Transition metals bond dissociation energies

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 ...

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...

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... 

Pensak and McKinney (28) [PM], using this method, have recently reported a systematic study of first-row transition metal carbonyl complexes for which experimental bond distances and angles were reliably reproduced, along with key bond dissociation energies. 

Co2(CO)q system, reveals that the reactions proceed through mononuclear transition states and intermediates, many of which have established precedents. The major pathway requires neither radical intermediates nor free formaldehyde. The observed rate laws, product distributions, kinetic isotope effects, solvent effects, and thermochemical parameters are accounted for by the proposed mechanistic scheme. Significant support of the proposed scheme at every crucial step is provided by a new type of semi-empirical molecular-orbital calculation which is parameterized via known bond-dissociation energies. The results may serve as a starting point for more detailed calculations. Generalization to other transition-metal catalyzed systems is not yet possible. 

Table II. Mean Bond Dissociation Energy Data for Some Early Transition Metal Complexesa and Estimated 5 Values for Thorium and Uranium.

Table 4.56. Statistical means and standard deviations (SD) for bond dissociation energies (BDEs) o/ M—H and M—Me bonds of saturated MH X (X = H, Me) transition-metal complexes from the first three series of the d block...

The Met+—L bond dissociation energies were measured for some Zn-containing gas phase cations using different methods ". Armentrout reviewed guided ion beam studies of transition metal-ligand thermochemistry and reported the following... 

Few transition-metal-alkyl bond-dissociation energies are known reliably (16). Potential approaches to the estimation of such dissociation energies encompass the following ... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

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