Complex ions robust complexes

In the case of the octahedral robust complexes of cobalt (III) and chro-mium(III), substitution in the first sphere is hindered. This type of complex ion is, therefore, especially suitable for studying association in the second sphere. The hexammine and tris(ethylenediamine) cobalt(III) ions have especially been used for this kind of study. For the association of these ions with anions, such as sulfate and thiosulfate, the ion-pair constant is of the order of magnitude of 10 at 7 = 0, somewhat smaller for Coena" than for Co(NH3)6 21)y but strongly dependent on the ionic strength. Thus Posey and Taube 37) y from spectrophotometric measurements in the ultraviolet, obtain the following expression for the association constant of the ion pair [Co(NH3)6]S04 in solutions with y/Jvarying from 0.04 to 0.3 ... 

In all these examples, both enzymic and model systems, it can be seen that the role of the metal ions is an active one. They do not merely hold the pieces together. Moreover, the model studies, especially on the preformed robust complexes, give clear indications on how the metal ion activates the substrate, provides a potent nucleophile at neutral pH, and helps the leaving group depart. 

A study of the optical rotatory dispersion and of circular dichroism of tris((+)j)-2,3-bn) complexes become particularly interesting in that rotatory strengths of complexes of known absolute configuration with centtal ions not forming "robust complexes such as Ni(II) and Cu(II) may thus be obtained for the first time. An example is shown in fig. 2. 

CERESTAT (aptiganel HCl, CNS 1102), a selective, non-competitive antagostist of N-methyl-n-aspartiate ion channel complex, has robust neuroprotective effects in animal stroke models (Gamzu 1996). 

As a more representative example of weaker neutral H-bonded species, let us consider the (HF)2 dimer, which offers a particularly clear contrast to the dipole-dipole expectations of classical electrostatics. The (HF)2 species is bound by about 5 kcal/mol (in the same range as water dimer and many other common H-bonded species) and exhibits a curiously bent equilibrium geometry, as shown in 1/0-9.1. Although HF has a robust dipole moment (calculated as p = 1.92 Debye) and F HF has the linear geometry expected for an electrostatic ion-dipole complex, the nonlinear geometry of (HF)2 clearly differs from the expected linear geometry of a dipole-dipole model. What s going on here ... 

In aqueous solution, the complexes of most metal cations exist in dynamic equilibrium with their components. If we disturb this equilibrium, another one is instantly formed. It is quite otherwise with robust complexes which persist for hours (or even days) under conditions favourable to their decomposition any biological properties that they may have are strikingly different from those of their components. Robust complexes are formed where metal ions have 3,4 (low spin), 5, or 6 d electrons provided that formation of the complex involves large values of ligand-field stabilization energy. Metals most prone to form robust complexes are the transition metals platinum, iridium, osmium, palladium, rhodium, ruthenium, also (but not so frequently) nickel, cobalt, and iron. The halide and, particularly, the cyanide anions most readily form robust complexes with these transi-... 

A review article entitled "Bulky amido ligands in rare-earth chemistry Syntheses, structures, and catalysis" has been published by Roesky. Benzamidinate ligands are briefly mentioned in this contexD The use of bulky benzamidinate ligands in organolanthanide chemistry was also briefly mentioned in a review article by Okuda et al. devoted to "Cationic alkyl complexes of the rare-earth metals S mthesis, structure, and reactivity." Particularly mentioned in this article are reactions of neutral bis(alkyl) lanthanide benzamidinates with [NMe2HPh][BPh4] which result in the formation of thermally robust ion pairs (Scheme 55). ... 

The lifetime of the excited state of fluorophores may be altered by physical and biochemical properties of its environment. Fluorescence lifetime imaging microscopy (FLIM) is thus a powerful analytical tool for the quantitative mapping of fluorescent molecules that reports, for instance, on local ion concentration, pH, and viscosity, the fluorescence lifetime of a donor fluorophore, Forster resonance energy transfer can be also imaged by FLIM. This provides a robust method for mapping protein-protein interactions and for probing the complexity of molecular interaction networks. 

The weak interactions that may exist between group 2 cations and anionic hydrocabyl ligands are demonstrated in the metal-in-a-box compounds such as [M(THF)6][Me3Si(fluorenyl)]2 (M = Ca 159 or Mg), which are formed by the addition of THF to solutions of the bis(fluorenyl) complexes in non-polar solvents. The box may be completed by the presence of aromatic molecules, as in 159 (Figure 84). The disruption of the metal-carbon bonds is thought to stem from a combination of robust M-THF interaction, the stability of the free [Me3Si(fluorcnyl)] ion, and the formation of numerous C-H- -7r interactions between THF and the anions. These and related examples are reviewed elsewhere. 

In addition to the discussed cyclotetramerizations, direct metallation of the metal-free ligands or metal exchange of a labile metal ion or ions for one held more robustly, the desired complexes may be prepared by the direct substitution, exchange or modification of substituents on preformed phthalocyanine derivatives. However, a review of works carried out on these types of transformations lies out of the scope of this chapter. 

A more robust way to write a rate law for a catalytically promoted reaction is to include the concentrations of one or more surface complexes, in place of the surface area As. In this case, the simulation can account not only for the catalyzing surface area, since the mass of a surface complex varies with the area of the sorbing surface, but the effects of pH, competing ions, and so on. 

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