Ligands distinguishing between

Bivalent Ligands Distinguish Between Related Proteins ConA and the Lectin from Dioclea grandiflora... 

The value of embodies the conformation-independent 3D arrangement of the atoms of the ligands of a chirality center in distance space and thus cannot distinguish between enantiomers. This distinction is introduced by the descriptor S , , . 

An achiral reagent cannot distinguish between these two faces. In a complex with a chiral reagent, however, the two (phantom ligand) electron pairs are in different (enantiotopic) environments. The two complexes are therefore diastereomeric and are formed and react at different rates. Two reaction systems that have been used successfully for enantioselective formation of sulfoxides are illustrated below. In the first example, the Ti(0-i-Pr)4-f-BuOOH-diethyl tartrate reagent is chiral by virtue of the presence of the chiral tartrate ester in the reactive complex. With simple aryl methyl sulfides, up to 90% enantiomeric purity of the product is obtained. 

It should be noted that, by convention, the ambidentate ligand is always written with its donor atom first, i.e. NO2 for the nitro, ONO for the nitrito, NCS for the A-thiocyanato and SCN for the 5 -thiocyanato complex. Differences in infrared spectra arising from the differences in bonding are often used to distinguish between such isomers. 

In order to distinguish between kinetic and thermodynamic phenomena it is convenient to refer to the former as the 7tr/ i-effect and the latter as the tra/u-influence or static /ra/u-effect". though this nomenclature is by no means universally accepted. However, it appears that to account satisfactorily for the kinetic /rau.s-effect , both it (kinetic) and a (thermodynamic) effects must be invoked to greater or les.ser extents. Thus, for ligands which are low in the Trans series (e.g. halides), the order can be explained on the basis of a u effect whereas for ligands which arc high in the series the order is best interpreted on the basis of a jt effect. Even so, the relatively high position of H , which can have no rr-acceptor properties, seems to be a result of a a mechanism or some other interaction. 

Timms and Atkins (181) reacted the novel ligand, PN (180) (generated by pyrolyzing P5N3), with a number of different metal atoms. With Cu, a bis(PN) complex was formed, although it was not possible to distinguish between the two isomers... 

The undisputed efficacy of benzodiazepines in relief of anxiety led to the question of whether this disorder could arise from abnormal concentrations in the brain of an endogenous ligand or a malfunction of the benzodiazepine/GABA receptor system. An important study, aimed at distinguishing between these possibilities, has been carried out in humans (Nutt et al. 1990) and was based on the premise that anxiety could be caused by either ... 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

IR spectroscopy is often used for distinguishing between unidentate and bidentate coordination of carboxylate (02CR) ligands. For monomeric carboxylate derivatives the separation between the symmetric and asymmetric C02 stretching bands, At = [t asym(C02) - t syJCOa)], provides a useful indication of the coordination mode complexes which exhibit values of greater than 200 cm-1 invariably possess unidentate coordination. Deacon, G. B. Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227. 

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