Outer-sphere association complex

Ion pairs are outer-sphere association complexes, which have to be clearly distinguished from the organometallic complexes discussed in Section 6. Ion pair formation appears to be much less important in biological membranes as compared with octanol, because the charge of the ions at the membrane interphase can be balanced by counter charge in the electrolyte in the adjacent aqueous phase. The reactions involved in ion pair formation are depicted in Figures 5b for acids and 5c for bases, and the equilibrium constant K ix is defined as follows ... 

The complexation reactions usually proceed by the Eigen mechanism (Diebler and Eigen 1959, Eigen 1963, Eigen and Tamm 1962). This mechanism involves two steps, the rapid formation of an outer-sphere association complex (i.e., an ion pair) and the subsequent rate-determining step in which the ligand displaces one or more water molecules. 

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... 

The nature of the ternary species is unclear but the addition of perchlorate to the complex causes no detectable shift in the position of the peaks in the visible spectrum. This may indicate that the anion effect involves an outer-sphere association or it may involve a weak axial inner-sphere coordination. 

Cobalt(lll).—Complexes. Ammine complexes. Optical activity can be induced in the complexes [Co(NH3) ] and [Cofenlj] by means of outer-sphere association with chiral anions, e.g. (- - )-tartrate. Circular dichroism is observed in the d-d bands of the cations and it is suggested that this is due to (a) direct interaction between the chiral anion and the metal f/-orbitals and (b) the preferred conformation adopted by the inner-sphere ligands in the presence of a helical outer-sphere ligand. 

Evidence for Outer-Sphere Association between Cationic Platinum Complexes and DNA... 

Studies with aqua species of Ni11 showed a marked preference for G-rich oligonucleotides (K0 2 X 105 M 1 for poly(dG)-poly(dC)), an association that is actually followed by the reversible binding of nickel to a G-N(7) [88], This case can be considered as intermediate between that of Mg11 and that of the aqua platinum(II) complexes which will irreversibly bind to a guanine after outer-sphere association. The corresponding kinetic equation can be written as in Scheme 3. The formation of the outer-sphere association is likely to be diffusion controlled in solution, in vitro. The coordination step (k) should be rate-determining [95]. 

It is to be noted that irrespective of the weaknesses of equation (7.3), it has been extensively used in kinetic studies, especially in kinetics of fast complex formation reactions. The above equation was tested for its applicability to outer-sphere association reactions by ultrasonic absorption methods [8]. 

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... 

Now consider mechanism (3). The rate determining step, k2, is preceded by an outer-sphere association with the activated complex having a weakly bound Y. Using the laws of mass action and conservation of mass, the rate of formation is equation (4.15). Under conditions of excess [Y ], kobs = 2k iP[Y ]/n + A jpfY-]) and kf = k2Kn>/( + ATip[Y—]). When A fY-] is small (second-order kinetics are observed. Thus we see that, under the defined conditions, both mechanisms (2) and (3) give second-order kinetics and, therefore, are not distinguishable. 

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

Outer-sphere complexes have been discussed by several authors in the context of ligand substitution reactions of metal complexes 29, 30). We consider here an example relevant to reaction (2), the outer-sphere association of TMPyP(- -4) with MA , x=0, 1, 2. 

It became evident during the discussion in Sections 3 and 4 that the inner sphere A of complexes MA determines the rates of metalloporphyrin formation to a high degree. Moreover, the conceptual separation of outer-sphere association from specific-site association is supported by the effects of solute components suchasTs, Hatir -, Fe(tir)i, Fe(tir)8 , which merely represent the most obvious cases with regard to association phenomena. It is justified to factorize a second-order rate constant according to... 

Figure 1.10. Surface hydroxyl groups (shaded) on kaolinite. Besides the OH groups on the basal plane, there are aluminol groups, associated with Lewis acid sites, and silanol groups protruding from the edge surface. The right side of the figure shows an outer-sphere surface complex between an ionized H2O and Na" ", as well as complexes between the silanol groups and OH (i.e., proton dissociation).

Solvent molecules may also be situated in the outer sphere of complex cations. For instance, Fung [Fu 67] used NMR examinations to demonstrate the outer-sphere association between the complex cation tris(ethylenediamine) cobalt(III) and the solvent (deuterated dimethyl sulphoxide-D20). 

In spite of the many-sided use of the various methods of examination, involving small or large instruments, it cannot be said that the understanding of the solvent effect in the various systems is simple. In non-aqueous solutions the conditions are perhaps even more complicated than in aqueous solutions. In most non-aqueous solvents the interaction between the solvent and the solute (the solvation) is not substantially less than in water. At the same time, the interactions of the components of the solutes with one another, the association reactions favoured by the lower relative permittivity (ion pair and complex formation, oligomerization and polymerization) are much more pronounced in non-aqueous solutions than in water. Non-aqueous solvents favour in particular the formation of the species with more complicated compositions (e.g., mixed ligand, polynuclear and outer-sphere type complexes). This also shows up in the more involved mechanisms of the reactions occurring in such systems. Nevertheless, as may be seen from the subject matter surveyed in this volume, more authors have dealt with the study of the simple formations, primarily mononuclear parent complexes and simpler mixed complexes, and with the reactions in which they participate, than with the study of the more compUcated systems, which occur more frequently in reality and which are of greater practical importance. 

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. ... 

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