Spin-state isomerism

It has been pointed out that the well-known and genuine phenomenon of spin-state isomerism, where molecules of the same composition and general structure will often show significant metrical differences in different spin states, should not be confused with the alleged bond-stretch isomerism.16... 

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. 

The nodal planes used qualitatively to discuss cis/trans isomerization clearly appear in the ELF profiles. For AA and BB at the crossing point, the picture reveals the numerical artifact of the adiabatic calculations. At the respective attractors, AA and BB show the expected nodal plane distribution. The method is not capable to distinguish between singlet and triplet spin-state ELF. This issue was discussed during the meeting. The reader may find appropriate discussions in this volume. 

Although interconversions of the two spin states under matrix-immobilized spectroscopic conditions have not yet been observed in the cases of the pyrrole derivative 46f and the bis(phenoxyl) 53, reversible thermal isomerizations of the singlet and triplet states have been reported in the cases of the bis(aminoxyl) 54 and the carbene 55. 

Nickel(II) complexes display a variety of equilibria which involve spin state changes. Planar four-coordinate complexes are invariably diamagnetic. These can undergo an intramolecular isomerization to paramagnetic tetrahedral four-coordinate species. Alternatively, the planar complexes can coordinate additional ligands to form five- and six-coordinate paramagnetic complexes. The additional ligand molecules can be Lewis bases in solution, or solvent molecules, or, in par-... 

If the electronic spin state change were the critical determinant of the dynamics of spin equilibria, then the AS = 1 equilibration between planar and tetrahedral nickel(II) isomers would occur more rapidly than equilibration of the octahedral AS = 2 spin states. This is not observed. Even though the AS = 1 transition is most likely adiabatic, the large coordination sphere reorganization energy requirement causes these nickel(II) isomerizations to occur relatively slowly, with relaxation times of the order of a microsecond. 

Only a few examples of these planar tetrahedral equilibria have been successfully investigated and there remain unanswered questions about the dynamics. Nevertheless, it seems reasonable to regard these equilibria as intramolecular isomerizations in which the spin state change is not an important factor in the dynamics. Electron spin affects the bonding, geometry, and thermodynamics of the two isomers, but there is apparently sufficient mixing of the singlet and triplet states to allow their interconversion to be adiabatic. 

Just as expansion of the coordination sphere by transition from the low-spin to the high-spin state enhances isomerization and racemiza-tion, so too should it enhance the rate of ligand substitution. Unpublished observations on the ligand substitution of [Fe(pyim)3]2+, which is coupled with its spin equilibrium, indicate that it is the high-spin state which preferentially undergoes the substitution reaction, as expected (117). 

A clear case of cyanide linkage isomerism was established for the pentanuclear TBP complex [Fe tmphen)2]3[Cr (CN)g]2 (174a). Unlike the similar complexes [Fe°(tmphen)2l3 [M ° CN)g]2 (M = Co or Fe), which exhibit temperature induced transitions between the high- and low-spin state at the equatorial Fe(II)... 

A is exothermic. An additional advantage is that one has a wider margin to select spin-state specific photochemical transformations. Representative examples include ds-trans-olefin isomerization (Eq. 49), olefin dimerization (Eq. 50), oxetane formation (Eq. 51), dienone rearrangement (Eq. 52), di-vr-methane rearrangement (Eq. 53), azoalkane denitrogenation (Eq. 54a, b), and photocyclization (Eq. 55). 

For diiron complexes Mossbauer spectroscopy allows to asses (1) oxidation and spin states of the iron atoms, (2) diamagnetism and ferromagnetism of the groimd state for diferric and mixed-valent oxidation levels and (3) valence (de)localisation in the solid state for mixed-valence complexes [2,3]. Isomer shifts (IS) in the range 0.35-0.60 mm/s are characteristic of 5- or 6-coordinate high-spin diferric p-hydroxo complexes [2,3], Tetrahedral high-spin ferric iron has lower isomeric shifts in the range of 0.22 mm/s [2,3]. For isolated ferric iron with S =... 

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