Octahedral substitution, mechanism

Octahedral substitution Mechanisms and reactive intermediates (Co111)... 

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

Activation parameters and reaction mechanism in octahedral substitution. T. W. Swaddle, Coord. Chem. Rev., 1914,14, 217-268 (231). 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

Most of the work on the kinetics and mechanism of aquation - the first step in octahedral substitution - has been done on cobalt(III) complexes, which are neither too inert nor too labile for exhaustive investigations. The aquation of Co(NH3)5X2+/3+ (the charge depends on whether X is neutral or anionic) has been studied in great depth. The rate law for such a process is found to take the form ... 

From the very limited evidence available, it appears that when an octahedral substitution proceeds via direct displacement, there is only a minor amount of d,l or cis-trans interconversion—that is, configuration is primarily retained. This is the case for a large number of conversions of the complexes of Pt(IV), and a smaller number of conversions involving complexes of Co(III), Cr(III), Rh(IlI). and Ir(III)—all of these may not involve the direct displacement mechanism, but some almost certainly do. Thus, we see a marked contrast to substitution reactions of tetrahedral carbon, where every act of displacement results in inversion of configuration. 

In conclusion, it seems that in MCM-1 besides the isomorphic substitution mechanism 2, which is found in SAPO s, mechanism 1 can also be operative. In MCM-1 synthesized with Pr4N-OH according to the latter mechanism A1 is replaced with Si. The presence of both octahedral and tetrahedral A1 in MCM-1 could be the reason for this unexpected behaviour. 

Substitution Reactions in Square Planar Complexes 538 Thermodynamic and Kinetic Stability 547 Kinetics of Octahedral Substitution 548 Mechanisms of Redox Reactions 557... 

Associative reactions are also possible in octahedral substitution, but are much less common. Table 12-5 gives data for both dissociative and associative interchanges for similar reactants. In the case of water substitution by several different anions in [Cr(NH3)5(H20)] ", the rate constants are quite similar (within a factor of 6), indicative of an Id mechanism. On the other hand, the same ligands reacting with [Cr(H20)6] show a large variation in rates (more than a 2000-fold difference), indicative of an mechanism. Data for similar Co(lll) complexes are not conclusive, but their reactions generally seem to have mechanisms. 

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]

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. 

Figure 1, A general scheme for octahedral substitution reaction products assuming a dissociative mechanism...

A schematic representation of the interchange mechanism for octahedral substitution. 

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