Outer-sphere complex formation

However, there are a number of other miscellaneous biological roles played by this complex. The [Co(NH3)6]3+ ion has been shown to inhibit the hammerhead ribozyme by displacing a Mn2+ ion from the active site.576 However, [Co(NH3)6]3+ does not inhibit ribonuclease H (RNase),577 topoisomerase I,578 or hairpin ribozyme,579 which require activation by Mg2+ ions. The conclusions from these studies were that an outer sphere complex formation between the enzyme and Mgaq2+ is occuring rather than specific coordination of the divalent ion to the protein. These results are in contrast to DNase I inhibition by the same hexaammine complex. Inhibition of glucose-induced insulin secretion from pancreatic cells by [Co(NH3)6]3+ has been found.580 Intracellular injection of [Co(NH3)6]3+ into a neurone has been found to cause characteristic changes to the structure of its mitochondria, and this offers a simple technique to label neuronal profiles for examination of their ultrastructures.581... 

The dashed line in the complex in (4.21) and (4.22) indicates an outer-sphere (o.s.) surface complex, Kos stands for the outer-sphere complex formation constant and kads [M 1 s 1] refers to the intrinsic adsorption rate constant at zero surface charge (Wehrli et al., 1990). Kos can be calculated with the help of a relation from Gouy Chapman theory (Appendix Chapter 3). 

We can now calculate the outer-sphere complex formation constant according to Eq. (4.24) ... 

DR. DAVID RORABACHER (Wayne State University) A point which is frequently overlooked is that the calculations generally applied for determining the extent of ion-pair (or outer-sphere complex) formation in substitution reactions may be overly simplistic. There are many types of interactions which tend to perturb the extent of outer-sphere complex formation relative to the purely statistical calculation commonly made which takes into account only the reactant radii and electrostatic factors. 

AK is estimated as 0 cm mol" for outer-sphere complex formation involving an uncharged species and =3 cm mol for reactants whose charge product is -2. ... 

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... 

A proper D mechanism requires that kx be identical to the rate constant for the exchange of solvent (due account being taken of any statistical correction when more than one solvent molecule is present) and the value of k2 (in reality the term fc2/fc i[S] is used because the constants cannot be separated) should be sensitive to the chemical nature of L rather than its size and charge (factors that control Kos in an interchange mechanism). The most convincing demonstration of a D mechanism would be found in cases where k2/k-1[S] is much larger than any value expected for an outer-sphere complex formation constant, but this is not a necessary requirement for the mechanism. 

Although the conceptual distinction between an /a and an A mechanism is that in the latter case there can be direct reaction between the substrate and the entering nucleophile, the rate law for the A mechanism can still be affected by outer sphere complex formation. If the substrate is in equilibrium with its outer sphere complex then, if the reaction is carried out under pseudo first-order conditions... 

A good example of outer-sphere complex formation is the adsorption of Co(CN -) ions on y Al203. As can be seen from Fig. 9.3, lowering the pH leads to a concomitant increase in the amount of anion adsorbed, while neutralizing the solution again induces the Co(CN) 3- ions to desorb again. The process is perfectly reversible, which is not usually the case for inner-sphere complex formation. 

The overall rate const kf is measured by the relaxation methods. Assuming that the outer-sphere complex formation is faster than the water substitution by ligand L, the interchange rate constant k can be calculated using the equation... 

The mechanism involving a fast outer-sphere complex formation step is not favoured by some schools of thought. If both the inner- and outer-sphere processes are of comparable... 

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. 

Diffuse layer metal retention and outer sphere complex formation involve electrostatic attractive forces, which are characteristically weaker than co-ordinative interactions leading to inner sphere surface complex formation. A number of factors influence metal interactions with surfaces, including the chemical composition of the surface, surface charge, and the nature and speciation of the metal ion. The importance of the pH of the aqueous phase in these interactions will be discussed further in Section 3.2.4.1. 

As an example of the first kind of outer-sphere complex formation, Werner (44) studied the interaction of thiocyanato complexes of cobalt (Ill)ammines with silver ions. In these complexes nitrogen is bound to the metal, and the sulfur atom in the thiocyanate group is free to complex... 

The dashed line in the complex in 5 and 6 indicates an outer-sphere (OS) surface complex ATqs stands for the outer-sphere complex formation constant... 

Figure 6. Two possible modes of hydration shell penetration in the conceptual outer sphere complex formation (24).

The insensitivity of the formation rate constants to the nature of the ligand is ubiquitous for non-transition and transition metal ions alike. This phenomenon is believed to be the result of either an SnI mechanism [Eq. (2, 3)], or a two-step, SNl-like mechanism [Eq. (4, 5)] involving first outer sphere complex formation followed by SnI loss of water from the aquo ion (7, 2). 

---