Metal ions outer-sphere complexes

Figure 1 A possible structure of a metal-ATP outer-sphere complex. The metal ion is octahedrally co-ordinated, and two water molecules not involved in hydrogenbonding to the ring have been omitted (Reproduced by permission from Biochemistry, 1971,10, 843)...

On the basis of these results it seems to the present author that inner and outer complexes can reasonably be assumed for the electron transfer to the diazonium ion, but that an outer-sphere mechanism is more likely for metal complexes with a completely saturated coordination sphere of relatively high stability, such as Fe(CN) (Bagal et al., 1974) or ferrocene (Doyle et al., 1987 a). Romming and Waerstad (1965) isolated the complex obtained from a Sandmeyer reaction of benzenediazonium ions and [Cu B ]- ions. The X-ray structural data for this complex also indicate an outer-sphere complex. 

Electron transfer between metal ions contained in complexes can occur in two different ways, depending on the nature of the metal complexes that are present. If the complexes are inert, electron transfer occurring faster than the substitution processes must occur without breaking the bond between the metal and ligand. Such electron transfers are said to take place by an outer sphere mechanism. Thus, each metal ion remains attached to its original ligands and the electron is transferred through the coordination spheres of the metal ions. 

Where solvent exchange controls the formation kinetics, substitution of a ligand for a solvent molecule in a solvated metal ion has commonly been considered to reflect the two-step process illustrated by [7.1]. A mechanism of this type has been termed a dissociative interchange or 7d process. Initially, complexation involves rapid formation of an outer-sphere complex (of ion-ion or ion-dipole nature) which is characterized by the equilibrium constant Kos. In some cases, the value of Kos may be determined experimentally alternatively, it may be estimated from first principles (Margerum, Cayley, Weatherburn Pagenkopf, 1978). The second step is then the conversion of the outer-sphere complex to an inner-sphere one, the formation of which is controlled by the natural rate of solvent exchange on the metal. Solvent exchange may be defined in terms of its characteristic first-order rate constant, kex, whose value varies widely from one metal to the next. 

Detailed kinetic data are rare for natural aquatic ligands. For simple, not strongly binding ligands, it has been shown [164,165,171] that the dehydration of M(H20)q+ subsequent to the formation of an outer sphere complex or ion pair (Eigen-Wilkens mechanism, equation (27)) is often the rate-limiting step in the formation of the metal complex, ML. This mechanism has often been applied to natural ligands [5,167,171] without further confirmation of its validity. 

As Fig. 2.4 illustrates, a cation can associate with a surface as an inner sphere, or outer-sphere complex depending on whether a chemical, i.e., a largely covalent bond, between the metal and the electron donating oxygen ions, is formed (as in an inner-sphere type solute complex) or if a cation of opposite charge approaches the surface groups within a critical distance as with solute ion pairs the cation and the base are separated by one (or more) water molecules. Furthermore, ions may be in the diffuse swarm of the double layer. 

The Homogeneous Case. Margerum (1978) and Hering and Morel (1990) have elaborated on mechanisms and rates of metal complexation reactions in solution. In the Eigen mechanism, formation of an outer-sphere complex between a metal and a ligand is followed by a rate limiting loss of water from the inner coordination sphere of the metal, Thus, for a bivalent hexaaqua metal ion... 

The separation of the two stages is easier to discern when the rates of the two processes are so different, but it can also be seen in the ultrasonic spectra of metal-sulfate systems (Sec. 3.4.4). Ultrasonic absorption peaks can be attributed to formation of outer-sphere complexes (at higher frequency, shorter t) and collapse of outer-sphere to inner-sphere complexes (at lower frequency). In addition to uv spectral and ultrasonic detection, polarimetry and nmr methods have also been used to monitor and measure the strength of the interaction. There are difficulties in assessing the value of ATq, the outer-sphere formation constant. The assemblage that registers as an ion pair by conductivity measurements may show a blank spectroscopically. The value of Aq at T" K may be estimated using theoretically deduced expres-... 

