Complexes chelate effect

The facile cyclopalladation of allylamine proceeds due to a chelating effect of the nitrogen. In MeOH, methoxypalladation take.s place to give the five-mem-bered chelating complex 507[460). The CO Insertion takes place readily in EtOH, giving ethyl 3-methoxy-4-dimethylaminobutyrate (508) in 50% yield[461). The insertion of alkenes also proceeds smoothly, giving the ami-noalkenes 509[462],... 

The carbopalladation of allylamine with malonate affords the chelating complex 510, which undergoes insertion of methyl vinyl ketone to form the amino enone 511[463]. The allylic sulfide 512 has the same chelating effect to give the five-membered complex 513 by carbopalladation[463.464]. 

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 term chelate effect refers to the fact that a chelated complex, i.e. one formed by a bidentate or a multidenate ligand, is more stable than the corresponding complex with monodentate ligands the greater the number of points of attachment of ligand to the metal ion, the greater the stability of... 

The formation of a single complex species rather than the stepwise production of such species will clearly simplify complexometric titrations and facilitate the detection of end points. Schwarzenbach2 realised that the acetate ion is able to form acetato complexes of low stability with nearly all polyvalent cations, and that if this property could be reinforced by the chelate effect, then much stronger complexes would be formed by most metal cations. He found that the aminopolycarboxylic acids are excellent complexing agents the most important of these is 1,2-diaminoethanetetra-aceticacid (ethylenediaminetetra-acetic acid). The formula (I) is preferred to (II), since it has been shown from measurements of the dissociation constants that two hydrogen atoms are probably held in the form of zwitterions. The values of pK are respectively pK, = 2.0, pK2 = 2.7,... 

The factors which influence the stability of metal ion complexes have been discussed in Section 2.23, but it is appropriate to emphasise here the significance of the chelate effect and to list the features of the ligand which affect chelate formation ... 

An investigation of the chelate effect the binding of bidentate phosphine and arsine chelates in square-planar transition metal complexes. D. M. A. Minahan, W. E. Hill and C. A. McAuliffe, Coord. Chem. Rev., 1984, 55, 31-54 (153). 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Two examples are noteworthy of the chelate effect in the formation of acyl complexes. The reaction of NaMn(CO)5 with MeSCH2CH2Cl at —78°C... 

Similar studies were carried out with methoxycyclohexanones.138 3-Methoxy groups showed no evidence of chelation effects with these reagents and the 2-methoxy group showed an effect only with Zn(BH4)2. This supports the suggestion that the effect of the hydroxy groups operates through deprotonated alkoxide complexes. 

The complex Cu(II)2(0-BISTREN) is much more acidic than the free Cu2+ ion, by a factor of more than three log units. This is primarily due to the presence of two Cu(II) ions, because the formation constant of the Cu2(OH)+ complex is not much less than that for the Cu2(0-BISTREN) complex with hydroxide. This is not a good indication of how well two free Cu2+ ions would bind hydroxide compared to the Cu2(0-BISTREN) complex, however, since one must take into account the dilution effect operative in the chelate effect to make the comparison more realistic (90). Thus, the formation constant for the Cu2OH+ complex above applies for the standard reference state of 1 M Cu2 +. In contrast, in 10 6 M Cu2+, for example, the pH at which Cu2(OH) + would form is raised from pH 5.6 to 11.6, ignoring the fact that Cu(OH)2(s) would precipitate out long before this pH as reached. By comparison, the acidity of the Cu2(0-BISTREN) complex is not affected by dilution and would still form the hydroxide complex at pH 3.9 if present at a 10"6 M concentration. 

Retard efficiently oxidation of polymers catalysed by metal impurities. Function by chelation. Effective metal deactivators are complexing agents which have the ability to co-ordinate the vacant orbitals of transition metal ions to their maximum co-ordination number and thus inhibit co-ordination of hydroperoxides to metal ions. Main use of stabilisation against metal-catalysed oxidation is in wire and cable applications where hydrocarbon materials are in contact with metallic compounds, e.g. copper. 

Kanemasa et al.63 reported that cationic aqua complexes prepared from the /ram-chelating tridentate ligand (i ,f )-dibenzofuran-4,6-diyl-2,2,-Mv(4-phcnyloxazolinc) (DBFOX/Ph) and various metal(II) perchlorates are effective catalysts that induce absolute chiral control in the Diels-Alder reactions of 3-alkenoyl-2-oxazolidinone dienophiles (Eq. 12.20). The nickel(II), cobalt(II), copper(II), and zinc(II) complexes are effective in the presence of six equivalents of water for cobalt and nickel and three equivalents of water for copper and zinc. 

The complexes of zinc with pyridylphenylketone and pyridylphenylmethanol are stabilized by the chelate effect. The complexes formed were dependent on anions present and the ratio of the... 

The second approach consists of synthesizing first the complex MLra 1(L X) with the desired ratio (L )/(M) this complex bears the reactive fragment X which then reacts with the surface of the silica. This method is of limited interest, because the synthesis and isolation of these functionalized complexes is not straightforward. One of the successful examples concerns the synthesis of nickel carbonyl complexes anchored to the surface via two bonds in an attempt to increase the stability through a sort of chelate effect. Initial attempts to achieve this by the methods described in Equation(5) (initial functionalization of silica) and Equation(6) (initial functionalization of complex) failed, as demonstrated by 29Si and 31P CP MAS NMR spectroscopies.51... 

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. 

Because of the chelate effect, ligands that can displace two or more water molecules from the coordination sphere of the metal generally form stable complexes. One ligand that forms very stable complexes is the anion ethylenediaminetetraacetate (EDTA4-),... 

Another factor that relates complex stability and siderophore architecture is the chelate effect. The chelate effect is represented by an increase in complex stability for a multidentate ligand when compared to complexes with homologous donor atoms of lower denticity. The effect can be observed when comparing the stability of complexes of mono-hydroxamate ligands to their tris-hydroxamate analogs, such as ferrichrome (6) or desferrioxamine B (4). However, the increase in stability alone is not sufficient to explain the preponderance of hexadentate siderophores over tetradentate or bidentate siderophores in nature, and the chelate effect is not observed to a great extent in some siderophore structures (10,22,50,51). 

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