Octahedral substitution

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... 

Complexes of Ir(III) are kineticaHy inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of prBr(NH3)3] " [35884-02-7] at 298 K has been measured at 2 x 10 ° (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid... 

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

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

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. 

We alluded earlier to the variety of structural modifications which may he observed in sheet silicates. Clearly it is a matter of considerable in jortance to he able to determine if, for example, the aluminium content within a clay arises p a ely from octahedral substitution (as in montmorillonite) or whether there is some tetrahedral component (as in heidellite). a1 MASNMR readily provides the necessary answers. Figvire 1 illustrates the a1 spectrum for a synthetic heidellite material with Na as charge balancing cation. Aluminium in two distinct chemical environments is observed, with chemical shifts corresponding to octahedrally and tetrahedrally co-ordinated aluminium. 

Exchange of organic ammonium cations. Exchange selectivity of monovalent alkyl ammonium cations in montmorillonites (40-41) and octahedrally substituted synthetic clay minerals (laponite) increases with their chain length (42) and along the series... 

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). 

There have been few studies of substitution in complexes of nickel(II) of stereochemistries other than octahedral. Substitution in 5-coordinated and tetrahedral complexes is discussed in Secs. 4.9 and 4.8 respectively. The enhanced lability of the nickel(II) compared with the cobalt(II) tetrahedral complex is expected from consideration of crystal field activation energies. The reverse holds with octahedral complexes (Sec. 4.8). 

Complexes of (( Ir(III) are kinetically inert and undergo octahedral substitution reactions slowly. The rate constant for aquation of [IrBr(NH3)5]2+ [35884-02-7] at 298 K has been measured at -2 x 10-10 s-1 (168). In many cases, addition of a catalytic reducing agent such as hypophosphorous acid greatly accelerates the rate of substitution via a transient, labile Ir(H) species (169). Optical isomers can frequently be resolved, as is the case of ot-[IrCl2(en)2]+ [15444-47-0] (170). Ir(III) amine complexes are photoactive and undeigo rapid photosubstitution reactions (171). Other iridium complexes... 

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... 

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 ... 

The maximum amount of Al3+ tetrahedral substitution that 2 1 clays minerals formed at low temperatures can accommodate appears to be 0.80—0.90 per four tetrahedra. While this appears to place an upper limit on the amount of R3+ octahedral substitution, it is not clear why the limit should be such a low value. The dioctahedral smectites can accommodate more substitution (R2 + for R3+) in the octahedral sheet than can the dioctahedral micas. The reverse situation exists for trioctahedral equivalents. In the latter clays octahedral R3+ increases as tetrahedral Al increases. Thus, as one sheet increases its negative charge, the other tends to increase its positive charge. This is likely to introduce additional constraints on the structure. In the dioctahedral clays substitution in either sheet affords them a negative charge and substitution in one sheet is not predicted by substitution in the other sheet thus, one might expect more flexibility. 

Fig.29. Ternary plot of tetrahedral R3+ and octahedral R3+ for 275 dioctahedral 2 1 clay minerals. The layer charge is assumed to be due to tetrahedral and octahedral substitution. The octahedral sheet is assumed to have 2.00 cations per 0,0 (OH),. = average.

Hydrated clay surfaces are acidic. When isomorphic substitution occurs in the tetrahedral layer, acid leaching or NH thermal decomposition may generate acidic surface OH. For clays whose negative charges are produced by isomorphic substitutions in the octahedral layer, mild dehydration removes the source of acidity, because of the reversibility of reaction (3). Deamination of the ammonium exchanged clay with octahedral substitution drives protons into the octahedral layer, as evidenced by the lowered temperature at structural dehydroxylation. 

OCTAHEDRAL SUBSTITUTION REACTIONS. LABILE AND INERT COMPLEXES... 

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. 

Ingold, C. K., Nucleophilic Octahedral Substitution, in Theoretical Organic Chemistry, Kekule Symposium, London, 1958, 84-102, Butter worth s Scientific Publications, London, 1959. 

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... 

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

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