Octahedral Substitution Base Hydrolysis

This chapter continues to be dominated by studies on cobalt(m) complexes, which are reported in the first section. A section on other oxidation state in studies is then followed by a consideration of other oxidation states. 

Studies of the hydrolysis of [Co(NOa)4(LL)] ions (LL=en or tmd) between pH 9.5 and 12.2 show that [Co(N02)3(OH)(LL)] ions are the products, and the reactions are believed to proceed via a Z)-mechanism (rather than iSNlcb) involving the five-co-ordinate intermediate [Co(N02)s(LL)]  

Acid and base hydrolyses of m-[Co(tmd)2(RNH2)Cl] + ions (R = H, Me, Ft, Pr , Bu , Bu, or PhCHa) proceed at ca. 10 times the rate for the corresponding CM-[Co(en)2(RNH2)Cl] + ions owing to a reduction in E by ca. 2.4 kcal moF. The ability of tmd molecules to twist more readily than en molecules in the trigonal-bipyramidal five co-ordinate transition state is invoked to account for this rate difference. The acid hydrolysis reactions are considered in Chapter 5 (ref. 48).  

Rate=A 2[Hgii][Co(en)2(RNH2Cl)2+], where k k K + k iK K [OH l Electrophilic catalysis by Hg3+ is more effective under basic conditions than under acidic conditions. The results are shown in Table 1. 

At 25 °C, the second-order rate constant for the base hydrolysis of cis-[Co(en)2(NH2R)Br]2+ ions (7=0.1 M NaC104) is for a range of 

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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. 

Since the early 1950 s when systematic studies of the substitution reactions of non-labile octahedral complexes were begun by Taube, Brown, Ingold and Nyholm and Basolo, there has been a preoccupation with features of aquation and base hydrolysis reactions. This work has been excellently summarised in all its features in a progressive series of and at least to this con-... 

The use of activation volumes in the diagnosis of mechanism has continued to provide much valuable information. Activation volumes for substitution at octahedral complexes have formed the subject of a well-referenced review,in which the importance both of intrinsic and of solvation contributions is recognized. The topics of most relevance to this chapter include isomerization and racemization reactions of cobalt(III) complexes, aquation of cobalt(III) and of iron(II) complexes, and base hydrolysis of cobalt(III) complexes. Merbach s continuing investigations into the effects of pressure on rates of solvent exchange at 2-h and 3+ transition metal cations, while not being always strictly... 

This chapter deals with substitution reactions, including aquation, base hydrolysis, formation, and ligand exchange and replacement, and isomerization of inert metal complexes in which the metal has a co-ordination number of five or more. In fact the great majority of the references reported are concerned with octahedral complexes references to complexes of other coordination numbers have been collected together at the end of this chapter (Section 10). 

Dr. Halpern This could be used in stabilizing, say an activated complex. The point about the hydrolysis observation is that this refers to the octahedral complex, whereas the explanations that have been offered for the effect of amide in the conjugate base mechanism are concerned, not with weakening of the binding, but with stabilizing a five-coordinated intermediate. I wondered if the role of the hydroxide in promoting water substitution might be of the same nature. 

Titanium-substituted silica-based molecular sieves, in particular TS-1 (MFI), have been the most intensively studied [6,7,9]. This generally involves controlled hydrolysis of a mixture of Si(OEt)4 and Ti(OEt)4 in the presence of the template, the tetrapropylammonium cation in the case of TS-1. Many workers have experienced problems in TS-1 synthesis and the various pitfalls haven been reviewed [9]. Small amounts of Na or orginating from commercial samples of the template suffice to prevent the substitution of Ti into the framework. Similarly, the presence of F leads to the formation of octahedral extra framework titanium. 

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