Ring chelation effect

On the basis of the values of AS° derived in this way it appears that the chelate effect is usually due to more favourable entropy changes associated with ring formation. However, the objection can be made that and /3l-l as just defined have different dimensions and so are not directly comparable. It has been suggested that to surmount this objection concentrations should be expressed in the dimensionless unit mole fraction instead of the more usual mol dm. Since the concentration of pure water at 25°C is approximately 55.5 moldm , the value of concentration expressed in mole fractions = cone in moldm /55.5 Thus, while is thereby increased by the factor (55.5), /3l-l is increased by the factor (55.5) so that the derived values of AG° and AS° will be quite different. The effect of this change in units is shown in Table 19.1 for the Cd complexes of L = methylamine and L-L = ethylenediamine. It appears that the entropy advantage of the chelate, and with it the chelate effect itself, virtually disappears when mole fractions replace moldm . ... 

The chromium-catalyzed coupling is extended to the intramolecular cycli-zation to the dials 43. The O-chelating silyl scavenger is used to control the stereoselectivity as shown in Scheme 25 [61,62]. The ring size effect on the stereoselectivity is also investigated. 

The hydroformylation of styrene using rhodium systems containing the four structurally related diphosphines dppe, dppp, (86), and (87) has been studied. A systematic analysis of the effect of the pressure, temperature, and the ligand metal molar ratio shows that the five- and six-membered ring chelating diphosphines behave differently from one another.347 An analysis of the effect of pressure, temperature, and ligand metal molar ratio on the selectivity of styrene hydroformylation catalyzed... 

The five-membered ring chelate ligands (CHIRAPHOS (7) and DIPAMP (9)) showed poor activity. DIOP (5) was found to be more effective than BINAP (6), while no real improvements in the levels of asymmetric induction were found by using cationic complexes [Rh(COD)(L-L)]+ instead of neutral systems. 

Because the ethylenediamine forms chelate rings, the increased stability compared to Nff3 complexes is called the chelate effect. For both ligands, the atoms donating the electron pairs are nitrogen atoms. The difference in stability of the complexes is not related to the strength of the bonds between the metal ion and nitrogen atoms. 

EQUILIBRIA IN RING FORMATION. MACROCYCLISATION EQUILIBRIA. THE CHELATE EFFECT... 

KMLJKM(Lh coincides with the EM for the chelate ring formation from M L L. In the general case, the ratio whose common logarithm defines the chelate effect has units of Mopen chain w-dentate ligand. 

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. 

Fig. 5.5c illustrates the effects of various ligands upon the dissolution rate, and that a surface chelate (ring structure of the ligand bound to the metal center at the surface) is more efficient in enhancing the dissolution rate. Furthermore, the acceleration increases in the series Salicylate > oxalate > malonate > phtalate > succinate which indicates that a 5-ring chelate is more efficient than a 6-ring or 7-ring chelate. 

The method chosen to achieve this stabilization is to have the olefinic group form part of a molecule containing a class b donor atom, usually P or As but occasionally 0 or S. It is found, as expected, that the strongest chelate effect occurs when the chelate ring has 5.5 or 6.5 members, that, is, when there are two or three carbon atoms between heavy atom and vinyl group. Typical ligands, with the abbreviations used here, are shown in Tables 1 and 2. This review excludes compounds such as the one in Fig. 1, where no true chelation, considered as the... 

The chelate effect is a celebrated feature of the chemistry of transition metal complexes. Ligands containing an increasing number of rings show several such ring order effects. 

Larger chelate rings should be easier to open and of course they are known to have a smaller thermodynamic chelate effect. 

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