The Associative Mechanism

Dissociation of a ligand is accelerated for bulky ligands. We shall see in Section 9.4 how this affects the dissociation of a phosphite from NiL4 in a key step in olefin hydrocyanation, an important catalytic reaction. The degree of dissociation can be predicted from the appropriate cone angles, and the bulky phosphite P(0-o-tolyl)3 makes one of the very best catalysts. Tri-phenylphosphine is very useful in a wide variety of catalysts for the same reason. 

Dissociation can sometimes be encouraged in various ways. For example, a chloride ligand can often be substituted in the presence of Ag, because AgCl is precipitated. T1+ is used in cases where Ag oxidizes the complex and is therefore unsatisfactory. Protonation can also be used to remove ligands such as alkyl or hydride groups. Weakly bound solvents are often useful ligands synthetically, because they can be readily displaced. As a tt donor, thf is a poor ligand for W(0)  

Substitution of halide for alkyl or hydride is often carried out with RMgX or LiAlH4. Cyclopentadienyls may be prepared from CpNa or CpTl, in which case the insoluble TlCl precipitates and helps drive the reaction. 

The slow step in associative substitution is the attack of the incoming ligand L on the complex to form an intermediate that rapidly expels one of the original ligands L. 

CARBONYLS, PHOSPHINE COMPLEXES. AND LIGAND SUBSTITUTION REACTIONS 

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The science of building aerodynamics considers the influence of wind forces over buildings and the associated mechanics of fluids these are complex in nature and are not considered here. It is sufficient to briefly consider Fig. 9.21, which shows how wind passing over a building produces a positive pressure on one side and a negative pressure on the other side. It is this pressure difference that produces airflow through openings. The combined wind and stack effects vary with the seasons. 

However, it is known that the direct correlation functions have an exact long-range asymptotic form, arising due to intramolecular correlations in clusters formed via the association mechanism. This asymptotics is not included in the Percus-Yevick approximation. Other common liquid state approximations also do not provide correct asymptotic behavior of Ca ir). 

This study is a good reference for the construction of fault/event trees of systems that are affected by valve performance. The valve failure modes are identified, the associated mechanisms are described in detail, and preventive measures are offered. 

In a CO matrix, therefore, the primary product involves an expanded coordination nunber ("ring-slippage") and it is argued that such a species is consistent with the associative mechanism proposed for room temperature substitution reactions of (13 -05115)Co-... 

It now appears that at least two mechanisms exist for the base-promoted homogeneous water gas shift reaction, differing in the method of hydride formation. The "associative mechanism", first proposed by Pettit and co-workers (1,4), involves nucleophilic attack on a carbonyl ligand and it has two variations. 

Fig. 5. The free-energy (G) profile of the associative mechanism at 423 K with the BP86 method and with pressure correction. Numbers in parenthesis are the highest estimated barrier for the H2/olefin loss.

Table XII contains the enthalpy and free-energy differences of the critical intermediate species for the anthraphos Rh catalyst. Although no experimental results are available yet, our predicted energies show a much smaller dissociation energy for H2 loss, the first step in the dissociative mechanism (8 is more stable by 13 kcal/mol relative to 4). In contrast, the oxidative addition intermediate of the first step in the associative mechanism, the M(V) species 5, is 14 kcal/mol less stable...

A few other interesting and potentially important consequences of the reversible formation of five-membered zirconacycles include stereo- and regioselective skeletal rearrangement, as exemplified by Scheme 1.57 [197], and 1,3-C=C bond and Zr migration (Scheme 1.58) [191,192], supporting the associative mechanism for alkene displacement (Generalization 22 ). 

Such conclusions have been initially regarded with reluctance by some experimentalists [40], Despite its potential interest, the new alternative does not fit the large body of experimental observations available so far. As pointed out by Fischer and Hofmann [40], kinetic studies are not consistent with the associative mechanism and are clearly in favor of a dissociative path. However, in a recent kinetic study, Waters, Bos, and Wulff (WBW) [41] have provided the first example of a bimolecular reaction of a... 

The results of the present work are summarized in Figure 10. As far as the initial part of the reaction is concerned, we have found that the associative mechanism is slightly more favored than the dissociative one. However, the small energy difference found between the two mechanisms and the possible effect of the solvent, which has not been included in the present study, precludes formulation of a definitive conclusion on the most effective reaction pathway. Most likely depending on the reaction conditions and the initial reactants the two mechanisms can be operative. In fact, experimental evidence in favor of both the dissociative [37, 38] and associative [41] mechanisms has been provided. 

In favor of the dissociative mechanism, but contradicting the exclusive involvement of the associative mechanism, is the fact that exchange... 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

Fig. 10. Isomerization and exchange of citf-2-buteno according to the associative mechanism.

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend to slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst (5). 

Later Greenhalgh and Polanyi (14) formulated kinetic expressions for this mechanism. They showed that the dissociative mechanism for exchange according to Farkas and Farkas would yield the same mathematical function as the associative mechanism if the combination of deuterium atoms with phenyl radicals was the slow step. However, they continued to favor the associative mechanism. 

Recently Harper and Kemball 90) have elaborated the associative mechanism in a manner which avoids the above geometrical limitations. Much of the reasoning upon which they base their ideas is recorded in papers by Gault et al. 91) and by Rooney 80). They postulate that TT-bonded benzene or molecular species such as B can combine either with a hydrogen atom from the surface or with a hydrogen molecule from the gas or physically adsorbed phase ... 

Fio. 22. Geometrical relationships involved in the associative mechanism for addition and exchange of an olefin. 

There have been extensive studies of the influence of an entering ligand on its rate of entry into a Pt(ll) complex.The rate constants for reaction of a large number and variety of ligands with trans-Pt(py)2C 2 have been measured (Table 4.13). The large range of reactivities is a feature of the associative mechanism and differentiates it from the behavior of octahedral complexes. The rate constants may be used to set up quantitative relationships. For a variety of reactions of Pt complexes in different solvents (Sec. 2.5.4) ... 

Fig. 4.9 Simplified reaction profiles for various situations in the associative mechanism for substitution in square planar complexes, focusing attention on the replacement M-X-l-Y —> M-Y + X(4.93).

For (a) k = AryAtjArlj [L,] when ring closure is fast. For (b) Ar bs = AtjlLJ. A large positive value for rules out the associative mechanism (b). 

Solvolysis studies of meta- and para-substituted phenyl phosphates (240) in anhydrous Bu OH and in Am OH have revealed that generally reactions of dianions are much faster in alcohols than in water. For example, the dianion of p-nitrophenyl phosphate (240 X = 4-NO2) reacts 7500- and 8750-fold faster in Bu OH and Am OH, respectively, than in water." The results of a theoretical study of the reactivity of phosphate monoester anions in aqueous solution do not support the generally accepted view that Brpnsted coefficients fhg = —1.23 and jSnuc = 0.13 determined more than 30 years ago for the uncatalysed reaction of water and a monophosphate dianion (241) represent conclusive evidence for the dissociative mechanism. It is suggested that, instead, the observed LFERs could correspond to a late transition state in the associative mechanism." ... 

In the case study presented here, the power of the HPMS technique as a probe for unimolecular dissociation kinetics and for elucidating the associated mechanism is thus evident. For this system, ample other experimental data are available to support the conclusions drawn, but in principle the same conclusions could have been reached solely on the basis of HPMS experiments. 

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