Dissociation D

In a dissociative (D) reaction, loss of a ligand to form an intermediate with a lower coordination number is followed by addition of a new ligand to the intermediate  

The stationary-state (or steady-state) hypothesis assumes a very small concentration of the intermediate, ML, and requires that the rates of formation and reaction of the intermediate must be equal. This in turn requires that the rate of change of [ML,] be zero during much of the reaction. Expressed as a rate equation, 

One criterion for this mechanism is that the intermediate, ML5, be detectable during the reaction. Direct detection at the low concentrations expected is a very difficult experimental challenge, and there are very few clear-cut dissociative reactions. More often, the evidence is indirect, but no intermediate has been found. Such reactions are usually classified as following an interchange mechanism. 

In an interchange (7) reaction, a rapid equilibrium between the incoming ligand and the 6-coordinate reactant forms an ion pair or loosely bonded molecular combination. This species, which is not described as having an increased coordination number and is not directly detectable, then reacts to form the product and release the initial ligand. 

When 2 -1 the reverse reaction of the first step is fast enough that this step is 

Examination of the possibility of a Z mechanism typically requires the steady-state approximation. This approximation assumes that a vanishingly small (and constant) concentration of the intermediate, ML5, is present during the reaction by assuming that the rates of formation and consumption of the intermediate are equal. If these rates are the same, the change in the [ML5] must equal zero (and this species cannot accumulate) during the reaction. Expressed as a rate equation, 

Evidence that the conditions associated with the steady-state approximation are present, and that a D mechanism may be operative, includes an inverse dependence between the rate of formation of [ML5Y] and the concentration of X as indicated in the rate law above. This derivation predicts that the presence of X should reduce the rate of formation of [ML5Y], as long as the intermediate ML5 reacts with X at a rate similar to or greater than Y reacts with ML5. 

A D mechanism on the basis of the derived rate law will exhibit a complicated dependence upon [X] and [Y] which has two limiting cases described below. Studies that systematically vary the concentrations of both X and Y provide the best evidence for a dissociative mechanism. At high [Y], the system will show saturation kinetics in which the reaction rate depends only upon [ML5X]. 

While experimental data that exhibit these relationships support both the validity of the steady-state approximation and a D substitution mechanism, the exceedingly low [ML5] concentration that forms on the basis of this approximation renders the detection of this intermediate impossible in many cases. 

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Spin-paired octahedral (P ions and spin-free octahedral (P ions appear to react by largely dissociative (D ot reactions, for example. 

Table 2.1. Dissociative (D) and molecular (M) chemisorption of CO on metal surfaces.

Simple examples of A, / and reactions are seen to have the same rate law. There remains another important mechanism that differs. This is the pathway through an intermediate of reduced coordination number that is possible if a transition state like d of Fig. 3 occurs. This path may be called dissociative, D. The mechanism may be represented as... 

Note added in proof Since this paper was written, it has been found that the two lines supposed to constitute group 12 are due to impurities. With this change in the data, the lower limit of the heat of dissociation, D, of the hydrogen molecule becomes 4.10 volts or 94,600 calories, the upper limit 4.50 volts or 103,800 calories and the probable value of D 4.34 volts or 100,100 calories. 

Fig. 3. Volume change (reactants-transition states-intermediates) for a dissociative D water exchange (top) and an associative A water exchange (bottom) estimated from ab-initio cluster calculations.

A = +14 cm3 mol-1 for both the forward and the reverse reaction. That this AV value is markedly less than the partial molar volumes of water and of ammonia (25 and 18 cm3 mol-1, respectively) indicates limiting dissociative (D) activation (133), as do the A values of close to +70JK-1mol-1 in both directions. Overall, the current situation with regard to thermal substitution at pentacyanoferrates(II) appears to be that an I,i mechanism can also operate for reactions of [Fe(CN)5(H20)]3-, whereas the D mechanism operates for all other [Fe(CN)5L]" complexes (134). 

See Section IV.B.2 above for a recent example of the balance between dissociative (D) and associative mechanisms, in the case of ring opening of platinum(II) chelates of hemi-labile ligands. 

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). 

A recent study shows that the (RAP)IrH2 complex can catalytically convert alkane to alkene at temperatures ranging from 150 to 250 °C through an acceptorless, thermal reaction. Three suggested reaction mechanisms, associative (A), dissociative (D) and interchange (I) will... 

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 ... 

There is strong evidence for a dissociative type of mechanism for base hydrolysis. There is an =10 -fold rate enhancement (steric acceleration) for base hydrolysis of Co(iso-BuNH2)5CP+ compared to Co(NH3)jCF (mainly residing in while the corresponding factor for aquation is only =<10, emphasizing the different degrees of dissociation D vs /j). There is, incidentally, a LFER for log Atqh vs log slope 1.0, for reactions of a series of Co(III) complexes. Finally, on the basis of a mechanism, the estimated pro-... 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

Dissociative, D. The M—X bond is folly broken before the M—Y bond begins to form. 

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... 

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