Unstable intermediate complexe

A comparison of equations 7.3.43 and 7.3.38 shows that they are of the same mathematical form. Both can be written in terms of four measurable kinetic constants in the manner of equation 7.3.40. Only the relationship between the kinetic constants and the individual rate constants differs. Thus, no distinction can be made between the two mechanisms using steady-state rate studies. In general, the introduction of unimolecular steps involving only isomerization between unstable intermediate complexes does not change the form of the rate expression. 

E21.10 The entropy of activation is positive, ruling out an associative mechanism (associative mechanisms have uniformly negative entropies of activation). A possible mechanism is loss of one dimethyl sulfide ligand, followed by the coordination of 1,10-phenanthroline, giving an unstable, intermediate complex that can be either five-coordinate (both phen donor atoms coordinated) or four-coordinate (only one phen donor atom coordinated). In the final step, the second dimethyl sulfide ligand leaves and rearranges to form a square-planar complex. 

Thibblin and Jencks (229) succinctly summarized this notion for simple chemical model systems when they suggested that the lowest energy pathway for a reaction that proceeds through an unstable intermediate complex containing... 

The role of a catalyst is, first, to form a complex molecule with reactant molecule coordination through its appropriate frontal orbital and thus fully weaken the H H bond and triple N=N bond. For example, H-H changes to two coordinated hydrogens and then may easily form new bond with the activated reaction intermediates. In other words, catalyst can participate in the reaction and form unstable intermediate complexes with reactants and form products at last. The activation energies required for every step are much lower than that for the reaction without catalyst, and therefore the reaction rate is accelerated. Consequently, the reaction pathway is changed by catalysts. For the case of ammonia synthesis reaction on Fe (111), Ertl, a winner of the Nobel chemistry prize in 2007, proposed a thermochemical kinetic profile, as shown in Fig. 2.1. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Stabilization of Unstable Intermediates. Transition metals can stabilize normally unstable or transient organic intermediates. Cyclobutadiene has never been isolated as a free molecule, but it has been isolated and fully characterized as an iron tricarbonyl complex (138) ... 

However, vinylacetyl chloride (CH2=CHCH2C0C1) reacts with [Co(CN)5H] to give propylene, presumably via the intermediate formation of the butenoyl complex (CH2=CHCH2COCo), which then eliminates CO to form the unstable allyl complex 111, 105). No test has apparently been made for the possible insertion of CO by organopentacyanides. 

It has been known from the earliest observations in gold chemistry that olefins form very weak 7r-complexes with gold in any of its oxidation states.1-3 Therefore, only a very few stable complexes of this type have been isolated. However, unstable 7r-complexes of olefins with gold compounds are believed to be important intermediates or transient species in gold-catalyzed reactions of unsaturated organic compounds.122-125,131,286-288,307-312... 

Direct routes from hazardous elements Routes at increased concentration or even solvent-free Routes at elevated temperature and/or pressure Routes mixing the reactants all at once Routes using unstable intermediates Routes in the explosive or thermal runaway regime Process simplification - e.g., routes omitting the need of catalysts or (complex) separation... 

Olefin metathesis is a catalytic process whose key step is a reaction between an olefin and a transition metal alkylidene complex, usually M=CHR (Eq. 1) or M= CH2, in a 2+2 fashion to give an unstable intermediate metalacyclobutane ring [1]. All possible reactions of this general type are reversible, possibly nonproductive, and in competition with one another, so the overall result depends heavily on relative rates, and in the case of formation of volatile or insoluble products, displacement of equilibria as those products form. 

The nature of species (II), whether an unstable intermediate or transition state, requires discussion because of its possible importance in catalytic reaction mechanisms. By analogy to homogeneous substitution reactions a distinction can be made between electrophilic and radical attack. Electrophilic substitution reactions, with a proton for example, appear to proceed via a charge-transfer complex... 

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