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Inorg. Chem. 1988, 27, 4412-4474
K1and K2 as in eq 4. Thevalue of k4 is 1.9 M-' s-l. Both schemes indicate that pentacyano and hexacyano complexes exist in basic aqueous solution and that their stability constants are not very large. Acknowledgment. This work was supported by National Institutes of Health Grant GM-12152 and by National Science
Foundation Grants CHE-8616666 and CHE-8720318. Registry No. Nilll(CN),(HzO),, 9701 1-73-9;Ni"'(CN):-, 9701 180-8; Ni111('3CN)4(Hz0)2-, 117226-39-8; Ni"'(CN),(NCO)?-, 9701 179-5;Ni"'(CN)4(C1)23-, 54003-01-9;Nilll(NCCHp)z;, 9701 1-77-3; Ni"'(py) MeOH > M G O ) was reported for solvolysis of the sterically hindered Pd(Et,dien)X+ c ~ m p l e x e s . ~ ' -The ~ ~ chemical shifts of the methyl protons indicated that smaller solvent molecules can interact better with the crowded Pd(I1) center than the larger ones. It was suggested on the basis of the AS*values that solvolysis is associative for water as solvent but dissociative for M e 2 S 0 and D M F as solvents.23 However, in later work2' the error limits of the AS* data were taken into account and the results (including AV data) were (18) Belluco, U.; Martelli, M.; Orio, A. Inorg. Chem. 1966, 5, 582. (19) Belluco, U.; Orio, A.; Martelli, M.Inorg. Chem. 1966, 5, 1370. (20) Belluco, U.; Graziani, M.; Nicolni, M.; Rigo, P. Inorg. Chem. 1967, 6, 721. (21) Palmer, D. A.; Kelm, H. Inorg. Chim. Acta 1980.39, 275. (22) Goddard, J. B.; Basolo, F. Inorg. Chem. 1968, 7 , 2456. (23) Roulet, R.; Gray, H. Inorg. Chem. 1972, 11, 2101.
interpreted in terms of an I, mechanism. The present data, however, exhibit a good agreement with data for nonhindered systems, and there is no reason to believe that a change in mechanism is at hand. The values of AV in Table I11 are all small and slightly negative and do not exhibit a specific trend with the nature of the solvent. The solvation of the Pd(Me5dien)py2+complex in the ground and transition states is expected to be very similar, since the entering and leaving groups are neutral and no substantial change in dipole moment is expected. This is in agreement with similar findings for solvolysis of Pt(I1) complexes,24where the solvent-substrate interaction is not of much kinetic significance because the nucleophilic discrimination factors change to approximately the same extent when the solvent is changed for various complexes. Our AV data, therefore, mainly reveal intrinsic effects in which the volume decrease associated with the binding of the entering solvent molecule is partially compensated for by the volume increase associated with the square-pyramidal to trigonal-bipyramidal transition as mentioned before. The partial molar volumes of the solvent molecules, as estimated from their densities at 20 OC, are 58.4 (EtOH), 40.4 (MeOH), 18.0 (H,O), 70.9 (Me2SO), 77.0 (DMF), and 52.2 cm3 mol-' (MeCN), from which it follows that there is no apparent correlation with the corresponding values of AV. Our earlier work has demonstrated that the extent of volume collapse during bond formation depends on the size of the entering solvent molecule, up to a point where this collapse is hindered by steric effects for large entering m ~ l e c u l e s . ' ~ It . ~ follows ~ that this volume collapse could be very small in the case of a large molecule such as Me2S0. The overall value of AV is therefore mainly controlled by steric and intrinsic effects. Finally, it should be mentioned that AV data of similar magnitude were reported for solvent exchange on Pd(H20)42+(-2.2 f 0.2 cm3 mol-')' and for solvolysis of Pd(H20)3Me2S02+(-1.7 f 0.6 cm3 mol-1):6 which are both believed to occur according to an associative mechanism. Acknowledgment. We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie and a DAAD fellowship to S.B. Registry No. Pd(Me5dien)py2+,117226-34-3;MeOH, 67-56-1; EtOH, 64-17-5; Me2S0, 67-68-5; DMF, 68-12-2; MeCN, 75-05-8. (24) Cattalini, L. Prog. Inorg. Chem. 1970, 13, 263. (25) Bajaj, H. C.; van Eldik, R. Inorg. Chem. in press. (26) Ducommum, Y.;Merbach, A. E.; Hellquist, B.; Elding, L. I. Inorg. Chem. 1987, 26, 1759.
Contribution from the Department of Chemistry, University of Mar del Plata, 7600 Mar del Plata, Argentina
ESR Studies of Intramolecular Electron Transfer in Malonic Acid Radical Chelates of Cerium( IV) M. A. Brusa, L. J. Perissinotti, and A. J. Colussi* Received January 5, 1988 The reduction of Ce(IV) complexed to dicarboxymethyl (malonic acid radical; 'CH(COOH),) has been directly investigated by kinetic electron spin resonancespectrometryin aqueous perchloric acid media at 298 K. The corresponding fust-order rate constant and Ce(1V)-chelated (M2) dicarboxymethyl radicals have slightly different has a value of k l l = (1.5 f 0.3) X lo3s-l. Free (MI) magnetic parameters [g(MJ = 2.0039, a(H.)(Ml) = 20.3 G, g(M2) = 2.0035, a(H,)(M,) = 19.6 GI and coexist in equilibrium, the ratio r = [M2]/[Ml] being a function of both [Ce(IV)] and [H+]. The stability constant of the radical chelate, K8 = 242 f 31 M-I, is considerably larger than that of malonic acid itself, K5 = 1.14 M-I. The product K 8 k l l ,which represents the overall rate constant for dicarboxymethyl radical oxidation by Ce(IV), has a value within the range of those measured for the direct oxidations of substituted secondary alkyl radicals by Ce(IV), Co(III), and Ti(1V).
Introduction Electron-transfer reactions between transition metal ations and alkyl radicals is an area of current They are relatively (1) Espenson, J. H.; Shimura,M.; Bakac, A. Inorg. Chem. 1982,21,2537.
fast processes with rate constants in the range 105-106 M-' s-' and can proceed in principle either by a direct mechanism or by (2) Bakac, A.; Espenson, J. H.; Lovric, J.; Orhanovic, M. Inorg. Chem. 1987, 26, 4096.
0020-1669/88/1327-4474$01.50/0 0 1988 American Chemical Society
