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J. Am. Chem. SOC.1984, 106, 4403-4411
Determination of the Metal-Hydrogen and Metal-Methyl Bond Dissociation Energies of the Second-Row, Group 8 Transition Metal Cations M. L. Mandich, L. F. Halle, and J. L. Beauchamp* Contribution No. 6909 from the Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125. Received December 19, 1983
Abstract: The gas-phase reactions of three metal ions, Ru', Rh+, and Pd+, with dihydrogen and ethane are studied in an ion beam apparatus as a function of relative kinetic energy. Analysis of the thresholds for the endothermic formation of metal-hydrogen ions in the reaction with dihydrogen yields the bond dissociation energies Do(Ru+-H) = 41 i 3 kcal/mol, Do(Rht-H) = 42 i 3 kcal/mol, and Do(Pd-H) = 45 i 3 kcal/mol. Similarly, analysis of the thresholds for the endothermic formation of metal-methyl ions in the reaction with ethane yields the bond dissociation energies Do(Ru+-CH3) = 54 f 5 kcal/mol, Do(Rh+-CH3) = 47 i 5 kcal/mol, and Do(Pd+-CH3) = 59 i 5 kcal/mol. The periodic trends for these bond energies are modeled semiquantitatively using simple covalent and electrostatic bonding models. The results of these model calculations indicate that the increased M+-CH3 bond strength relative to the Mt-H bond is most likely caused by a resonant charge stabilization of the metal cation by the methyl ligand. Almost certainly for Ru, Rh, and Pd these M+-R bonds are predominately covalent in character with the metal contribution to the bond being mostly d-like. Contributions from M2+-R- type structures appear to be unimportant.
Introduction Bond dissociation energies provide a basis for predicting stable molecular structures, designing rational syntheses, proposing reaction mechanisms, and testing theoretical chemical bonding models. Although bond strengths have been measured and calculated for a great many organic species, the number of bond strengths known for organometallic moieties is extremely By far the most useful quantity is the dissociation energy of a particular metal-ligand bond as opposed to an average over several metal-ligand bond energies. Calorimetric measurements generally yield only the latter quantities.' Determination of activation parameters in thermochemical kinetic studies and the direct use of spectroscopic methods have provided limited results for individual metal-ligand bond dissociation e n e r g i e ~ . ~ One - ~ method whereby a single metal ligand bond can be isolated for study is to examine an appropriate organometallic fragment which has no other ligands attached. Even though these species are coordinatively unsaturated and may not directly resemble isolatable organometallics, they permit the desired thermochemical studies while facilitating the development of theoretical models for describing metal-ligand bonds. Ion beam reactive scattering methods afford a means of studying the reactions of metal ions with neutral molecules to form organometallic fragments. By adjusting the relative kinetic energy, the threshold energy for formation of a particular organometallic bond can be ascertained, leading to a direct measure of the dissociation energy of that bond. This experimental methodology can also provide thermochemical data for neutral fragments,6v7 but they have been less extensively investigated. In this work, we use these techniques to determine the metal-hydrogen and metal-methyl bond dissociation energies for Ru', Rh', and Pd'. A comparison of our results to the predictions of simple covalent and electrostatic bonding models serves to highlight the importance (1) J. A. Connor, Top. Curr. Chem., 71, 71 (1977). (2) P. B. Armentrout, L. F. Halle, and J. L. Beauchamp, J . A m . Chem. Soc., 103, 6501 (1981). (3) K. P. Huber and G. Herzberg, "Molecular Spectra and Molecular Structure. IV. Constants of Diatomic Molecules", Van Nostrand Reinhold, New York, 1979. (4) 'JANAF Tables", J . Phys. Chem. Ref.Dum, 4 (1975). (5) A. G. Gaydon, "Dissociation Energies and Spectra of Diatomic Molecules", Chapman and Hall, London, 1968. (6) L. F. Halle, P. B. Armentrout, and J. L. Beauchamp, J . A m . Chem. SOC.,103, 962 (1981). (7) L. F. Halle, F. S . Klein, and J. L. Beauchamp, to be submitted for
publication.
0002-7863 I841 1506-4403SO1SO10
Table I. Lower Electronic States of Rut, Rh', and Pdt and Their Relative Ion Populations at 2500 K'
re1 ion Ru'
stateb X4F(4d7) 4P
energyc (eV) 0.18 1.06
6D(4d65s')
1.27
popula. . tion, %
99 1
99 Rh' X'F(4d8) 0.4 1.01 'D > De(M2'-CH
