Octahedral stereochemical changes

Stereochemical changes during octahedral substitution reactions. R. D. Archer, Coord. Chem. Rev., 1969, 4, 243-272 (148). 

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. 

C tereochemistry has played a major role in the development of chemistry, and it continues to be most significant. Werner made extensive use of the information available to him on the stereochemistry of metal complexes in developing his coordination theory. He made the first meaningful attempt to understand the mechanisms of substitution reactions of these systems on the basis of the stereochemical changes accompanying such reactions. The paper 49) he wrote in 1912 is a real milestone and should be read by anyone interested in octahedral substitution reactions. It is valuable because of the large amount of experimental data it contains on reactions of cis and [Pg.408]

Although Werner s interpretation of the cause of stereochemical changes during substitution and how they take place adequately explains the experimental facts, it has almost no predictive value. This is not intended as a criticism for we will see in the discussion which follows that we are still largely unable to predict the steric course of a substitution reaction of an octahedral metal complex. This is true despite the availability of much more experimental data and more sophisticated theories of bonding. Excellent reviews (27) have been written on the stereochemistry of octahedral substitution reactions. The discussion that follows deals almost exclusively with cobalt (III) complexes, but the principles involved are generally applicable to other octahedral systems. 

Before presenting some experimental observations on stereochemical changes accompanying octahedral substitutions, it is desirable to consider some of the changes that appear to be theoretically possible. In order to do this, it is convenient to consider what may happen for two types of mechanisms (1) dissociation (S Ulim) or SatI) with a decrease in coordination number and (2) displacement (SAr2(lim) or Sat2) with an increase in coordination number. 

Stereochemical changes taking place in the course of substitution reactions of octahedral complexes have been reviewed. An improved method for estimating crystal field activation energies has been described. ... 

The occurrence of stereochemical change is quite common in octahedral systems, and some examples are summarised in Table 3.4. 

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