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Heats of Reaction of Triphenyl Phosphite with Some Rhodium-Olefin Compounds Walter Partenheimer” and Edgar F. Hoy Contribution from the Department of Chemistry, Clarkson College oj Technology, Potsdam, New York 13676. Received September 29, 1972
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The heats of the following reactions (in CH2C12) are reported: [RhCl(olefin),12 2P -+ [Rh2C12P2(olefin),] n(olefin), [Rh2Cl~P2(olefin),l 2P [RhClP& n(olefin), and [RhC1P2l2 2P -+ 2[RhC1Pa],where olefin is norbornadiene, 1,5-~yclooctadiene,1,3,5,7-~yclooctatetraene,dicyclopentadiene, benzoquinone (n = l), and cis-cyclooctene (n = 2), and P is triphenyl phosphite, and [Rh(acac)(olefin)] 2P + [Rh(acac)P2] olefin (in CH2C12or CCl,) where olefin is 1,5-~yclooctadiene or 1,3,5,7-~yclooctatetraene.Relative displacement energies have been calculated, evidence given that the solvation terms largely cancel to zero, and an identical ordering is found for rhodium@)and palladium(I1) compounds: norbornadiene > 1,5-cyclooctadiene > 1,3,5,7-cyclooctatetraene > dicyclopentadiene. Replacement of two monoolefins by a chelating diolefin in rhodium(1) compounds is strongly exothermic. The similarity of the displacement energies to the heats of adsorption on metallic surfaces and the far-ir spectra of these compounds are briefly discussed. Abstract:
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here has been a n impressive growth in the number and variety of metal-olefin compounds in the last 20 years. 1-3 Interest in metal-olefin compounds stems largely from attempts to elucidate the nature of the metal-olefin bond, their use as model compounds in homogeneous catalysis, and as catalysts in a variety of olefin isomerizations and polymerizations. Thermodynamic information relating to the metal-olefin bond is therefore of great interest. Thermodynamic data for palladium-, platinum-, and silver-olefin compounds have been reviewed. 2 , 4 The primary reason for obtaining thermodynamic data has been to elucidate the nature of the metal-olefin bond. This has been achieved by varying the electronic, steric, and strain characteristics of the olefin and observing how this affects the metal-olefin interaction. These effects are most directly manifested by enthalpic changes. It is therefore noteworthy to mention that the authors are unaware of any systematic studies in which accurate enthalpic changes have been measured in relatively nonpolar solvents until our first report dealing with palladium(I1) compounds.5 Many studies report only equilibrium constants usually measured in polar solvents such as water and alcohol^.^-^ Interpretation of the equilibrium constants are difficult due to solvation effects and the unknown entropy term. Indeed, different orderings of the stability of olefins toward the argentous ion have been found in water and in ethylene glycol,1o and vinyl fluoride is less stable toward rhodium(1) than ethylene, but probably has a stronger bond than ethylene. l 1 The enthalpic changes which are available are either inaccurate, measured in polar solvents, 1 2 - 1 4 or measured in different phases. 1 5 , 1 6 (1) J. H. Nelson and H. B. Jonassen, Coord. Chem. Reu., 6,27 (19711, see also other review articles quoted therein. (2) C. P. M. Beverwijk, G . J. M. Van der Kerk, A. J. Leusink, and J. G. Noltes, Organometal. Chem. Reti., Sect. A , 215 (1970). (3) U. Belluco, B. Crociani, R. Pictropaulo, and P. Uguagliati, Inorg. Chim. Acta Rec., 3 , 19 (1970). (4) F. R. Hartley, Chem. Rec., 69,799 (1969). ( 5 ) W. Partenheimer, Inorg. Chem., 11, 743 (1972). (6) J. R. Joy and M. Orchin, J . Amer. Chem. Soc., 81, 305 (1959). (7) S.I. Shupack and M. Orchin, ibid., 86, 586 (1964). (8) H. C. Volger, B. Gaasbek, H. Hogeveen, and I
