Outer-sphere association

Determination of stability constants towards outer-sphere association reactions for cis-[Ir(phen)2Cl2]X, X = C1 , Oac-, 11 COO, gives the following decreasing order Cl >Oac > HCOO. 50... 

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 ... 

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... 

Another factor which increases outer-sphere association and leads to an increase in formation rate is caused by stacking interactions (27). In 1966 Wilkins (28) observed that the rate... 

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. 

Cerium(IV) oxidations of organic substrates are often catalysed by transition metal ions. The oxidation of formaldehyde to formic acid by cerium(IV) has been shown to be catalysed by iridium(III). The observed kinetics can be explained in terms of an outer-sphere association of the oxidant, substrate, and catalyst in a pre-equilibrium, followed by electron transfer, to generate Ce "(S)Ir", where S is the hydrated form of formaldehyde H2C(OH)2- This is followed by electron transfer from S to Ir(IV) and loss of H+ to generate the H2C(0H)0 radical, which is then oxidized by Ce(IV) in a fast step to the products. Ir(III) catalyses the A -bromobenzamide oxidation of mandelic acid and A -bromosuccinimide oxidation of cycloheptanol in acidic solutions. ... 

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... 

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. 

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... 

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]. 

Scheme 3. Kinetic Scheme for DNA-Platination Involving the Preliminary Formation of an Outer-Sphere Association. Pt stands for Ptn bound to amino and eventually chloro...

More work is needed to clearly establish the role of outer-sphere association in DNA-platination. We can infer its influence on the rate of platination according to the relation kp = kK0 [N]/(l + K0 [N]) (with N = nucleotide-binding sites of Pt, i.e., N G, [N] [Pt]) (Scheme 3). It could also influence the selectivity of platination via selective association between the cationic species and the sites of higher negative electrostatic potential. To test this hypothesis one will have to analyze the influence of various sequences, of different types of platinum ligands, and of the ionic status of the DNA medium. 

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]. 

Reference to Fig. 7.22 shows that the exchange rate constant increases in proportion to [DMF] for which the following scheme involving outer sphere association is proposed. 

Since substantial amounts of free trimethylaluminium i.e. Al2Me6) are present in normal MAO preparations, cationic trimethylaluminium adducts Cp 2Zr-( r-Me)2AlMe2, in outer-sphere association with their MeMAO counteranions, are in general the dominant species in MAO-activated catalyst systems [D. E. Babushkin, N. V. Semikolenova, V. A. Zakharov, E. P.Talsi, Macromol. Chem. Phys. 2000, 201, 558]. While being quite stable against destructive side reactions MAO-activated catalyst systems are thus often less active than those obtained with the cationization reagents described above. 

In equation (5), is the equilibrium constant for the outer-sphere association of the donor and acceptor, is the electronic transmission coefficient (the probability that products form once the nuclear configuration of the transition state is achieved), Vnu is the effective frequency for nuclear motion along the reaction coordinate in the neighborhood of the transition state, and the nuclear transmission coefficient nu is the classical exponential function of the activation energy. The weak-coupling limit corresponds to the limit in which Kei < 1, and for the strong-coupling limit /Cei = 1. 

The concept of coordination in the second sphere was introduced by Werner. All authors agree that such outer-sphere association exists in solution, hut they disagree about the kind and the extent of this association. Some advocate a second-sphere coordination which is closely analogous to the inner-sphere coordination. The data which support this hypothesis are not very convincing and can be criticized in various ways. The present author finds that the electrostatic theories of N. Bjerrum, Fuoss, and Kraus, according to which the formation of the ion-associates is a result of coulombic attraction, both qualitatively and quantitatively, give the most trustworthy picture of the outer-sphere association. However, this does not exclude the fact that some preferred mutual orientation exists in the ion pairs. 

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. 

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