Ligand apical potentiality

The conclusion from these extensive elegant stereochemical studies is that in five-membered ring systems the apicophilicity of endocyclic ligands does not conform to Trippett s relative apicophilicity scale nor to Holmes s preference rules. Clearly, apicophilicity is a redundant term in such species. Relative apicophilicity is an empirically derived parameter formulated for universal application to phosphorus TBP species. The apicophilicity of a particular ligand is independent of the other phosphorus substituents and the nature of the TBP and is a transferable parameter. Since this definition of apicophilicity breaks down in five-membered TBP intermediates. Hall and Inch defined the alternative term apical potentiality . Thus, in the... 

In their review of phosphorus stereochemistry. Hall and Inch (1980b) have catalogued many nucleophilic substitution reactions at phosphorus whose product distribution and overall stereochemistry are dependent not only on solvent but also on other reaction conditions, in particular the presence of metal ions. They further conclude that mixed reaction pathways may be followed, with the possibility that initial attack of nucleophile may be directed apical to more than one specific ligand. This situation is easy to rationalize in five-membered rings with endocyclic N, O and S ligands, in which there is a fine balance between apical potentialities. 

Hall and Inch (1981) have amply demonstrated the similar apical potentialities of oxy and azo ligands the higher effective electronegativity of an amido nitrogen presumably results in a higher apical potentiality for the nitrogen ligand in this system. This analysis is not incompatible with the rationale presented for the reactivity of the cyclic phosphonyl urea esters [24],... 

An early study on the hydrolysis of the cyclic phosphoramidate [65] in base demonstrated exclusive endocyclic P—O bond cleavage, in contrast with the P—N bond cleavage observed for the corresponding phosphonamidate [66] (Brown et al., 1976) (Scheme 24). This observation is consistent with the similar apical potentialities of oxy and azo ligands derived by Hall and Inch in their elegant stereochemical studies of similar cyclic compounds (p. 144). 

Since the apical potentialities of oxy and azo ligands are finely balanced, the influence of one or more of these effects will be sufficient to alter the observed product distribution. Sinnott (1988) has advanced a further rationale involving SP intermediates. Although this cannot be dismissed, it seems unnecessary to invoke such a mechanism. 

Even though there is a possibility that the apical methionine ligand may control the reduction potential, the definite role still remains ambiguous (75). Synthetic studies with model compounds, in conjunction with protein work by site-directed mutagenesis, should be undertaken to further address this point. 

The closely related ligand (67) is potentially septadentate, and although full details are not published, it seems that the Mn11 compound is indeed seven-coordinate, with an Mn—N (apical) distance of 2.79 A.173... 

The interesting potential septadentate tripod ligand, obtained by the condensation of 5-chlorosalicylaldehyde with tris(2-aminoethane)amine, has been shown to yield an almost undistorted octahedral Mn111 compound, MnN303, with the apical N atom of the ligand unbound.660... 

Semiclathrochelate N3S3- and Nc-complexes with one of the tetrapodal units as apical substituent were prepared by the interaction of cobalt(III) ion with potentially octadentate tetrapodal N4S4- and Ns-ligands (Scheme 62). Cyclization of these complexes... 

The E values depend on the electron density on the encapsulated metal ion and on donor atoms, and as a result, correlate a sum of the inductive constants of apical substituents. The slope of three linear correlation regressions for Ns-sarcophaginates (including a sepulchrate) and for sarta.cn and capten (higher potentials) and absar (lower potentials) ligand complexes is the same hence, in each group the E values are governed by an inductive effect. 

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