Dissociation energies, transition metal metals

Table L Bond Dissociation Energies for Transition Metal Carbonyls...

Figure 4.44. Contracted energy diagrams for CO methanation over Ni, Ru, and Re (Left). BEP-relation for CO dissociation over transition metal surfaces (right-top) and the corresponding volcano-relation for the turnover frequency (right-bottom). Adapted from [55,140].

Staley and coworkers6 tyave provided direct measurement of HSAB effects in gas-phase dissociation energies between transition metals (where the principle has... 

In 1984 Krauss and Stevens described tests and applications of the effective potential method used to gain knowledge of the electronic structure of the molecules in order to analyze the accuracy of the experimentally deduced dissociation energies of refractory metal salts [3]. They used the development of ab initio theoretical methods for the calculation of potential energy surfaces, which further allowed the direct computation of certain rate constants. Transition state theory was also utilized for this computation of some rate constants. However, as discussed by Krauss and Stevens, as of the mid 1980 s computational techniques were not yet readily applied to atmospheric science. Computing power and theoretical methods since these seminal reports have been greatly advanced. 

The dissociation energies of alkali-metal clusters show variations due to even-odd effects and shell closures. This is not the case for mercury. The dissociation energies and the mass spectra show no trace of similar rapid variations. The dot-dashed line is from the tight-binding calculation of Pastor et a/. - for neutral Hg clusters. It deviates from the experimental result already for n = 3 and shows a much smoother transition. The experiment points to a more abrupt transition between the different regions of binding. 

In order to discuss the physical meaning of a, we need to introduce the concept of early and late transition states. In the previous section we discussed in detail the transition state for CO dissociation over transition-metal surfaces and described the reaction as an example of a late transition state. The transition state is late along the reaction coordinate since the transition-state structure is close to the final dissociated state. Transition states which are early along the reaction coordinate are called early transition states and thus resemble the initial reaction states (see Chapters 4 and 7 for the definition of the pretransition state). The activation energies for the protonic zeolite reactions correlate with deprotonation energies (see Fig. 2.9) and are examples of intermediate transition states that also vary with the energies of the initial states . When a 0.5 (a 0), the transition state is early AS fv 0... 

In summary, symmetry considerations based on frontier orbital theory enable a good understanding of H2 dissociation on transition-metal clusters. For a single atom, electron promotion energy and hybridization are important variables. In diatomics, the relative position of d versus s electrons determines whether the bonding symmetric s orbital built from s-atomic orbitals is occupied. Occupation of this... 

Several useful reviews have appeared on different aspects of the metal-carbon or bond. Halpern has collected data on bond dissociation energies of transition metal-alkyl bonds. An extensively referenced review on the breaking of organometallic bonds, R-M, in reactions in which R2, RH, and... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Table 3. Bond lengths (A), bond dissociation energies (kcal/mol), a- and n-bond strengths (kcal/mol), charges on phosphorus (e), and orbital energies (eV) for first row transition metal complexes ML =PH ...

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