The effect of pressure on the hydrolysis equilibria of aqueous metal ions

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The logarithm of the pseudo-first-order rate coefficient k for the aquation of CO(NH,),X'~-"'+ can be represented by a quadratic equation in the pressure P, or, better, bywhere Pis in kbar, AV,* is the volume of activation at P = 0, and xis the increase in the number of water molecules solvating the complex as it goes to the transition state. For Xn-= SO:-, Cl-, NO;, and N; at 25' (75' for N;) and ionic strength I = 0.1 M LiC104/HC104, A G = -18.5, -10.6, -9.2, -6.3, and + 16.8 cm3 mol-', and x = 8.0,4.1,3.9, 1.9, and -4.2; for X"-= NCS-, the mean A V* from P = 0.001 to 2.5 kbar at 88" is -4 cm3 mol-'. Detailed consideration of these data, especially their correlation with the molar volume of reaction by a straight line of unit slope for X"-= SO:-, C1-, Br-, NO;, and H,O, provides strong evidence for a dissociative interchange mechanism. For Xn-= N;, the separating entity is probably HN, rather than N;. For Xn-= NCS-, aquation is incomplete, at practical complex concentrations; at 88.0°, 1 bar, and I =

Section: Discussionmentioning

confidence: 99%

Mechanistic Information on the Aquations of Complexes of the Pentaamminecobalt(III) Series from Kinetic Studies at High Pressures

Jones¹,

Carey²,

Swaddle³

1972

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“…Neither is it clear whether the measured AVT value given in ref. 6 refers to the pressure dependence of an experimental rate constant (first order in cr"' (total) and cr", inversely first order in [H']), or to exchange between the actual conjugate species C~(H?O)~OH'+ and cr"; in the former case, 3.8 Z 0.7 cm3 11101-I should be subtracted from Av: to allow for the volume change accompanying conjugate base formation (38), bringing A v f into excellent agreement with the value calculated for an inner-sphere process, as in the Fe""" case (28).…”

Section: Self-exchange Reactions In Aqueous Solution: Llanomalous9' Cmentioning

confidence: 99%

Reflections on the outer-sphere mechanism of electron transfer

Swaddle¹

1996

Abstract:The quantitative efficacy of the Stranks-Marcus-Hush theory of volumes of activation A v~ for outer-sphere electron transfer between metal complexes in solution is assessed. The theory predicts A v S accurately for several couples in aqueous solution, but is satisfactory for polar nonaqueous solvents only at pressures of ca. 100 MPa and above, and accuracy is not improved when the molecular nature of the solvent is allowed for through the Mean Spherical Approximation approach. At low pressures, the calculations become numerically unstable when the isothermal compressibility of the solvent is high and its relative permittivity is low, particularly for the more highly charged couples. For aqueous systems, departures from the predicted A v t afford insights into the role of the counterions, the incursion of inner-sphere pathways, the enhanced reactivity of CO""" cage complexes relative to conventional chelates, and the question of "spin forbiddenness" of electron transfer processes that involve a large change in spin multiplicity.Key words: redox kinetics, inorganic reaction mechanisms, pressure effects, Marcus-Hush theory, activation volumes. prkdites fournissent des renseignements sur le r6le des contre-ions, sur les incursions de voies impliquant les sphkres internes, I'augmentation de la rCactivitC des complexes de cage du CO""" par rapport aux chClates conventionnels et la question de l'<

“…These were made irz sirzl in a Cary Model 15 spectrophotometer, using the high-pressure optical cell assenlbly previously described (15). The temperature of the highpressure cell was maintained at 25.0 & 0.1' by circulating water from a thermostat bath through the walls of the spectrophotometer sample compartment, thzrmal contact being made through an aluminum mounting-block, and the temperature was measured with a calibrated Atkins thermistor inserted in a narrow well drilled close to the solution cavity.…”

Section: Kinetic Measuren?enfsmentioning

confidence: 99%

Pressure Effects upon the Rates of Aquation of Halopentaamminechromium(III) Ions: Evidence for an Associative Mechanism

Guastalla¹,

Swaddle²

1973

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For the replacement of X-by H 2 0 in Cr(NH3),X2+, AVOx (the value of the volume of activation AV* at zero pressure) at 25.0" in 0.1 m NH,CIOL is -10.8, -10.2, and -9.4 cm3 mol-' respectively for X -= Cl-, Br-, and I-, while A V, the molar volume of reaction, is -8.4, -7.2 and -6.0 cm3 mol-'. The pressure-dependence of AV" can be expressed in terms of the number x of water-molecules which are electrostricted as the systems go from ground-state to transition-state; for X-= C1-, Br-, and I-, x = 1.9, 2.0, and 1.7. These data, combined with AVv = -5.8 cm3 mol-' for the aquo-exchange of Cr(NH,j,-OH,,+, indicate an associative mechanism for these reactions, as does the contrast with AVO* data for the formally analogous, but dissociatively-activated, reactions of CO(NH,),X(~-")-.Dans le remplacement de X -par H z O dans Cr(NH,),X2+, AVO* (valeur du volume d'activation AV* B la pression zkro) a 25.0' dans NH,CIO, 0.1 n1 est de -10.8, -10.2, et -9.4 cm3 mol-' respectivement pour X -= CI-, Br-, et I-, tandis que AV, volume molaire de la reaction, vaut -8.4, -7.2, et -6.0 cm3 mol-'. Z'influence de la pression sur A V* peut Etre exprimee en fonction du nombre x de molecules d'eau, conditionnk electrostatiquement lorsque le systZme passe de l'etat fondamental a l'itat de transition; pour X-= C1-, Br-, et I-, x = 1.9, 2.0, et 1.7. Ces donnees, combintes a AV* = -5.8 cm3 mol-' pour I'Cchange a i'eau de Cr(NH3),0H,3+, indiquent un mecanisme associatif pour ces reactions, de m&me que les donnees contraires sur AVO* concernant les analogues formels, mais actives par dissociation, des rkactions de CO(NH,),X(~-"'+.[Traduit par le journal]Can 3 Chem.. 51. 821 (1973) We have recently shown (1-5) that the effect of pressure P upon the rates of substitution reactions of transition-metal complexes in aqueoi;s solution can lead to useful mechanistic information if either the reaction is suitably simple or there is an independent criterion with which the derived parameters (the volun~e of activation, AV", and its pressure dependence, if significant) can be compared. Thus, for the ideally simple aquo-exchange reaction of CO(NH,),OH,~+, AV* is + 1.2 cm3 mol-' and is pressure-independent within the experimental uncertainty (6), which suggests a dissociative mechanism. This conclusion is reinforced by the observation that for X n -= SO,*-, Cl-: Br-, NO3-, and FI,O in the reaction there is an excellent linear correlation of slope I .O between A V,'$ ( = A Yq' at zero pressure) and the molar volume of reaction AV> which indicates that the Co-X is completely broken, and