Activation energies coordination

Figure 6 A poor choice of reaction coordinate can lead to a poor estimate of the activation energy and related rate constant. Because of the discrete nature of the reaction pathway, it is possible to step over the bamer. This leads to an underestimate of the activation energy.

Examination of cyclopropanation through RT6 and RT7 reveals that a less conventional explanation may be required to rationalize the high reactivity of zinc car-benoids (Fig. 3.29). The structure of RT6 represents a pseudo-dimer as shown in RTS that has been further activated by coordination of zinc chloride to the oxygen of the chloromethylzinc alkoxide. This mode of activation is reminiscent of that observed in RTl. Cyclopropanation proceeding from RT6 through TS3 has an activation energy of 27.8 kcal mol . This represents a negligible decrease in the barrier to methylene transfer when compared to reaction from RTS. 

The Lewis acid-catalyzed reaction of nitrone 21 with ethyl vinyl ether 22 (Scheme 8.8) was also investigated for BH3 and AlMe3 coordinated to 21 [32]. The presence of BH3 decreases the activation energy for the formation of 23 by 3.1 and 4.5 kcal mol to 9.6 kcal mol for the exoselective reaction and 11.6 kcal-mol for the endo-selective reaction, respectively, while the activation energy for the formation of 24 increases by >1.4 kcal mol , compared to those for the uncatalyzed reaction. The transition-state structure for the BH3-exo-selective 1,3-dipolar cycloaddition reaction of nitrone 21 with ethyl vinyl ether 22 is shown in Fig. 8.19. 

Several attempts to relate the rate for bond scission (kc) with the molecular stress ( jr) have been reported over the years, most of them could be formally traced back to de Boer s model of a stressed bond [92] and Eyring s formulation of the transition state theory [94]. Yew and Davidson [99], in their shearing experiment with DNA, considered the hydrodynamic drag contribution to the tensile force exerted on the bond when the reactant molecule enters the activated state. If this force is exerted along the reaction coordinate over a distance 81, the activation energy for bond dissociation would be reduced by the amount ... 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

Given a certain metal, what can we do to alter its reactivity First there is the structure of the surface. More open surfaces expose atoms of lower coordination. This narrows the d band, and shifts its position (up if it is more than half filled, down if the d band is less than half filled). To illustrate the point Tab. 6.2 shows experimentally determined activation energies of NO dissociation on the (111) and (100) surfaces of rhodium. 

Figure 13. Dependence of ethane hydrogenolysis TOF and apparent activation energy on Pt particle size. TOFs decrease by two orders of magnitude over the size range, while the apparent activation energy increases. Coordinatively unsaturated surface atoms in small particles have a higher reactivity and subsequently a smaller barrier for hydrogenolysis than highly coordinated surface atoms of larger particles. TOFs were measured at 20 Torr C2H6, 200 Torr H2, and 658 K [16]. (Reprinted from Ref [16], 2006, with permission from American Chemical Society.)...

E, the activation energy for the transition between these two states along the coordinate. 

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