Kinetics of the Oxidation of Metal Complexes by Manganese(III) Aquo Ions in Acidic Perchlorate Media: The Mn(H20)62+-Mn(H20)63+ Electron-Exchange Rate Constant

Inorganic Chemistry

Volumn 24, Issue 21, 1985, Pages 3403-3409

  Macartney, Donal H ^a   Sutin, Norman ^a  

^a BROOKHAVEN NATIONAL LABORATORY   (United States)

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EID: 33845379176     PISSN: 00201669     EISSN: 1520510X     Source Type: Journal    
DOI: 10.1021/ic00215a022     Document Type: Article

References (70)

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44. Beattie, J. K.; Best, S. P.; Skelton, B. W.; White, A. H. J. Chem. Soc, Dalton Trans. 1981, 2105. The structure of the Mn(H20)63+ does not reveal appreciable Jahn-Teller distortion; the manganese-oxygen bond lengths are similar, and there is no evidence for exceptionally large thermal parameters.
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46. In these calculations a value of 1.56 V was used for the reduction potential of the unhydrolyzed Mn(H20)63+ ion.29'31'4411 For the reduction potentials of the ML33+ complexes, the values measured in 2.0 M H2SO.,45 or extrapolated to 2.0 M H2S04 from 0.5 M H2S0446 were employed, (b) Ciavatta, L.; Grimaldi, M. J. Inorg. Nucl. Chem. 1969, 31, 3071.
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49. This assumption seems reasonable since the electronic coupling in aquo or ammine systems in which a σ∗d electron is transferred is, in general, enhanced relative to πd systems by mediation via the ligands.22 See also: Logan, J.; Newton, M. D.; Noell, J. O. Int. J. Quantum Chem., Quantum Chem. Symp. 1984, 18, 213.
50. Macartney, D. H., unpublished observations.
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61. In a superexchange treatment of the three-site system abc the electronic coupling of the end sites a and c by b is equal to Hab/ΔE∗ provided that ΔE∗ » Hab and H bc(ΔE∗ = (Ec - Ea) = (Ec - Eb) at the intersection of the surfaces for a and c), i.e., the extent of mixing of an excited state configuration with the ground-state configurations is inversely proportional to the vertical energy separation. See, for example: Halpern, J.; Orgel, L. E. Discuss. Faraday Soc. 1966, 29, 32. Kuznetsov, A. M.; Ulstrup, J. J. Chem. Phys. 1981, 75, 2047. (b) Although a superexchange framework is being used, this is equivalent to the three-center bonding description used earlier38 and the same criteria that were considered necessary for a strong three-center interaction are also relevant here.
62. Based on the E° of 1.89 V for the -OH/OH" couple60b and a pKa of ≤-20 for H2O+.60c (b) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1984, 88, 3643. (c) Schwarz, H. A., personal communication.
63. Winkler, J., unpublished observations, (b) Note that the LMCT transition in Co(H20)63+ produces a low-spin Co(II) and it is necessary to correct for the energy difference, E∗(Co(II)), between the vibrationally relaxed low-spin state and the ground state of Co(II) in order to obtain the ΔG° for reaction 19. The estimate of ΔG° is very approximate largely because of the uncertainty in the X value for H20+/H20. An assumption underlying this approach is that the transition in the far-UV spectrum of Co(H20)63+ solutions is ligand to metal rather than solvent to metal in character.
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65. Winkler, J. R.; Rice, S. F.; Gray, H. B. Comments Inorg. Chem. 1981, 1, 47.
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67. Under normal circumstances the spin conversion is likely to be sufficiently rapid for the preequilibrium to be maintained. See, for example: Dose, E. V.; Hoselton, M. A.; Sutin, N.; Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1978, 100, 1141. (b) The diffusion-controlled and the maximum activation-controlled rate constants for the preequilibrium spin-change mechanism are KaKskciv., and Kskdiff (M_1 s~'), respectively, where Ks is the spin conversion equilibrium constant. In this model, reactions with faster rate constants involve direct reaction with the ground state in an "ordinary" (but superexchange-enhanced) outer-sphere mechanism.
68. See, for example: Sutin, N. Acc. Chem. Res. 1968. 1, 225. Haim, A. Prog. Inorg. Chem. 1983, 30, 273.
69. No structures in which a water molecule is the sole bridging group have been reported. However, double-bridged structures, in which one bridging group is a water molecule and the other a fairly basic oxygen, have been described. The two iron(II) centers in Fe3(P04)2-4H20 (Ludlamite) are bridged by a water molecule and a phosphate oxygen (iron-iron distance 3.27 Å): Abrahams, S. C.; Bernstein, J. L. J. Chem. Phys. 1966, 44, 2223. See also: Abrahams, S. C. J. Chem. Phys. 1966, 44, 2230. Evidence has recently been presented for the presence of a double μ-aquo, μ-oxo bridge in a binuclear copper(II) system (copper-copper distance 3.03 Å): Chadhuri, P.; Ventur, D.; Wieghardt, K.; Peters, E.-M.; Peters, K.; Simon, A. Angew. Chem., Int. Ed. Engl. 1985, 24, 57. (b) The rate constant for protonation of a coordinated water molecule in Cr(H20)63+ has been estimated to be 5.0 ± 10" M"1 s_1, ΔH∗ = 0.5 ± 0.5 kcal mol-1, and ΔS∗ = -36 ± 2 cal deg-1 mol-1: Swift, T. J. Stephenson, T. A. Inorg. Chem. 1966, J, 1100. (c) Theoretical calculations show that the formation of a hydrogen bond to the oxygen in H30+ is endergonic: Newton, M. D. J. Chem. Phys. 1977, 67, 5535.
70. The reorganization of the inner-coordination shells, even for relatively large displacements as in the Co(H20)62+/3+ exchange, is adequately calculated by harmonic oscillator expressions (with nuclear tunneling corrections as appropriate). For the Co(H20)62+/3+ exchange, the difference between the inner-shell reorganization energy for the outer-sphere mechanism calculated with harmonic and Morse potential functions for the M-OH2 bonds is less than 0.3 kcal mol-1 (with the Morse potential giving the larger reorganization energy).68b Interestingly, despite the similarity in the reorganization energies, the nuclear configuration of the outer-sphere transition state is quite different in the harmonic and Morse calculations. The calculation of the inner-shell reorganization energy is somewhat more complicated for the bridged mechanism. The reorganization energy for the ten nonbridging water molecules is presumably similar to the value for the outer-sphere mechanism, while the reorganization energy for the bridging water molecule is quite sensitive to the assumptions made about the cobalt-cobalt separation and the cobalt-oxygen distances in the precursor and transition states. However, calculations68b using reasonable values for these parameters and Morse functions show that the inner-shell reorganization energy in the water-bridged mechanism is unlikely to be more than 1 kcal mol-1 lower than the value for the outer-sphere mechanism. (b) Brunschwig, B. S., unpublished calculations.