Chelate rings coordination compound stability

It is believed [1135,1136] that the decomposition of metal complexes of salicyaldoxime and related ligands is not initiated by scission of the coordination bond M—L, but by cleavage of another bond (L—L) in the chelate ring which has been weakened on M—L bond formation. Decomposition temperatures and values of E, measured by several non-isothermal methods were obtained for the compounds M(L—L)2 where M = Cu(II), Ni(II) or Co(II) and (L—L) = salicylaldoxime. There was parallel behaviour between the thermal stability of the solid and of the complex in solution, i.e. Co < Ni < Cu. A similar parallel did not occur when (L—L) = 2-indolecarboxylic acid, and reasons for the difference are discussed... 

The effect of /rfive-coordinate aluminum alkyls have been discussed.94 Studies have also focused on the equilibrium between four-coordinate and five-coordinate isomers in compounds of the type [R2Al /i-0(GH2) ER1x ]2 (n = Z, 3 ER1x = OR1, SR1, NRX2) (shown in Figure 7(h) for n = Z). Factors that control this equilibrium and hence the coordination around the aluminum include the steric bulk of the substitutent at the aluminum (R), and the Lewis base donor R1, the basicity of the neutral donor group ER1, and the chelate ring size (as determined by n).95... 

Later, Martell and Calvin (11) noted the high stabilities of the alkaline earth-EDTA chelates and suggested that the heats of coordinate bond formation in solution (i.e., relative to the aquo metal ion) must be negligible, and that the stabilities of the aquo chelates must be due to a favorable entropy change associated with formation of the metal chelate compound. The stabilizing effect of the chelate ring was therefore concluded to be an entropy effect. 

Functionalized tertiary aryl phosphines play an important role in transition metal coordination chemistry. These compounds have been used as ligands in synthesis, catalysis, mechanistic studies, and in the study of coordination compounds as structural models. In this contribution the syntheses of two new types of these ligands, tertiary aryl phosphines functionalized by an amide group, are detailed. The published coordination chemistry of these compounds includes the study of intramolecular N—H oxidative addition, the synthesis of chelates stabilized amido complexes, and the preparation of complexes with both ftve- and six-membered chelate rings. ... 

It is well known [26, 27] that coordination of one bidentate ligand to a metal ion is energetically more advantageous than the coordination of two monodentate ligands with analogous donor atoms, due to formation of a chelate or metallocyclic ring. This phenomenon is known as the chelate effect [28]. Moreover, in a series of related compounds, stability constants increase with an increase in the number of... 

Chugaev conduded that five- or six-membered chelate rings are the most favored in coordination compoundsl and this conclusion is supported by quantitative data. Compounds with seven-membered rings are close in thermodynamic stability to analogous coordination compounds without... 

Thermal stability is enhanced in chelates thus dimethyl-2-methy1pentane-2,4-dio1titanium [23916-35-0] (22) is much more stable than (CH2)3Ti(OCH(CH2)2)2 (68)- The stmcture of the former has been shown by x-ray diffraction to be dimeric and five-coordinate through oxygen bridges. The more highly substituted the six-membered ring, the mote thermally stable the compound. 

---