Spin equilibrium

In the final section of this chapter we briefly review some phenomena which, although involving no new concepts, nonetheless lead to some unexpected magnetic results. In Chapter 3 we met two forms of isomerism which have 

The literature on this subject tends to divide between the old, which provides the best available treatment of the classical (including the classical quantum mechanical), and the recent, which provides an up-dating of the earlier. Older are  

Problems in the effects of magnetic coupling are reviewed in Magnetochemistry a research proposal R. L. Carlin, Coord. Chem. Rev. (1987) 79, 215. 

A detailed account of magnetic phases and phase transitions at low temperatures is Magnetic phase transitions at low 

More recent, and concentrating on systems of two transition metal ions, is Magnetism of the Heteropolymetallic Systems O. Kahn, Struct. Bonding (1987) 68, 89. 

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Trivalent iron dithiocarbamate complexes have been extensively studied, because of the existence of "spin equilibria in these complexes. Table II outlines the tris(l,l-dithiocarbamate) iron(III) complexes and, some of their physical properties. 

Nickel(II) complexes of (505) exhibit spin equilibria in solution.1355 With the bidentate analogues (506), complexes [Ni(506)2] have been isolated.1356 When Rj = Ph, the complex is tetrahedral in solution. It has a temperature independent magnetic moment of 2.75pB- When R = Me, the complex exhibits square planar-tetrahedral equilibrium in solution. Both are, however, diamagnetic in the solid state. 

Beattie (1988) Dynamics of spin equilibria in metal complexes [223]. 

The study of the dynamics of spin-state changes is important for the understanding of the kinetics of bimolecular electron transfer reactions ° and racemization and isomerization processes (Sec. 7.5.1). Low spin — high spin equilibria, often attended by changes in coordination numbers, are observed in some porphyrins and heme proteins, although their biological significance is, as yet, uncertain. 

Magnetic orbitals, 38 430 direct exchange, 38 435 Magnetic resonance, spin equilibria dynamies investigation, 32 14-16... 

Mott transition, 25 170-172 paramagnetic states, 25 148-161, 165-169 continuum model, 25 159-161 ESR. studies, 25 152-157 multistate model, 25 159 optical spectra, 25 157-159 and solvated electrons, 25 138-142 quantitative theory, 25 138-142 spin-equilibria complexes, 32 2-3, see also specific complex four-coordinated d type, 32 2 implications, 32 43-44 excited states, 32 47-48 porphyrins and heme proteins, 32 48-49 electron transfer, 32 45-46 race-mization and isomerization, 32 44—45 substitution, 32 46 in solid state, 32 36-39 lifetime limits, 32 37-38 measured rates, 32 38-39 in solution, 32 22-36 static properties electronic spectra, 32 12-13 geometric structure, 32 6-11 magnetic susceptibility, 32 4-6 vibrational spectra, 32 13 summary and interpretation... 

A change in the spin state of a metal ion also can accompany a change in coordination number. Again, in some cases conditions may be established in which an equilibrium exists between two complexes with different coordination numbers and different numbers of unpaired electrons. Some of the concepts which are used to describe intramolecular spin equilibria can be extended to the description of these coordination-spin equilibria. Examples include equilibria among four-, five-, and six-coordinate nickel(II) complexes and equilibria involving coordination number changes in iron porphyrin complexes and in heme proteins. 

This article examines the dynamics of spin-equilibrium processes, principally from studies in solutions. The properties of the complexes which are relevant to the dynamics studies are first reviewed. Then the techniques used to observe these rapid processes are described. Some aspects of solid-state dynamics are mentioned. Finally, some implications for the description of intersystem crossing processes in excited states and for spin equilibria in heme proteins are described. 

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