Metal bond dissociation energy

A second, shorter progression is observed in the resonance Raman spectrum of each ion, based on one quantum of the V2 (MX), A jg, stretching mode in each case (see Section 2.8). However, again it is the (MM) mode which acts as the progressionforming mode. Cross terms X12 have therefore been evaluated in these cases. The resonance Raman results on these ions, and on other metal-metal bonded species, are summarised in Table 9. More extensive tabulations of data on metal-metal stretching frequencies are available elsewhere (86). Metal-metal bond dissociation energies (Dq) for diatomic species can be estimated from the Birge-Sponer extrapolation. 

Considerable difficulties arise when attempting to apply this expression to polyatomic molecules, owing to possible coupling between different modes of the same symmetry. Nevertheless, the data in Table 9 allow one to estimate the metal-metal bond dissociation energy in [M2X8]" ions (84) to be around 500kJmol, which is a very substantial value, exceeded among homonuclear units only by those of C C and N=N. 

The homolytic thermal dissociation of R—M, previously used by Paneth in generating and studying free alkyl radicals, occurs with the more electronegative alkyls, such as those of mercury, lead, and the metalloids. Currently there is much interest in evaluating carbon-metal bond dissociation energies from such pyrolyscs (Section III.B). 

The predominant species observed in SIMS spectra are singly charged atomic and molecular ions [51], However, inorganic and organic cluster ions can also be formed. If the sample consists of a simple single-component metal, then clusters such as M, M, etc., are observed in addition to M+ [52], Oxidation of the metal results in formation of MO ", MO/, M Oll", etc. The relative yield of MO+ to M+ depends on the bond dissociation energy of the oxide [52], For a two-component, oxidized metal, clusters of the type M/", M N, MjO, and M N O/ are observed [51]. 

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 ...

Figure 4. Comparison of theoretical and experimental bond dissociation energies for first row diatomic metal hydride ions. Data from reference 27.

The addition of several ligands to the metal system can reverse relative metal methyl and metal hydrogen bond dissociation energies. For example, reaction 5 is observed to be a last exothermic process,... 

The application of newer methods to studies of gas phase organometallic reactions will lead to the development of routine techniques for determination of the thermochemistry of organometallic species. The examples discussed above demonstrate that an analysis of kinetic energy release distributions for exothermic reactions yields accurate metal ligand bond dissociation energies. This can be extended to include neutrals as well as ions. For example, reaction 15 has been used to determine accurate bond dissociation energies for Co-H and C0-CH3 (57). 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

Lewis et al,16 DH° 40 Kcal/mole, but is nevertheless consistent with trends found in other transition metal carbonyls, i.e. first bond dissociation energies are typically greater than second bond dissociation energies. See Table I. Note that the DH° for... 

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

Finally, it is worth noting that all of the research described here is greatly facilitated when accurate values are available for metal-ligand bond dissociation energies. Only limited data of this type is presently available and further work along these lines is certainly warranted. 

Atomic metal ion-hydrocarbon reactions bond dissociation energies for fragments, 15,16t endothermic reactions, 13,15,17f Atomic transition metal ion reactions development of approach for real-time measurements of dissociation kinetics, 39 ion beam apparatus, 12,14f studies of... 

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