermic, i.e., a temperature increase favors formation of the low-spin form. 

Brautigam, D.L., Feinberg, B.A., Hoffman, B.M., Margoliash, E., Peisach, J., and Blumberg, W.E. 1977. Multiple low spin forms of the cytochrome c ferrihemochrome. EPR spectra of various eukaryotic and prokaryotic cytochromes c. The Journal of Biological Chemistry 252 574-582. 

The discussion above has been directed principally to thermally induced spin transitions, but other physical perturbations can either initiate or modify a spin transition. The effect of a change in the external pressure has been widely studied and is treated in detail in Chap. 22. The normal effect of an increase in pressure is to stabilise the low spin state, i.e. to increase the transition temperature. This can be understood in terms of the volume reduction which accompanies the high spin—dow spin change, arising primarily from the shorter metal-donor atom distances in the low spin form. An increase in pressure effectively increases the separation between the zero point energies of the low spin and high spin states by the work term PAV. The application of pressure can in fact induce a transition in a HS system for which a thermal transition does not occur. This applies in complex systems, e.g. in [Fe (phen)2Cl2] [158] and also in the simple binary compounds iron(II) oxide [159] and iron(II) sulfide [160]. Transitions such as those in these simple binary systems can be expected in minerals of iron and other first transition series metals in the deep mantle and core of the earth. 

For five-membered heterocycles other than thiazole, (such as pyrazole [27], imidazole [28], and triazole [29]) the effect of replacement of just one pyridine moiety in 1 is greater and the [Fe N6]2+ derivatives in these instances show crossover behaviour. The [Fe N6]2+ derivative of 2-(pyridin-2-yl)imidazole 19 (Dq(Ni2+) 1150 cm-1 [22]) was shown relatively early on to be a crossover system [28]. In solid salts and in solution the transition is continuous and centred above room temperature. The dynamics for the 5T2— Ai relaxation for this system have been investigated by a number of techniques [30-32] and Beattie and McMahon have shown that in solution there is not only a spin equilibrium but also a ligand dissociation process, very reasonably ascribed to the high spin form of the tris complex [32]. 

The observation of a single set of resonances in the NMR spectra of [Fe(HB(pz)3)2], spectra that are clearly obtained for a mixture of the high-spin and low-spin forms of the complex, indicates that the equilibrium between the two states is rapid on the NMR time scale [27]. Subsequent solution studies by Beattie et al. [52, 53] using both a laser temperature-jump technique and an ultrasonic relaxation technique have established that the spin-state lifetime for [Fe(HB(pz)3)2] is 3.2xl0 8 s. These studies also established... 

Observation by NMR of both the high-spin and low-spin forms of a complex in solution is unusual. As outlined above [27], [Fe(HB(pz)3)2] shows only averaged spectra upon cooling to 243 K. Given that the two spin states differ in their solid state Fe-N bond distances by ca. 0.2 A, slow exchange is expected. 

The Fe(II) complex of this ligand shows crossover behavior both in solution and in the solid state. The complex has a distinct green color derived from the ligand field transition 2=620 nm) of the low-spin form of... 

The structure of [Fe(bzpa)2]C104 has been determined for the low spin form at 140 K, as well as for the high spin form at 290 K (Table 3) [134]. The gradual spin transition is complete as has been confirmed by the time-aver-... 

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