Creation of a Vacant Site

The catalyst is HCo(CO)4, which must lose one molecule of CO in order to make room for an alkene molecule (Fig. 4.17). 

A high CO pressure would shift equilibrium (4.3) to the left and the catalytic reaction would become slower. In this complex CO is a far better ligand than an alkene. On the other hand the reaction uses CO as a substrate, so it cannot be omitted. Furthermore, low pressures of CO may lead to decomposition of the cobalt carbonyl complexes to metallic cobalt and CO, which is also undesirable. Finally, the product alcohol may stabilize divalent cobalt species which are not active as a catalyst  

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Another way of looking at the question of creation of a vacant site and coordination of the substrate is the classical way by which substitution reactions are described (Figure 2.1). Two extreme mechanisms are distinguished, an associative and a dissociative one. In the dissociative mechanism the ratecontrolling step is the breaking of the bond between the metal and the leaving ligand. A solvent molecule occupies the open site, which is a phenomenon that does not appear in the rate equation. Subsequently the solvent is replaced by the... 

Creation of a vacant site by CO dissociation is favored at low CO pressures, whereas the CO insertion step seems not to be very sensitive to pressure. Above 150°C the cyclopentadienyl ruthenium complex was destroyed, and many complexes were formed, with [Ru3(CO),2] as the major component. This mixture of compounds is of poor activity, and [Ru3(CO),2] tested under the same conditions presents the same low level of catalytic activity. 

Subsequent studies on the reactivities of neutral and cationic alkyl- and aryl- palladium complexes revealed that the creation of a vacant site adjacent to the alkyl or aryl ligand causes marked enhancement in reactivity toward j8-hydrogen migration. The implications of these results on the fundamental processes of the transition metal alkyls and aryls with the mechanisms of Pd-catalyzed organic synthesis, such as arylation of olefins and carbonylation of aryl halides, have been discussed. 

Sakaki. In fact, the product from the oxidative addition is a saturated hexacoordinated Pd(IV) complex which would require creation of a vacant site in order to enable the coordination of an alkene. The activation ofB2cat2 by [Pd(II)(NHC)Br] resulted more favorably through o-bond metathesis providing [Pd(NHC)(Bcat)] andBrBcat (Scheme 17B) with 3.4 kcal/mol above the reactants (Scheme 17B). Alternatively, the dicationic complex [Pd(II)(NHC)] could form a very stable o-complex with B2cat2, with 32.9 kcal/mol below the two isolated reagents, which promoted the oxidative addition of the diboron (Scheme 17C). 

Still the most common picture for j6-H elimination is this sequence Creation of a vacant cis site, reversible j6-H elimination via a planar four-center transition state, and displacement or loss of the coordinated alkene. If the lack of a vacant cis site explains the stability of many complexes, the inherent difficulty to produce a planar four-center transition state is probably responsible for the stability of metallacycles, as compared to simple alkyls. The decomposition rates of Pt(CH2) (PPh3)2 (Eq. 6.14) illustrate this fact dramatically The complexes with n = 4, 5 are 10" times more stable than the complex with n = 6, because the former are less flexible, and the corresponding planar four-center transition state... 

When AI2O3 is incorporated into MgO, due to the similarity in ionic radii, it is assumed that the A1 will substitute for Mg, with charge balanced achieved by the creation of a vacant Mg site, through the following equation ... 

The latter reaction is usually described as an oxidative addition reaction. Thus, Rh(lII) dihydride species, very active as hydrogenation catalysts, are formed by oxidative addition to the rhodium(I) compound RhCKPPhsls. In contrast, the analogous IrCKPPhala, which also reacts with hydrogen, is cataljrtically inactive. This is due to the fact that the formed IrHzCKPPhsls is very stable and cannot, under normal conditions, dissociate phosphine to allow the creation of a vacant coordination site. 

The energies and configurations of interstitials and vacancies in the B2 (cP2) ordered compounds NiTi and FeTi have been calculated by Lutton et al. (1991) using atomistic simulation. In NiTi, the stable configuration of a vacancy after the removal of a Ni atom was a vacant Ni site, and in FeTi the removal of an Fe atom resulted in a vacant Fe site. In both compounds, removal of a Ti atom led to the creation of a vacant Ni or Fe site... 

For example, using this system, V-Na indicates a vacant site normally occupied by a Na+ ion so the site has a charge of—1, Fe+Fe indicates an Fe+++ substituting for an Fe++ so the site has a charge of + 1, and Ag°Ag indicates a silver ion on its proper site. Reactions can be written with this system. For example, formation of a Frenkel defect in NaCl can be written as Na°Na - Na+ + V-Na and the creation of a Schottky defect can be written Null V" Na + V+C1. 

The creation of the vacant oxygen sites leads to a change in the cation to anion ratio, i.e., to nonstoichiometry. The equilibrium constant for the reaction is... 

The ultimate purpose of mechanistic considerations is the understanding of the detailed reaction pathway. In this connection it is important to know the structure of the active catalyst and, closely connected with this, the function of the cocatalyst. Two possibilities for the action of the cocatalyst will be taken into consideration, namely, the change in the oxidation state of the transition metal and the creation of vacant sites. In the following, a few catalyst systems will be considered in more detail. 

The decrease in 0 jq occurs for two reasons a) inhibition of the readsorption of desorbing NO as a consequence of H2 adsorption and b) the consumption of adsorbed NO by reduction. It should be noted that the dissociation of NO, and hence the rate of NO reduction is accelerated by the creation of vacant sites. The increase in 0V, seen in Fig. 13, can be ascribed to a consumption... 

In the mid 1970s, Falconer and Madix observed a surface- kinetic explosion for the decomposition of formic acid (HCOOH) [23] and acetic acid (CH COOH) [24] on the Ni(llO) surface, characterized by very narrow product desorption peaks in TPRS. Such autocatalytic reactions have also been observed in the decomposition of acetic acid on Pd(llO), Rh(llO), Rh(lll), and even supported Rh catalyst by Bowker et al. [70-75]. In general, these reactions exhibit accelerations in rate as the reaction proceeds to completion. Earlier work hypothesized that decomposition of the carboxylate species formed following adsorption of the acids on the surface was initiated at vacancies (i.e. bare metal sites) and propagated by the further creation of vacancies as the products desorbed from the surface [23, 24]. The rate of decomposition was well described by the rate equation r = -k(C / Cj )(Cj - c+/Cj), in which C is the instantaneous surface concentration of carboxylate, C, is the initial surface concentration, and/is the density of initiation sites. Since the decomposition produced an ever-increasing concentration of vacant sites, a kinetic explosion occurred. 

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