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Con-N bond lengths. But, the Co-N stretching frequency of Com(taptacn) is 519 cm"1 (identified by an 8-cm"1 shift upon deuteriation of the primary amines) compared to 479 cm'1 for Coln(tacn)2.45 The greater force constant can then lead to a greater inner-sphere reorganization energy.It has been shown recently that the self-exchange parameters of Co3+/2+ couples may be treated in the framework of the Marcus-Sutin model for outer-sphere electron transfer.38 For complexes of the type under consideration the preequilibrium constant (A0) is 0.04 (/ = 0.1 M) and radii (/•) are 9.0 Á (with reaction thickness = 0.8 Á). From these values, a self-consistent set of parameters for the Marcus-Sutin model has been determined for the Co(taptacn)3+/2+ couple: (/ * = 13.8 kcal/mol; <7 * = 5.2 kcal/mol, and d2-d} = 0.165 A, yielding kmc = 0.04 M"1 s'1 as observed in the present study. The somewhat smaller Ad value calculated for the present complexes is in keeping with the Co(III) species being slightly distorted toward the Co(II) structure (45) Boeyens,

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The dependences of the electrode potentials E of aqueous Fe(H20);+/'+, ~e(~hen),)+/'+, F~(cN)~)-/"-, and ~o ( s e~) ' + / ' + upon pressure P (up to ca. 200 MPa) have been measured at 25.0°C relative to an AgCl/Ag(satd. KCI) electrode by cyclic voltammetry. For ~o ( s e~)~+ / ' + at ionic strength I = 0.28 and 1.00 rnol kg-', (dE/dP), is independent of P as well as I, giving a volume of reaction AV = + 13.7 ? 0.4 cm3 mol-' relative to AgCl/Ag; similarly, for F~( H~o ) ; + /~+ at I = 0.28 rnol kg-' (CF3S03H), AV = +5.0 * 0.3 cm3 mol-'. For F~(cN);-/~-in KC1 media, (dE/dP), is independent of P and I at I = 0.28 and 0.51 rnol kg-', giving AV = -36.1 ? 1.0 cm3 mol-', but shows some slight dependence on P at I = 1.00 rnol kg-' (low-pressure limit AVO = -38 * 1 cm3 mol-'; (dAV/dP), = +0.030 cm3 mol-' MP~-I).For ~e (~h e n )~~+ / ' + , (dE/dP), is markedly pressure and medium dependent, with AVO = +14.2 ? 0.5 and +6.2 ? 0.5 cm3 mol-' in KNO, media at I = 0.25 and 1 . OO rnol kg-', respectively. The peak-to-peak separation of the cyclic voltammograms increases with increasing P for Fe(cN)$-I4-, but decreases slightly for the cationic couples; these trends can be accounted for in terms of the volumes of activation for the corresponding self-exchange reactions.

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