High spin states

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... 

The soluble products are able to form charged high-spin states after chemical and electrochemical oxidation. The high-spin character is the result of the lack of conjugative interaction between the highly distorted, orthogonally arranged aromatic subunits (decoupled rr-systems) [68]. 

K+ is generally used as the reversibly intercalating ion since it leads to insoluble compounds for all the forms. In the mixed-valence Prussian Blue compound, Fe is in the high-spin state and coordinated octahedrally with the N ends of the cyaiudes, whereas Fe is low-spin and octahedrally coordinated with the C ends of the... 

Relaxation of Thermally Quenched or Optically Excited High-Spin States Resulting... 

The first test case was the ferrous high-spin state (Fe, S = 2) in the picket-fence porphyrin acetate complex [Fe(CH3COO)(TPpivP)] [13, 23], which is a model for the prosthetic group termed P460 of the multiheme enzyme hydroxyl-amine oxidoreductase from the bacterium Nitrosomonas europeae. Both the picket-fence porphyrin and the protein P460 exhibit an extraordinarily large quadrupole splitting, as observed by conventional Mossbauer studies [56]. 

In NiFe204, an inverse spinel Fe +[Ni2+Fe3+]004, the spins of the octahedral sites are parallel with one another the same applies to the tetrahedral sites (Fig. 19.8). The interaction between the two kinds of sites is mediated by superexchange via the oxygen atoms. High-spin states being involved, Fe3+ (d5) has five unpaired electrons, and Ni2+ (ds) has two unpaired electrons. The coupled parallel spins at the octahedral sites add up to a spin of S = + =. It is opposed to the spin of S = of the Fe3+ particles at the tetrahedral sites. A total spin of S = 1 remains which is equivalent to two unpaired electrons per formula unit. 

MnAs exhibits this behavior. It has the NiAs structure at temperatures exceeding 125 °C. When cooled, a second-order phase transition takes place at 125 °C, resulting in the MnP type (cf. Fig. 18.4, p. 218). This is a normal behavior, as shown by many other substances. Unusual, however, is the reappearance of the higher symmetrical NiAs structure at lower temperatures after a second phase transition has taken place at 45 °C. This second transformation is of first order, with a discontinuous volume change AV and with enthalpy of transformation AH. In addition, a reorientation of the electronic spins occurs from a low-spin to a high-spin state. The high-spin structure (< 45°C) is ferromagnetic,... 

Note In this table all metal ions are in high-spin states and liganding atoms are small O, N donors. S-donors favour lower co-ordination numbers. Ligand-field theory, that is polarisation of and binding by the core electrons and orbitals of the metal ion compounds, can explain the above observations see inorganic chemistry textbooks in Further Reading . 

EXAFS (Extended X-ray Absorption Fine Structure) measurements using synchrotron radiation have been successfully applied to the determination of structural details of SCO systems and have been particularly useful when it has not been possible to obtain suitable crystals for X-ray diffraction studies. Perhaps the most significant application has been in elucidating important aspects of the structure of the iron(II) SCO linear polymers derived from 1,2,4-triazoles [56]. EXAFS has also been applied to probe the dimensions of LIESST-generated metastable high spin states [57]. It has even been used to generate a spin transition curve from multi-temperature measurements [58]. 

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. 

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