Iron complexes tetrahedral

By analogy with iron complexes, let us discuss the tetrachlorocobalt-ate(II) dianion [C0CI4]2-, the tetrahedral geometry of which is illustrated in Figure 80.123... 

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... 

Pearson, in elaborating upon these ideas, distilled the essence of the tr competition theory to two soft ligands in mutual irons positions will have a destabilizing influence on each other when attached to class (h) (soft) metal atoms. He also provided additional examples of the rule that symbiosis prevails in octahedral complexes. antisymbiosis in square planar complexes. Tetrahedral complexes are expected to show antisymbiosis but on a much reduced scale compared with the square planar complexes.92... 

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. 

However, in acetonitrile with the ratio PhS Fe 5 1, the mononuclear tetrahedral iron complex [Fe(SPh)4]2 is formed first, which reacts with sulfur to form the iron(III) dimer [Fe SPh),]2-according to equations (67) and (68). No further reaction takes place, but addition of methanol facilitates the reductive elimination of PhSSPh from the dimer to give the 2Fen2FenI cubane complex as in equation (69). Although this series of reactions has been written for PhS-, it appears similar reactions take place with alkyl as well as aryl thiolates. 

Fig. 25-E-4. The tetrahedral iron complex found in rubredoxin. The tetrahedron is very...

As a consequence of these points, the spectra of octahedral Mn high-spin and of tetrahedral iron complexes consist of a number of bands of width 500 cm with e 0.1 lying at fairly high energies [those for Mn(H20)6 " commence at 18000 cm ] and whose positions are not very sensitive to the details of the ligands, Probably because the chemical species present are not well defined in solution, the spectra for octahedral Fe complexes are diffuse and rarely interpretable, Because a number of bands are present for the other cases, it may be possible to deduce several of the ligand field and interelectronic repulsion parameters and even X, although the uncertainty in Dq may be substantial. 

H2ML4 (M = Fe or Ru L = a wide range of phosphines, phosphites, phos-phinites, and phosphonites) and have shown two types of complexes to exist by n.m.r. One type is present as the cis-isomer with essentially no trans-form present, the other type has both cis- and trans-isomers present in equilibrium. Barriers to rearrangement for the iron complexes are largely insensitive to steric and electronic changes in L and such rearrangements are discussed in terms of the tetrahedral jump model. ... 

In contrast to the analogous Ni and Co complexes Tp MCl that react with allyl magnesium bromide during the formation of Tp MCj/ -CsHs), Tp TeCl gives under the same reaction conditions the tetrahedral 14-electron j/ -allyl iron complex Tp Fe(fj -CH2CH = CH2). ... 

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