Octahedral complexes Hydrolysis

RATES OF HYDROLYSIS REACTIONS OF SOME OCTAHEDRAL COMPLEXES AT 25 °C... 

These parameters often parallel one another since they are related to similar characteristic of the system (ehange in number of particles involved in the reaction etc.). The catalyzed hydrolysis of CrjO by a number of bases is interpreted in terms of a bimolecular mechanism, and both AS and AK values are negative. In contrast the aquation of Co(NH2CH3)5L (L = neutral ligands) is attended by positive AS and AK values. The steric acceleration noted for these complexes (when compared with the rates for the ammonia analogs) is attributed to an mechanism.There is a remarkably linear AK vs AS plot for racemization and geometric isomerization of octahedral complexes when dissociative or associative mechanisms prevail, but not when twist mechanisms are operative (Fig. 2.15). For other examples of parallel AS and AF values, see Refs. 103 and 181. In general AK is usually the more easily understandable, calculable and accurate parameter and AK is... 

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. 

Hydroxide ion also may have an appreciable effect on the rate of hydrolysis of octahedral complexes. The rate constant for hydrolysis of [Co(NH3)5CllJ in basic solution is a million times that found for acidic solutions. Furthermore, the reaction is found to be second order and dependent on tbe hydroxide ion concentration ... 

Complexation228 of Zn2+ with the quadridentate 1,5-diazacyclooctane-N, A -diacetic acid (H2L) leads to formation of the trigonal prismatic [Zn(L)H20] species. The hydrolysis constant of the coordinated H20 (pKa = 8.6) is much lower than that of the coordinated H20 in the octahedral complex formed by ethylenediaminediacetate (pXa = 10.5). This increased acidity of coordinated H20 with decreasing coordination number has also been previously noted in the [Zn(Me6tren)H20]2+ cation,229 and has been attributed in part to the hydrophobic environment provided by the Me groups. This finding has important implications for the role of the... 

The structure of titanium complexes affects the formation of hydrated titanium dioxide structure, since rutile and anatase lattices are composed of TiO octahedrons connected in definite manner. The formation of anatase structure occurs when two octahedral complexes form a common vertex. When two octahedrons are united via their edges, rutile structure is formed. Based on this assumption, it is considered that if titanium (IV) complexes with one reactive centre are formed during hydrolysis, anatase structure is formed if there are two reactive centres, then rutile structure is formed. 

Hydroxide ion also may have an appi eciable effect on the rate of hydrolysis of octahedral complexes. The rate constant for hydrolysis of in basic... 

The bimolecular racemization of ethylenedinitrilotetraacetatocobalt-ate(III) 29j 36) should be noted. This racemization suggests that bimolec-ularity should not be excluded in mechanistic considerations of octahedral complexes. Base hydrolysis studies of other complexes without ionizable protons would be of considerable value, provided they are of intermediate field strength. 

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 formal similarity of these ruthenium and osmium compounds to the octahedral Group VI carbyne compounds would lead to an expectation of reactivity towards nucleophiles. In fact the neutral octahedral complexes prove to be rather unreactive compounds but the cationic complexes do react with nucleophiles. Mention has already been made of the hydrolysis of [Ru(=CPh)ClI(CO)(PPh3)2]I to RuPhCl(CO)2g Ph3)2. A very clear-cut example is provided by the reaction of [0s(sCR)Cl2(CNR )(PPh3)2]C104 (R=p-dimethylaminophenyl, R =p-tolyl) with NaSH. 

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... 

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