Dissociative D Mechanism

In a dissociative mechanism, the metal ion dissociates X in the first step to form a five-coordinate intermediate and then adds Y in the slow step to form the substituted product The rate law for the D mechanism is given by Equation (17.25) and depends on the concentration of the intermediate [MLj]. Applying the steady-state approximation to [MLj], as shown in Equation (17.26), we obtain the overall rate law in terms of only the reactant concentrations given by Equation (17.27). 

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  

The rate of substitution varies only a little with the nature of the incoming ligand Y , as shown by the data in Table 17.2. This would support a dissociative or Ij mechanism. 

The rate of substitution varies over five orders of magnitude with the nature of the leaving group X, as shown in Table 17.3. The weaker the Co-X bond, the faster the reaction. These results support a dissociative intermediate, as the loss of X occurs in the RDS of the D mechanism. 

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The first step in the reaction of ran,.s-[Fe(salpn)(H20)2l+, salpn=A(A7v-propylene-l,2-bis-salicylidiniminate, with sulfur(IV) is the formation of [Fe(S03)(salpn)(H20)], with the pH-rate profile showing greater trans-labilization by hydroxide than by water, in that traras-[Fe(salpn) (H20)2]+, reacts 10 times less rapidly than traras-[Fe(salpn)(OH)(H20)]. A limiting dissociative (D) mechanism is proposed for reaction of the latter formation of the sulfito complex is followed by a slow intermolecu-lar redox reaction (346). A similar situation prevails for the analogous irans-[Fe(salen)(H20)2]+/sulfur(IV) system (347). 

Associative (A) mechanisms are extremely rare and it is uncertain whether an authentic example exists. Dissociative (D) mechanisms are more common although difficult to establish. Some examples were cited in Secs. 4.2.5 and 4.2.6. Thus interchange ( ) mechanisms dominate the scene. This leads to the following generalizations ... 

If this reaction proceeds by a dissociative (D) mechanism, the first step is breaking the metal-water bond, followed by formation of the metal-L bond ... 

The dissociative (D) mechanism, in which the intermediate of lower coordination number lives long enough to equilibrate its environment and hence be consumed in a way that is independent of its mode of formation... 

A dissociative /d mechanism has been established for substitution reactions of low-spin [Fe(cyclam)(NCS)X]+ ions (X=NCS, CH2CICO2-, CHClaC02-, CF, or Br ). Of the complexes studied, only the [Fe(cyclam)Br] + ion was found to be in a high-spin/low-spin equilibrium (ca. 71% low-spin at 20 °C). A plot of the logarithm of the rate constant for the loss of ligand X (A x) against the logarithm of the equilibrium constant for its formation K) is linear over 5 log units with a slope of 1.00+ 0.05. The data are summarized in Table 15. 

It has been shown that axial base-ligand exchange in alkyl-(base)cobaloximes occurs by a purely dissociative (D) mechanism in chloroform but it was not possible to distinguish between this and an 7d mechanism for the reaction of pyridine with a series of alkylcobaloximes in water. 

Reaction (1) is reported to follow a dissociative (D) mechanism for a variety of phosphines and phosphites at room temperature in THF. In view of the known tendency of metal nitrosyls to replace CO with PRj via... 

Substitution reactions of these complexes generally proceed by a limiting dissociative (D) mechanism (Scheme 13). Kinetic investigations of such reactions... 

Kinetics of aquation of [Ru(LLLL)X2], with LLLL = cyclam, 2,3,2-tet, en2, or (NH3)4, and X = Cl or Br, have been followed by cyclic voltammetry. Rate constants, and activation parameters (A// and A5 ) have been evaluated, and compared with kinetic parameters for reactions of analogous compounds of ruthenium(III) and cobalt(III). Similar trends obtain for all three sets of complexes. There is retention of stereochemistry, rates decrease as the extent of chelation in LLLL increases, and trans complexes are less labile than cis analogs. Reactivities are determined by solvation of the initial and transition states, by nephelauxetic effects, and by a-trans effects. A limiting dissociative (D) mechanism is proposed for the ruthenium complexes, with square-pyramidal geometry for the transient intermediate [cf. rhodium(III) photochemistry below. Section 5.8.10]. Differences in isomer lability have also been described for... 

Substitution at pentacyanocobaltate(III), as at pentacyanoferrate(II) (Section 5.4.1), is generally thought to proceed by a limiting dissociative, D, mechanism. Activation volumes of +8.4, +9.4, and +8.2 cm mol for reaction of [Co(CN)5(OH2)] with bromide, iodide, and thiosulfate, respectively, are consistent with a D mechanism, in that they are equal within experimental uncertainty and are markedly positive. Such estimates for the partial molar volume of [Co(CN)5] " as can be derived from these anation results and from the other substitution results detailed elsewhere (Section 5.7.1 above) are all equal, as they should be for the operation of a D mechanism in all cases, with the same intermediate. 

The results of a stopped-flow study of the substitution of axial acetonitrile by imidazole and A -methylimidazole in two complexes of the type [FeL(AN)2] (where L is a 14-membered tetra-aza macrocyclic ligand) in acetonitrile (AN) and acetone are consistent with a dissociative (D) mechanism. The ratedetermining step in the reaction between ammineaquonickel(II) complexes and the tridentate ligand l-(2-hydroxyphenyl)-3,5-diphenylformazan in a 50% (by weight) ethanol-water mixture appears to be ring closure. 

The dissociative (D) mechanism for the substitution of one ligand for another in an octahedral complex, ML5X (L = inert ligands, X = labile ligand, and Y = incoming ligand). 

How might this substitution actually take place on the molecular level Or, to restate the question, what is the reaction mechanism—that is, the sequence of molecular-level steps involved in the reaction At first blush, there are at least two major possibilities the dissociative (D) mechanism and the associative A) mechanism. 

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. 

Two studies of the rraw-[Co(en)2(OHa)2] + ion at high pressures indicate that the water exchange rate proceeds by a dissociative /d mechanism involving a square-pyramidal intermediate [A K =+5.9 0.2 cm mol(pressure-independent) at... 

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