Anions, tendency toward complex formation

The stabilization of a complex due to cyclization has already been noted, and it might be assumed that where one ring is good, several rings should be better. In the vast majority of cases, stability is indeed increased by incorporation of more than one ring into a structure. The Ca2+ ion, one of the ions with the least tendency toward complex formation, easily snuggles into the hexadentate anion of ethylenediaminetetraacetic add (versene, V) while the anion twists itself into a five-ringed structure (Fig. 6-2). So stable is this chelate that a 0.001 molar solution in water is but 0.01 percent dissociated. 

Another possibility is to change the steric factors. Thus, Shaw et al. (5) have shown that, with respect to transition metal complexes, there is a relationship between the tendency toward hydride formation and the bulkiness of the phosphine ligands so that larger phosphines increase the tendency toward the formation of hydride. This may be explained by the fact that the hydride anion is the smallest conceivable ligand, and thus for purely steric reasons hydride formation is facilitated when the other ligands are very large. 

Complex ions are formed in solution by the combination of a metal cation with a Lewis base. The formation constant Kf measures the tendency toward the formation of a specific complex ion. Complex ion formation can increase the solubility of an insoluble substance. Qualitative analysis is the identification of cations and anions in solution. 

The majority of metals of practical interest form divalent ions in solution. The kinetics of their deposition and dissolution have been extensively studied for the purpose of elucidating reaction mechanisms ever since the development of transient methods enabled measurements under conditions of pure activation control to be achieved. However, most divalent cations show a tendency toward complexing even with simple anions in solution. Moreover, strong interactions with OH ions lead to some hydroxo complex formation even at low pH values, which complicates the kinetic behavior. 

Of the two contrasting properties of the Cr(CO)3 group, i.e., donor and acceptor, with respect to an aromatic side-chain, the inductive acceptor effect has been more widely explored with regard to its synthetic consequences. In particular, temporary complexation of an organic substrate favors proton abstraction from a carbon chain in basic media. This increased tendency toward anion formation thus permits the alkylation of substrates whose free ligands have little or no reactivity. 

The stoichiometries of these complexes are most likely determined by the tendency of XeF6 towards formation of stabilized cations containing XeF5] + moieties and of the anions towards formation of octahedral hexafluoro-anions, [MFg] (viz. [MFg]2-). The list of XeF6 complexes is large, but we shall refer here only to those where more than passing reference is made to their salt-like nature. 

Such alkali metal ion pairs are capable of two electron transfer from the potassium anion towards a suitable substrate, e.g. p-butyrolactone with formation of a respective carbanion. The strong tendency to two electrons transfer is due to the unusual oxidation state of potassium anion bearing on its outer s orbital a labile electron doublet shielded from the positive potassium nucleus by inner orbitals. Using 5 -enantiometr of P-butyrolactone as a monomer and potassium supramolecular complex as catalyst, enolate carbanion is formed as the first reactive intermediate which induces polymerization, yielding poly-(R)-3-hydroxybutanoate. The resulting biomimetic polyester has the structure similar to native PHB produced in nature, except for acetoxy-end-groups, which are formed instead of the hydroxyl ones typical for natural PHB. 

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