Stereochemical consideration, ligand

As a result of the recognized role of transition metal hydrides as l reactive intermediates or catalysts in a broad spectrum of chemical reactions such as hydroformylation, olefin isomerization, and hydrogenation, transition metal hydride chemistry has developed rapidly in the past decade (J). Despite the increased interest in this area, detailed structural information about the nature of hydrogen bonding to transition metals has been rather limited. This paucity of information primarily arises since, until recently, x-ray diffraction has been used mainly to determine hydrogen positions either indirectly from stereochemical considerations of the ligand disposition about the metals or directly from weak peaks of electron density in difference Fourier maps. The inherent limi-... 

Some general considerations governing the nature of selective enantiomeric interactions for both gas and liquid chromatographic phases (at least of the bonded monomeric ligand type) have been forthcoming [721,742,754,756,781,782,790). It is generally assumed that three points of simultaneous interaction at least one of which must be stereochemically controlled, are required to distinguish the chirality of a molecule. These... 

Another compound of considerable radiopharmaceutical relevance and based on dithiolato donors is the Tc complex of dimercaptosuccinic acid (H2dmsa, (95)). The ligand (95) has both a meso and optically active forms, leading to complexes of different stereochemical configuration, which have very different in vivo behavior. Whereas the complex from racemic (95) leads to a product that is renally excreted without accumulation in any other organ, the complex prepared from... 

The relative frontier molecular orbital (FMO) energies of the reagents are very important for the catalytic control of 1,3-dipolar cycloadditions. In order to control the stereochemical outcome of a reaction with a substoichiometric amount of a ligand-metal catalyst, it is desirable that a large rate acceleration is obtained in order to assure that the reaction only takes place in the sphere of the metal and the chiral ligand. The FMO considerations will be outlined in the following using nitrones as an example. 

If we consider the geometry of the [M(en)3]"+ complex ion, we have further possibilities to consider. Whereas an octahedral complex with six identical ligands can only exist in one form, one with three didentate chelating ligands is chiral and can exist as two (non-superimposable) enantiomers (Fig. 2-7). The incorporation of polydentate ligands into a co-ordination compound may well lead to a rather considerable increase in the complexity of the system, with regard both to the stereochemical properties and any related chemical reactivity. 

Just as one divides stereoisomers into two sets, enantiomers (Greek enantios = opposite) and diastereomers, so it is convenient to divide heterotopic (non-equivalent) groups or faces into enantiotopic and diastereotopic moieties. Enantiotopic ligands are ligands which find themselves in mirror-image positions whereas diastereotopic ligands are in stereochemically distinct positions not related in mirror-image fashion similar considerations relate to planes of double bonds. 

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