High-spin complexes Fe

The EPR-silent [Fe(Papy3)(NO)] complex is very stable, allowing for the preparation of crystaUine solids. The NO-release represented by reversing reaction (10) is not available thermally, but it becomes possible upon illumination (21). In a similar way, the compoimd [Fe(bpb)(N0)(N02)] complex was synthesized upon bubbhng NO into an oxygen-free acetonitrile solution of the low-spin [Fe (bpb)(py)2] ion (a nonheme planar analog of the iron(III)-porphyrinate) (57). Notice that the high-spin complex [Fe (bpb)(Cl)2] did not afford any isolable nitrosyl, a fact that has been employed to propose that the spin state of the iron center of the precursor complex dictates its affinity toward NO. 

Tris(2,4-pentanedionato)iron(III) [14024-18-1], Fe(C H202)3 or Fe(acac)3, forms mby red rhombic crystals that melt at 184°C. This high spin complex is obtained by reaction of iron(III) hydroxide and excess ligand. It is only slightly soluble in water, but is soluble in alcohol, acetone, chloroform, or benzene. The stmcture has a near-octahedral arrangement of the six oxygen atoms. Related complexes can be formed with other P-diketones by either direct synthesis or exchange of the diketone into Fe(acac)3. The complex is used as a catalyst in oxidation and polymerization reactions. 

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]. 

Thus, for example, high-spin d5 Fe(m) in an axial ligand field should show a resonance around g = 2 and another resonance near g - 6 when B is perpendicular to the symmetry axis. In solution, where the complex tumbles rapidly and averages the g-values, the resonance is expected to be so broad as to be undetectable, but, in frozen solution, both resonances should be observable. 

As was mentioned above, the [Fe(HB(3,5-(CH3)2pz)3)2] complex represents a classic example [27, 28] of an iron(II) spin-state crossover that may be induced in a high-spin complex upon cooling. The room temperature crystal structure of this complex [26] reveals a structure rather similar to that of [Fe(HB(pz)3)2], but with a substantially longer average iron-nitrogen bond... 

The complex [Fe 24](C104)2.xH20 (0high-spin at room temperature with an average Fe-N distance of 2.25 A. All the Fe-N distances are very similar. The complex exhibits spin crossover in the temperature range 150-250 K. Above 300 K the water solvate is lost, forming a purely high-spin complex. 

The spin state of iron(II) in substituted 2,2 6, 2"-terpyridine, (72), complexes depends on the position of bulky substituents—phenyl substituents in both the 6 and 6" positions give a high-spin complex phenyl substituents in the 4 and 6 positions give a complex which exists both in high-spin and in low-spin forms. Crystal structure determinations gave Fe—N bond distances in both forms of the 4,6-diphenyl complex and of the 6,6"-diphenyl complex. Each ligand in the latter com )lex has one terminal pyridine very weakly bonded to the iron, with... 

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