The extractabilities of metal-organic complexes depend on whether inner or outer sphere complexes are formed. Case 1, section 4.2.1, the extraction of ura-nyl nitrate by TBP, is a good example. The free uranyl ion is surrounded by water of hydration, forming U02(H20)f, which from nitric acid solutions can be crystallized out as the salt U02(H20)6 (N03), though it commonly is written U02(N03)2(H20)6. Thus, in solution as well as in the solid salt, the UOf is surrounded by 6 HjO in an inner coordination sphere. In the solid nitrate salt, the distance du.o(nitrate) between the closest oxygen atoms of the nitrate anions, (0)2N0, and the U-atom is longer than the corresponding distance, du-o(water), to the water molecules, OH2, i.e., du.o(nitrate) > 4u.o(water) thus the nitrate anions are in an outer coordination sphere. 

This account is concerned with the rate and mechanism of the important group of reactions involving metal complex formation. Since the bulk of the studies have been performed in aqueous solution, the reaction will generally refer, specifically, to the replacement of water in the coordination sphere of the metal ion, usually octahedral, by another ligand. The participation of outer sphere complexes (ion pair formation) as intermediates in the formation of inner sphere complexes has been considered for some time (122). Thermodynamic, and kinetic studies of the slowly reacting cobalt(III) and chromium(III) complexes (45, 122) indicate active participation of outer sphere complexes. However, the role of outer sphere complexes in the reactions of labile metal complexes and their general importance in complex formation (33, 34, 41, 111) had to await modern techniques for the study of very rapid reactions. Little evidence has appeared so far for direct participation of the... 

Until now, only a few theoretical studies of porphyrin metalation by divalent metal ions in solution have been reported (72,94,95). In the first theoretical work (94,96) on this topic, insertion of Fe2+ and Mg2+ into the porphyrin ring was studied by DFT methods. The authors followed the reaction from the outer-sphere complex formation via stepwise displacement of the solvent molecules until... 

In coordination chemistry two types of complex can occur between metals and complexant ligands. Outer-sphere complexes are relatively weak electrostatic associations between a hydrated metal ion and a complexant ligand, and in which both of the charged species retain a hydration shell. In contrast, inner-sphere complexes are stronger interactions in which a covalent bond is formed between a metal ion and a ligand. 

The incorporation of metals into metal-free porphyrins has been reported to proceed by a dissociative mechanism.278 The porphyrin and metal form an outer-sphere complex, the porphyrin nucleus deforms to provide a suitable configuration to complex with the metal ion, and then dissociative exchange occurs as shown ... 

Other types of complexes in aqueous as well as in nonaqueous solutions can also be successfully studied by means of X-ray diffraction and their structures can often be derived, completely or in part (7,12-15). Structural changes in the coordination sphere of a metal ion, caused by the stepwise replacement of solvent molecules on addition of a ligand, can be followed. Under favorable conditions structures can be derived for the whole series of complexes formed, from the solvated noncom-plexed metal ion to the complex in which all solvent molecules have been replaced. Structural changes beyond the first coordination sphere can sometimes be determined, making it possible to differentiate between inner- and outer-sphere complexes. Even when the information that can be obtained from X-ray diffraction measurements is not sufficient for an unambiguous determination of a structure, it may still give significant structural features, which, in combination with information from other sources, can lead to the complete structure. 

In recent years, there has been a great deal of interest in the mechanisms of electron transfer processes.52-60 It is now recognized that oxidation-reduction reactions involving metal ions and their complexes are mainly of two types inner-sphere (ligand transfer) and outer-sphere (electron transfer) reactions. Prototypes of these two processes are represented by the following reactions. 

The adsorption reaction that occurs between metallic ions and the charged surfaces of clay-organics may involve formation of either relatively weak outer-sphere complexes, or strong inner-sphere complexes. 

Interaction 1 denotes electrostatic forces between humic substances (negatively charged) and metal ions (positively charged). It is a relatively weak interaction (outer-sphere complex) and the cation can be readily exchanged by other weakly bonding cations,... 

Outer and Inner Sphere Complexes. Outer sphere complexation involves interactions between metal ions and other solute species in which the co-ordinated water of the metal ion and/or the other solute species are retained. For example, the initial step in the formation of ion pairs, where ions of opposite charge approach within a critical distance and are then held together by coulombic attractive forces, is described as outer sphere complex formation. 

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