Outer-sphere association constant

Rate constants for the formation of complexes from the aquometal ion and various chelating ligands are often predicted from the expected outer-sphere association constant (K ) and... 

In Eqs. (5.22-5.23), coordination water molecules are omitted and A ass indicates the ion pair (or outer sphere association) constant. In general is a small number (<10) and Eq. (5.23) simplifies to... 

Cl, Br [Pt(en)3] X- Measured outer-sphere association constants. Bromide interacts with [Pt(en)j] less strongly than Cl . Values higher than an earlier estimation e f... 

A ligand such as SCN shows a relatively higher tendency to inner complex formation than does S04 (36). Fronaeus and Larsson (16) assume that the C—N stretching frequency is nearly the same for the free ligand as for thiocyanate bound in the outer sphere, and under this assumption they estimated from infrared absorption measurements the inner-and outer-sphere association constant in the nickel(II)-thiocyanate system at average ionic strengths to be ... 

The factors that may affect the value of the outer-sphere association constants for aqueous solution are discussed in Ref. 118. It can be inferred that both electrostatic forces and hydrogen bonding contribute significantly to trication complexes. With mono- and... 

Calculated outer-sphere association constants for Ni(II) complexes [105]... 

The pseudo-first-order rate constant (/robs) for the base hydrolysis of amino-carboxylato-complexes of Mo, and vary with [OH ] according to the general equation /robs=(/ri-l-/r2i ro[OH-]")/(l-l-J5 o[OH ] ) with n=2 for and n=l for Mo i and W i, where Ko is the outer-sphere association constant and k and k are the rate constants for reaction of the rapidly formed ion-pair with HgO and OH ion respectively. Complexes with ida , edda , nta , and edta ions were investigated, and non-bonded carboxylate arms were found to increase values of k and k by factors of 10 —10 . An analytic procedure has been proposed, based on these reactions, which allows concentrations of Mo, and to be estimated to an accuracy of ca. 5 % at molarities in the region of 10 M. ... 

The reaction of Mg + with pyrophosphate is about twice as slow in D2O as in H2O (at 15 °C). This difference is attributed to a change in the outer-sphere association constant rather than to a change in the interchange rate constant. Kinetics of solvolysis of [Fe(bipy)3] + in D2O lend support to the mechanism of dissociation via a unidentate-bipyridyl transient intermediate postulated, for aqueous solution, many years ago. ... 

The dependence of rate constant on ionic strength is still widely used, often in conjunction with the dependence of rate constant on dielectric constant, as an indicator of substitution mechanism. Recent instances of this classical approach include the reaction of /ra j-[Co(dmgH)2(SCN)(tu)] with thiourea (tu), aquation of trans-[Rh(dmgH)2Cl(tu)], aquation of the [Co(02CCHaCl)(NH3)6]"+ cation, and substitution at the [Fe(CN)6(OH2)] anion by nitrite, thiocyanate, sulphite, or nitrosobenzene. Salt effects on observed rate constants for the reaction of nickel(n) with pyrophosphate operate via the outer-sphere association constant rather than via the interchange rate constant. ... 

Eigen-Tamm-Wilkins mechanism for complex formation.The same mechanism is thought to operate in the formation of the oxalate complex from cis-[Co(en)2(NH3)(OH2)]. Again all three forms of the ligand, ox , oxH", and 0XH2, react with the aquo complex. The outer-sphere association constant for the oxH" anion is 1.5, for the ox anion 5.8 dm mol ° The kinetic results here are compared with those established several years ago for the reaction of [Co(NH3)5(OH2)] with oxalate.The formation of the sulfito complex [Co(NH3)5(S03)] is mentioned in Section 5.7.7. below. 

There have been a few reports over the years on nitrate catalysis of substitution at cobalt(III) and at chromium(III). Now it has been shown that nitrate has a catalytic effect on oxalate anation of a-c/5-[Co(edda)(OH2)2] - The outer-sphere association constant appears to be larger when nitrate is present. The possibility of the formation of a transient five-coordinate intermediate under the influence of the nitrate (cf. the end of the previous section, 5.7.3.) should not be ruled out. ... 

As indicated in Section 5.7.4.3, the effects of ion pairing on rates of dimethyl sulfoxide exchange with the [Co(NH3)5(dmso)] cation have been established.The small accelerating effects of ion pairing are listed in Table 5.14. Activation enthalpies and activation entropies for dimethylformamide exchange with [Co(NH3)5(dmf)] both decrease as the proportion of dimethylformamide increases in water + dimethylformamide solvent mixtures. These trends are discussed in terms of an interchange mechanism and the effects of interactions between free and coordinated dimethylformamide molecules on the outer-sphere association constant. 

Further evidence has been obtained for the enhancement of the rate of ternary complex formation between octahedral [L Ni(II)] and L (where L and L are both aromatic) through the unusually large outer-sphere association constants attributable to the stacking interactions between the incoming and bound ligands. On the other hand, the substitution of the axial water molecule in the... 

Outer-sphere association constants measured for the [Fe(dipic)2] reduction of binuclear cobalt(III) complexes (2), [Co(III)2] with stacking ligands,... 

Kinetic data for formation of pentacyanoferrate(II) complexes [Fe(CN)5L]" from [Fe(CN)5(OH2)f" are shown in Table 8.2. The Eigen-Wilkins mechanism applies here overall formation rate constants kf are determined by outer-sphere association constants that is, mainly by electrostatics. However, there is an unexpected inconsistency if one compares N2H4/N2H5 with en/enH, though this may well arise in part from solvation effects (en much less hydrophilic and thus less heavily solvated than N2H4) (cf. previous paragraph),There seems to be some difficulty... 

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