Substitution mechanisms octahedral complexes

Since we shall not obtain the comparable amount of detailed information on the mechanisms of substitution in octahedral complexes from the studies of more complicated substitutions involving chelation and macrocycle complex formation (Secs. 4.4 and 4.5) it is worthwhile summarizing the salient features of substitution in Werner-type complexes. 

S213 As discussed in Section 21.6, the Eigen-Wilkins mechanism for substitution in octahedral complexes suggests the formation of an encounter complex [V(H20)6] Cl , in the first, fast step ... 

The mechanism may be associative (A or f) or dissociative (D or f), and it is not at all easy to distinguish between these, even though the rate laws are different. An associative mechanism involves a 7-coordinate intermediate or transition state and, sterically, an associative pathway seems less likely than a dissociative one. Nevertheless, activation volumes do sometimes indicate an associative mechanism (see Table 25.4). However, for most ligand substitutions in octahedral complexes, experimental evidence supports dissociative pathways. Two limiting cases are often observed for general reaction 25.22 ... 

Let us now look further at experimental trends that are consistent with dissociative (D or If) mechanisms for substitution in octahedral complexes is supported in very many instances. 

Although most substitutions in octahedral complexes involve D or pathways, we consider the stereochemical implications only of the D mechanism since this involves a 5-coordinate species which we can readily visualize (equation 25.36). 

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... 

B.23 Assuming no experimental complications, explain, in your own words, some ways in which one might expect to differentiate between an associative (A) and dissociative (D) mechanism for the substitution of octahedral complexes. 

Theoretical aspects of substitution in octahedral complexes have been discussed from a quantum mechanical viewpoint, and according to the role of symmetry. ... 

Mechanisms of substitution of octahedral cobalt(III) amine complexes. C. K. Poon, Inorg. Chim. Acta, Rev., 1970, 4, 123-144 (131). 

Several authors have suggested that the pathway may prove to be the most common mechanism in substitution reactions of octahedral complexes generally. However, the D path can be clearly demonstrated in some cases including at least two examples from Co(III) chemistry. The path (I - III - IV, Fig. 7) through the fivecoordinate intermediate would lead, in the case of rate studies in the presence of excess anionic ligand, to observed first-order rate constants governed by equation (13)... 

Under (i) the square and pyramidal complexes are often easier to substitute than the octahedral complexes for the obvious reason that they have open residual coordination sites, looking upon all the complexes as derived from an octahedron. The mechanism of substitution can then be the typical organic Sn2 attack. More usually the reactions of complex ions proceed by predissociation, SnI, so that the important consideration is that c and d should be at least relatively good leaving groups. 

Explain the difference with respect to the size of the neighboring groups on substitution in an octahedral complex by associative and dissociative mechanisms. 

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. 

Substitution reactions in octahedral complexes may proceed by D, Idt Ia, or A mechanisms and it is often difficult to distinguish between them because the rate law by itself does not allow the distinction to be made. Consider the replacement of water by ligand L under neutral conditions ... 

Because of the inertness of Co(III) and Cr(III) complexes, their substitution reactions were the first among those of octahedral complexes to be extensively studied. Most evidence supports the fd mechanism for substitution in Co(fll) complexes. First, there is little dependence of reaction rates on the nature of the incoming ligand, if bond making were of significant importance, the opposite would be expected. Data are presented in Table 13.4 for the anation reaction of penta-ammineaquacobaltdll) ... 

The addition and dissociation of pyridine and substituted pyridine molecules to a planar nickel(II) complex with a quadridentate N202 ligand have been studied by the microwave temperature-jump technique in chlorobenzene solvent (38). The data were interpreted with the assumption of mechanism C (Fig. 7), i.e., that k65 is the smallest rate constant. Subsequently, however, 14N NMR was used to measure the rate of pyridine exchange from the octahedral complex (138). The rates are the same for the two different experiments within a factor of two. This observation excludes mechanism B and is consistent with either mechanism A or C. The rate constants have consequently been presented in Table VI as k64. 

Although direct complex formation is observed kinetically (stopped flow) and spectrophotometrically, where X = Br or Cl, the reaction with I results in an oxidation of the halide. The reactions are rapid and there is the question of inner- or outer-sphere electron transfer, for the [14]aneN4 complex. However, further studies (140) using ligand substituted (dimethyl) complexes reveal that for the rac-Me2[14]aneN4 isomer, two processes are observed, k = 2.9 x 104 M-1 sec-1 and a subsequent redox step, krci = 5.5 x 103 M-1 sec-1, both of which are iodide dependent. The mechanism proposed involves the formation of an octahedral complex which further reacts with a second mole of I- in the redox step ... 

Figure 5-38. The prototypical ligand substitution reaction in octahedral complexes. In principle, the reaction could proceed by associative or dissociative mechanisms.

The electron theory of Lewis made a considerable contribution in understanding not only reaction routes, but also reaction mechanisms with participation of Lewis acids and bases [20,31,50]. In particular [31], substitution (exchange) reactions of ligands in octahedral complexes include the acid-base interaction (1.1). Oxidative addition reactions can occur when a complex behaves simultaneously as a Lewis acid and a Lewis base [the metal provides electrons for ligand binding and has vacant coordination sites to accommodate two additional ligands, Scheme (1.10)] [34b] ... 

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