Complex stability ring size

Experimental studies show that chelating spectator ligands impart a degree of stability to complexes of type 23 (Scheme 13.10) [42]. If monodentate phosphine ligands are used decomposition is rapid at 20°C, however, using dppp no decomposition is detected after 24 h [19]. It was found that the rate of decomposition could be linked to the chelate ring size at 65°C, with dppp decomposition was complete after 6 h, with dppe only a small amount of decomposition occurred after this time [42]. 

Effect of Chelate Ring Size on Complex Stability Comparison of Stabilities of Complexes of EDTA (Chelate Ring Size Five Involving the Two Nitrogens) and TMDTA (Analogous Chelate Ring Size Six ) ... 

Effect of Chelate Ring Size on Complex Stability with Metal Ions of Different Sizes Oxalate (Ring Size Five) and Malonate (Ring Size Six)°... 

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

A number of novel dioxo variants of the tetraazamacrocycles have been synthesized and stability constants were measured with Cu2+.114 A few of these were further evaluated as chelating agents with 64Cu.115 Structural differences included ring size (from 12 to 14) and placement of the oxo groups. Of the six variations synthesized, only one, l,4,8,ll-tetraazacyclotetradecane-3-9-dione(14N402-3,9), readily formed a complex (Figure 9). 

Romeo et al. (1978) clearly indicate that complexes of divalent metal ions with 1,2-diaminoethane are more stable than those with 1,3-diaminopro-pane. Moreover, in a thorough discussion of the relations between the chelate effect and the ring size, Anderegg (1980) has listed thermodynamic data of complex formation between divalent metal ions and ligand [45], showing that almost invariably the stability of chelate rings decreases with increasing n in the order 5 > 6 > 7. 

In contrast to the ionic complexes of sodium, potassium, calcium, magnesium, barium, and cadmium, the ease with which transition metal complexes are formed (high constant of complex formation) can partly be attributed to the suitably sized atomic radii of the corresponding metals. Incorporated into the space provided by the comparatively rigid phthalocyanine ring, these metals fit best. An unfavorable volume ratio between the space within the phthalocyanine ring and the inserted metal, as is the case with the manganese complex, results in a low complex stability. 

Surface-active crown ethers are distinctly differ from usual type of nonionics in salt effect on the aqueous properties, due to the selective complexing ability with cations depending on the ring size of the crown. As shown in Figure 3 (22), the cloud point of the crowns is selectively raised by the added salts. This indicates that the degree of cloud point increase is a measure of the crown-complex stability in water (23). 

Frensdorff (3) has shown substituent groups to have a smaller effect on complex stability than changing ring size where the donor atom remains the same. Results for derivatives of 18-crown-6 and selected crown-4 and crown-5 compounds, given in Table 12, generally bear out this observation. No study of the variation of log K with the nature and location of substituent has been reported. 

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