Metal-ligand bonds, bond energies

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. 

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. 

Klassen JK, Selke M, Sorensen AA, Yang GK. Metal-ligand bond dissociation energies in CpMn(CO)2L complexes. J Am Chem Soc 1990 112 1267-1268. 

Scheme 7. Thermochemical cycle for the determination of relative metal ligand bond-dissociation energies in 18- and 19-electron complexes.

Figure 1. Metal ligand ion bond energies vs. 4s 3d" promotion energy for first row transition metals. Data are shown for (open circles), MCH3 (closed circles), MCH2 (closed triangles), M(0113)2 (open triangles), and (closed squares). Lines are...

Metal—Ligand Bond Dissociation Energies in CpMn(CO)2L and Cr(CO)g(olefin) Complexes... 

KLASSENETAL, Metal-Ligand Bond Dissociation Energies... 

Mass spectrometry is a unique method that allows study of the reactivity of isolated metal-containing ions in the gas phase in the absence of solvent. A number of fundamental thermochemical characteristics of organometallic molecules and ions, such as ionization energies, proton affinities, electron affinities and metal-ligand bond dissociation energies, can be determined from mass spectrometry experiments. 

Corderman, R.R. and Beauchamp, J.L. (1976) Quantitative metal-ligand bond dissociation energies in the gas phase by ion cyclotron resonance spectroscopy. J. Am. Chem. Soc., 98, 3998-4000. 

As briefly stated in the introduction, we may consider one-dimensional cross sections through the zero-order potential energy surfaces for the two spin states, cf. Fig. 9, in order to illustrate the spin interconversion process and the accompanying modification of molecular structure. The potential energy of the complex in the particular spin state is thus plotted as a function of the vibrational coordinate that is most active in the process, i.e., the metal-ligand bond distance, R. These potential curves may be taken to represent a suitable cross section of the metal 3N-6 dimensional potential energy hypersurface of the molecule. Each potential curve has a minimum corresponding to the stable... 

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