Of octahedral substitution

Considerable investigation of the octahedral carbonyl complexes has been carried out. To a certain degree this is because definitive evidence for associative substitution in the case of type A complexes has been conspicuously lacking whereas for the type B compounds there seem to be several well-substantiated examples. A general summary of the main types of octahedral substitutions which have been kinetically examined is given in Table 15. 

It is a matter of some significance catalytically to be able to ascertain whether the aluminum present in a natural clay or its synthetic analogue is in a state of octahedral substitution (as in montmorillonite) or whether there is some tetrahedral substitution (as in beidellite). 27A1 MAS NMR readily provides the necessary answers. For example, Diddams et at. (462) in a study of the synthesis, characterization, and catalytic performance of synthetic beidellites and their pillared analogues, monitored the fate of AI from the gel precursor to the sheet silicate and to its pillared state by 27A1 MAS NMR (see... 

Another pathway that is important in the discussions of octahedral substitution and distinct from interchange is considered below. In this pathway the encounter with the entering ligand does not precede but follows the slow step. The scheme of the pathway may be written as... 

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

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 considering the results of previously studied complexes, it would seem wise to consider a general scheme for classifying the products of octahedral substitution reactions in general. 

To state categorically that isomorphous substitution in the montmorillonites concerns only the octahedral layer would be misleading. Some tetrahedral replacement could and does occur, but for most minerals within this class the observed exchange capacity correlates closely with the degree of octahedral substitution. 

It was clear in the 1950s that there was a need for detailed kinetic studies of ligand substitution reactions of platinum(ii) complexes, and our laboratory was prepared to do this because it was engaged in such studies of octahedral substitution. However, only a brief account of our studies is given in this article. At about this time Martin and his students initiated their investigations of aquation reactions of chloroammineplatinum(ii) complexes. 

This promise has been only partially fulfilled because of the difficulty of interpreting anation mechanisms where second order kinetics, first order in entering anion and first order in complex, are often found because of ion association which contributes a term in anion concentration to the rate law. A further difficulty, emphasised by Archer in his recent review on the stereochemistry of octahedral substitution reactions, is found in cobalt(III) chemistry because of the difficulty in isolating trans solvent-containing species. This results in continued doubt in the study of such systems as ... 

The majority of octahedral substitution reactions are believed to occur by a D or Ij type mechanism, although it can be difficult to distinguish between these two mechanisms based on the similarity of their rate laws in the absence of the observation of any intermediates. In general, the rates of dissociative reaction mechanisms are largely independent of the nature of Y, increase with steric bulk around the metal ion of the overall coordination, show a positive AS- (as there are more species in the intermediate than in the reactants), and exhibit a positive sign for AV. ... 

The classic studies of octahedral substitution reactions involved Co(lll) ammine compounds, such as the example already shown previously in Equation (17.11). For this particular class of compounds, the experimental data yielded the following results ... 

MECHANISM OF OCTAHEDRAL SUBSTITUTIONS ON TRANSITION METAL COMPLEXES,... 

Other aquation reactions give further evidence concerning the most favored mechanism of octahedral substitution reactions. Consider the data for the aquation of various bidentate amine complexes of cobalt(III) as shown in Equation (5.33) and Table 5.3 ... 

S. Asperger, Mechanism of Octahedral Substitutions on Transition Metal Complexes. Attempts to Distinguish between D and f Mechanisms, in Advances in Solution Chemistry, Plenum Press, New York 1981, pp. 105-114. 

An inert metal ion must have at least one electron in the t2g orbital, and none in the eg orbital. Such ideas about labile and inert complexes can explain many experimental data of the rates of octahedral substitutions. Thus, the low spin complexes of chromium(III) (t2g ), cobalt(III) (t2g ), iron(II) (t2g ), and iron(III) (t2g ) are inert. This approach explains why vanadium(III) (t2g ) is more labile than vanadium(II) (t2g ). 

All research carried out on the mechanisms of octahedral substitutions speaks in favor of the dissociative mechanism limiting rates at high concentrations of entering ligands, independence of the reaction rates of the nature of entering ligands, increase of reaction rates with steric crowding of constituents. However, it has been established that the reaction ... 

---