Substitution dissociative

The reactions of phosphines with metal carbonyls, investigated by Basolo, form the basis for our understanding of organometallic substitution reactions in general. The phosphine is usually refluxed with the carbonyl in an otganic solvent, such as ethanol or toluene. One can distinguish two extreme mechanisms for substitution, one dissociative, labeled D, and the other associative, labeled A. Intermediate eases are often labeled I U if closer to A and if closer to D.  

The dissociative extreme involves a slow initial loss of a CO to generate a vacant site at the metal, which is trapped by the incoming ligand L. In general, a dissociative step precedes an associative step. Because the rate-determining step is dissociation of CO, the reaction is usually independent of the concentration of L, and the rate is the same for any of a series of different L ligands. This leads to a simple rate equation  

In some cases, the back reaction, becomes important, in which case the intermediate, L M— , partitions between the forward and back reactions. Increasing the concentration of L docs now have an effect on the rate because k2 now competes with The rate equation derived for Eq. 4,28 is shown in Eq. 4.29. It reduces to Eq. 4.27, if the concentration of CO, and therefore the rate of the back reaction, is negligible. 

The overall rate is usually controlled by the rate at which the leaving ligand dissociates. Ligands that bind less well to the metal dissociate faster than does CO. For example, Cr(CO)5L shows faster rates of substitution of L in the order L = CO PhjAs py. For similar ligands, say, phosphines, the larger the cone angle, the faster the dissociation  

This mechanism tends to be observed for 18e carbonyls. The alternative, initial associative attack of a phosphine would generate a 20e species. While it is not 

The dissociative extreme involves a slow initial loss of a CO to generate a vacant site at the metal, which is trapped by the incoming ligand L. Because 

This mechanism tends to be observed for 18e carbonyls. The alternative, initial attack of a phosphine, would generate a 20e species. While it is not forbidden to have a 20e transition state (after all, NiCp2 is a stable 20e species), the 16e intermediate of Eq. 4.28 provides a lower-energy path in many cases. This is reminiscent of the SnI mechanism of substitution in alkyl halides. The activation enthalpy required for the reaction is normally close to the M—CO bond strength, because this bond is largely broken in going to the transition state. A5 is usually positive and in the range 10-15 eu (entropy units), as expected for a dissociative process in which the transition state is less ordered. 

Steric bulk is easily achievable with NHCs because the R groups point toward the metal, not away as in M-PR3. For example, in IMes, the mesityl groups play this role, leading to easy access to low coordinate complexes, such as [PtMe(IMes)2]. The / of this naming convention means that an imidazole ring is involved and the Mes refers to the mesityl substituents at N. 

The mechanisms of CO substitution by PR3in metal carbonyls are the basis for the understanding of organometallic substitution in general. Two extreme mechanisms are invoked, one dissociative, D, and the other associative, A. In the D mechanism, a CO first dissociates, leaving a vacant site at which PR3 subsequently binds. This is typical of 18e complexes because the intermediate is then 16e after CO loss. In the A mechanism of Section 4.5, PR3 binds first and only subsequently does the CO depart. This is typical of 16e complexes because 

In the D mechanism, initial CO loss to generate a vacant site at M is usually slow, followed by fast trapping by the incoming ligand L. The rate-determining step is thus independent of the concentration and identity of L. This leads to the simple rate equation of Eq. 4.27. 

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This equilibrium applies to a mixture of an acid HA and its salt, say MA. If the concentration of the acid be ca and that of the salt be c5, then the concentration of the undissociated portion of the acid is (cfl — [H + ]). The solution is electrically neutral, hence [A ] = cs + [H + ] (the salt is completely dissociated). Substituting these values in the equilibrium equation (18), we have ... 

Dissociative substitution at square-planar centers (231), particularly platinum(II), was a matter of controversy for several years, with the... 

A further example concerns the substitution reactions of the co-balamins (vitamin Bi2). Here the usually inert Co(III) center is labi-lized by the corrin ring, which induces a dissociative substitution behavior. A series of detailed studies of the effect of pressure on com-... 

The tetrahedral complexes of the d ° Ni(0) system undergo dissociative substitution (Table 4.15). Kinetic data are shown in Table 8.12.243 Infrared monitoring methods feature prominently in these studies (Sec. 3.9.2). 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Disproportionation, and metal hydride stability, 1, 301 Dissociative substitution, in 18-electron complexes, 1, 96... 

Both associative and dissociative substitution pathways can operate, in both the absence and the presence of acid. 

Ideally, the measurement of obs as a function of Xe concentration would have provided strong evidence for the type of mechanism, since for a dissociative mechanism, obs l/[Xe], whereas an associative mechanism would have no dependence on [Xe], However, it is not possible to vary the concentration of Xe in liquid Xe, and so pre-exponential factors A obtained from Arrhenius plots were used to differentiate the two mechanisms. The value of logA was found to lie within the expected range for a unimolecular dissociation reaction, and it was concluded that this reaction occurs by a dissociative substitution mechanism in liquid Xe. The Arrhenius plot therefore gave an estimate of the W—Xe BDE, AHw—Xe = 35.1 0.8 kJ mol . ... 

A more recent addition to the half-sandwich chemistry of ruthenium is given by a number of complexes where the central metal obeys a 16 valence electron count. These coordinatively unsaturated (see Coordinative Saturation Unsaturation) metal centers are widely invoked as intermediates or transition states in dissociative substitution processes or in catalytic transformations at transition metal centers. Such complexes are not, however, easily isolated. The most common way to stabilize such species is by coordinating sterically bulky ligands to the metal, thereby preventing further ligand addition. They can also be stabilized in the form of dimeric complexes. 

The dissociative substitution of d10 ML4 complexes has been well studied, particularly with regard to M = Ni (equation 7.18). 

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