Octahedral Substitution Formation

Any detailed description of the mechanism of an octahedral substitution must also account for the stereochemical changes that accompany reaction. Werner recognized this and made use of it in his discussions of the stereochemistry of reactions of cobalt(III) complexes. The available experimental results can be explained on the basis of possible molecular rearrangements and some cautious predictions can even be made. The base hydrolysis of cobalt III)ammines appears to be unique in that it often occurs with rearrangement it also affords the few known examples of optical inversion. These results can be explained by formation of a 5-coordinated species with a trigonal bipyramidal structure. Optically active metal complexes racemize by either an intramolecular or an in-termolecular process. Substitution reactions of platinum metal complexes often occur with retention of configuration. 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

As with substitution in octahedral complexes, in chelation in square planar complexes the formation of the first bond is usually rate-determining (see (4.101)). 

Minnesotaite resembles talc with Fe substituting for some of the Mg the octahedral sheet (Fe, Mg)3Si40 o(OH)2. Fibers of this material are abundant in the rock formations mined for iron in Minnesota (Gruner, 1944)— hence the name. In its fibrous form, about three-fourths of the octahedral sites are occupied by Fe, rather than Mg. Although also a layer silicate, minnesotaite has a complex structure that is not yet fully understood. 

One of the interesting points connected with this type of coordination springs from the fact that nucleophilic substitution on phosphoranes probably occurs via the formation of an octahedral complex242-245. Further, it should be recalled that the mechanism of irregular stereomutations of pentacoordinated structures in basic media may also involve a hexacoordinated intermediate. Figure 26 shows just a few of the numerous and diverse structures known (196-203) but, even then, some general observations can be made. 

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