Iron complexes octahedral

To illustrate the effect of geometry on the Cl shift. Table IV shows some data for several pairs of high spin tetrahedral and octahedral iron complex ions. In each case the shift of tetrahedral ion is more negative than that of the octahedral ion, and the difference in shift is rather substantial. We feel that in these cases the differences in shifts can be ascribed to differences in 4 covalency six electronegative ligands are more able to draw off 5 electron density than are four. Apparently it makes little difference whether the six ligands belong exclusively to a... 

The available evidence thus suggests that relaxation times for planar-tetrahedral equilibria in nickel(II) complexes in solution at room temperature fall in the range 0.1-10 /isec, corresponding to rate constants of the order 105-107 sec-1. These relaxation times are several orders of magnitude longer than those observed for octahedral spin equilibria. The reaction coordinate for the planar-tetrahedral equilibria is characterized by large enthalpies of activation for the reaction in both directions, in contrast with a relatively low enthalpy of activation for the high-spin to low-spin process in octahedral iron complexes. 

There have been no reports of complexes of " JV-substituted thiosemicarbazones derived from 2-formylpyridine, but 2-acetylpyridine JV-methyl-thiosemicarbazone, 3a, formed [Fe(3a-H)2]C104 and [Fe(3a-H)2]FeCl4 [117]. The nature of these two species was established by partial elemental analyses, molar conductivities, magnetic moments, electronic, infrared, mass and electron spin resonance spectra. A crystal structure of a related selenosemicarbazone complex confirmed the presence of a distorted octahedral iron(III) cation coordinated by two deprotonated anions so that each ligand is essentially planar and the azomethine nitrogens are trans to each other the pyridyl nitrogen and selenium donors are both cis. 

The rate of the spin state change for the octahedral cobalt(II) complexes is expected to be faster than that observed for the iron(II) and iron(III) complexes. In the cobalt(II) case the spin state change involves only one electron, that is AS = 1. The 2E and 4T states are directly mixed by spin-orbit coupling (10, 163). The spin state transition should be adiabatic, with k = 1, without any spin-forbidden barrier. Furthermore, the coordination sphere reorganization involves a change in bond length of 21 pm along only two bonds, instead of all six bonds as in iron complexes. Both of these factors lead to the prediction of rapid spin state interconversion. 

The dynamics of spin equilibria in solution are rapid. The slowest rates are those for coordination-spin equilibria, in which bonds are made and broken even these occur in a few microseconds. The fastest are the AS = 1 transitions of octahedral cobalt(II) complexes, in which the population of the e a antibonding orbital changes by only one electron these appear to occur in less than a nanosecond. For intramolecular interconversions without a coordination number change, the rates decrease as the coordination sphere reorganization increases. Thus the AS = 2 transitions of octahedral iron(II) and iron(III) are slower than the AS = 1 transitions of cobalt(II), and the planar-tetrahedral equilibria of nickel(II) are slower again, with lifetimes of about a microsecond. 

The spin equilibria of octahedral iron(II) complexes are the best studied examples in both the solid state and in solution. Both the low-spin lA and the high-spin 5T states are regular octahedra, so the... 

X-ray structural analysis of 2,2-dimethyl-3-phenyl-l-methylenecyclopropane tungsten pentacarbonyl reveals an octahedral complex with characteristic W—C bond distance of 238 pm. The typical bond distances within the organic ligand are 138 (complexed C=C), 148 (proximal C—C), 154 (distal C—C) pm, compared e.g. with 140, 148 and 154 pm, respectively, for the Feist s ester iron complex analogue (see above). 

Iron as a cofactor in catalysis is receiving increasing attention. The most common oxidation states of iron are Fe2+ and Fe3+. Iron complexes are nearly all octahedral, and practically all are paramagnetic (as a result of unpaired electrons in the 3d orbital). The most common form of iron in biological systems is heme. Heme groups (Fe2+) and hema-tin (Fe3+) most frequently involve a complex with protoporphyrin IX (fig. 10.19). They are the coenzymes (prosthetic... 

The best known example is enterobactin (otherwise called enterochelin), which is produced apparently by all enteric bacteria. It has three 2,3-dihydroxybenzoyl groups attached to a macrocyc-lic lactone derived from three residues of L-serine condensed head-to-tail. The structures of enterobactin and its iron complex are shown in Figure 45, which shows that the iron is bound by six phenolate oxygen atoms in an octahedral environment. Enterobactin has the highest known affinity for Fem, with log K = 52 at pH 7.4.1182 The iron(III) complex can exist as isomeric forms, which may be associated with selectivity in binding to the receptor site. 

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