Complexes from free ions

Where a complex is formed there is an intimate chemical interaction between the ions. Some electronic rearrangement is occurring resulting in covalent interactions, in contrast to the physical coulombic electrostatic interactions involved in the formation of an ion pair. 

If complexes and chelates involve intimate chemical interactions, the extent of association should reflect the chemical nature of the ions involved. Equilibrium constants should be different, and possibly even very grossly different, for equilibria which superficially seem very similar and alike, for instance, association of one species with ions of similar size and charge. The situation is reminiscent of that found for the dissociation constants of weak acids and bases where the magnitude of the equihbrium constants depends on the chemical nature of the species involved. It is in stark contrast to that expected for the formation of ion pairs, where the magnitude of the association constant is expected to be independent of the chemical nature of the ions involved. 

On the other hand, it is beheved that the interaction of the same metal ions with SO (aq) results in an ion pair. Here the association constants are very similar, ranging from 1.9 X lO to 

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Ion-pair formation lowers the concentrations of free ions in solution, and hence the conductivity of the solution. It must be pointed out that ion-pair formation is not equivalent to the formation of undissociated molecules or complexes from the ions. In contrast to such species, ions in an ion pair are linked only by electrostatic and not by chemical forces. During ion-pair formation a common solvation sheath is set up, but between the ions thin solvation interlayers are preserved. The ion pair will break up during strong collisions with other particles (i.e., not in all collisions). Therefore, ion pairs have a finite lifetime, which is longer than the mean time between individual collisions. 

In a study of the bioaccumulation of metals as colloid complexes and free ions by the marine brown shrimp, Penaeus aztecus [29] the colloids were isolated and concentrated from water obtained from Dickinson Bayou, an inlet of Galveston Bay, Texas, using various filtration and ultrafiltration systems equipped with a spiral-wound 1 kDa cutoff cartridge. The total colloidal organic carbon in the concentrate was found to be 78 lmgdm 3. The shrimps were exposed to metals (Mn, Fe, Co, Zn, Cd, Ag, Sn, Ba and Hg) as radiolabelled colloid complexes, and free-ionic radiotracers using ultrafiltered seawater without radiotracers as controls. The experiments were designed so that the animals were exposed to environmentally realistic metal and colloid concentrations. 

The combining of two or more substances or molecular entities to yield a single substance or molecular entity, a process that involves either covalent or noncovalent bonding. Included in this definition is the formation of ion pairs from free ions, the noncovalent aggregation of monomers to form polymeric structures or complexes, as well as colligation. The opposite of association is dissociation. 

The value of (fcobsd — kc.dc) at a given concentration of bromide anion depends on the association constant for formation of the ion pair from free ions (Aias = fcd/fc d), and on the relative reactivity of the ion-pair and free carbocation toward addition of solvent (k Jks). For example, if Kas is small, then the concentration of the ion-pair intermediate will be low and its reaction will make a small contribution to fcobsd or, if (k jks) is small then the ion pair intermediate will be unreactive compared with the free cation.32 Equation (7) shows the relationship between the experimental parameters, K s and (ks/kls) where fcobsd — Kale = (3 1) X 10 5s 1 (equation (6)) ksolv = 0.049 s-1 [(fcsolv = kik-d/(k-i + fc d)] kBl/ks = 77 M-1 (kBl/ks = kdk-,/ [ks(k-1 + k-d)], see equation (4)) and = kd/k-d for formation of an encounter complex between 1+ and Br-. 

The reactivity of zinc complexes supported by a variety of tridentate amine donor ligands (Fig. 44) with diphenyl 4-nitrophenyl phosphate in 20% (v/v) acetonitrile-water, and with 2,4-dinitrophenyl diethyl phosphate in 1 % (v/v) in methanol-water, has been investigated. 5 For the former reaction, the second-order rate constant for the hydrolysis of 2,4-dinitrophenyl diethyl phosphate correlates linearly with the —AH value for the formation of the [(ligand)Zn(OH2)]2 + complex from free chelate ligand and aqueous zinc ion. This indicates that in this series of complexes, faster hydrolysis of 2,4-dinitrophenyl diethyl phosphate corresponds to weaker chelate... 

In the presence of a solid phase the distribution logarithmic diagrams are called solubility log concentration diagrams. From these diagrams it is possible to find the liquid phase composition (distribution of complexes and free-ion form) of the areas with predominating existence of the particular forms, total and minimum solubility of the solid phase and pH, or precipitant concentration required for separation of the solid phase at a given pH... 

The L and S values are those from which the / value was formed via the vector coupling rule. These formulae strictly apply only for the magnetism of free-ion levels. They provide a good aproximation for the magnetism of lanthanide complexes, as we shall note in Chapter 10, but provide no useful account of the magnetic properties of d block compounds. 

Again, however, this is strictly applicable only for free ions. Even though spin-orbit coupling is much less important for the first row of the d block, this formula provides a far less good approximation for d -block complexes than Eq. (5.6) does for lanthanide complexes. The reason is that the ground, and other, terms in these d complexes differ grossly from those of the corresponding free ion. These differences are one result of the crystal field. 

Fig. 15-11 Effects of strong complexation on metal ion toxicity, (a) Increasing concentration of NTA, a strong multi-dentate complexing agent, decreases the toxicity of Cd to grass shrimp. All systems have equal concentrations of total Cd. (b) When the results are replotted showing survival as a function of Cd concentration, the data for all concentrations of NTA collapse to a single curve. (Reprinted with permission from W. G. Sunda et al. (1978). Effect of chemical speciation on toxicity of cadmium to grass shrimp, Palaemonetes pugio importance of free cadmium ions. Environ. Sci. Technol. 12,409-413, American Chemical Society.)...

Fig. 5.8 Molecular radial wavefunctions for the ferric complex FeCLt compared to the radial wavefunctions of the free ions Fe ", Fe ", and Fe " (taken from [79])...

However the second question, whether the Cr+3 species either underwent some chemical change so that they became inert in the solution or Cr+3 ions were not available to DPC for complexation from the existing dichromate ions remain to be explained. Since either oxidation (c) or reduction (b) would occur in the solution in the given set of experimental condition, another experiment was performed to ascertain the cause of decomposition of Cr-DPC complex resulting into the decolourisation. A current of N2 gas was purged into the decolourised solution for about 10 min to remove all dissolved 02 gas from the solution and create an oxidation free atmosphere in and above the solution in the flask. The solution was sealed and left for an hour. The colourless solution changed to feebly pinkish colour and intensified over night (about 10 h). This confirmed the restoration of chromium ions to +3... 

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