Asymmetric carbon atom atomic coordinates

As in organic chemistry, there are several sources of chirality at a metal center. As for an asymmetric carbon atom in an organic molecule, the coordination of the metal ion by four different monodentate hgands in a tetrahedral con-... 

Even metals like Cu, Pt, or Pd which form tetrahedral coordination compounds also from asymmetric compounds. In all these cases, therefore, the centre of asymmetry has a tetrahedral configuration just like an asymmetric carbon atom. 

The coordination chemistry of macrocyclic ligands has been extensively studied and aspects of isomerism have been considered in numerous systems.241 Methods whereby two diastereomers of complexes of tetra- N-methylcyclam may be isolated have been discussed previously.184 This, however, is a relatively simple system and it is usually necessary to consider isomerism due to the presence of asymmetric atoms in the chelate arms, as well as that due to asymmetric donor atoms that may be rendered stable to inversion by coordination. An example of a system exhibiting this level of complexity is afforded by the nickel(II) complexes of the macrocyclic ligands generated by reduction of the readily prepared macrocycle (46). These ligands contain two asymmetric carbon atoms and four asymmetric nitrogen atoms but, because AT-inversion is rapid, it is conventional to consider that only three separable stereoisomers exist. There is an enantiomeric pair, (47a) and (47b), which constitutes the racemic isomer (R, R ), and an achiral (R, S ) diastereomer (47c), the meso isomer. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

One should not only classify different types of diastereoisomers according to the combination of chiral elements, but also different types of diastereoselectivity. Usually such a distinction is not made, which might often lead to some confusion. The formation of [M(asp)2] (M = Ni2+, Cu2+) shows no or only little selectivity considering a difference in stability between the optically active forms [M(R-asp)2 ]-or [M(S-asp)2] and the meso-form [M(R-aspXS-asp)]-13 On the other hand we may assume that each aspartic acid molecule coordinates in a stereospecific manner in labile as well as in inert complexes14 The asymmetric carbon atom allows the coordination of aspartic acid as a tridentate ligand only in a way producing def-... 

Note The palladium(II) carbene complex now has the asymmetric carbon atoms in close proximity to the catalytically active metal centre. However, there are no readily available free coordination sites for a substrate. Provision of free coordination sites only becomes possible after Pd-N bond fissure, accompanied by loss of chiral information on the catalytic centre. 

Employing a somewhat similar approach. Paiaro and Panunzi and co-workers (137, 455, 458, 459, 462) have shown that diastereoisomeric pairs are produced when an olefin which does not contain symmetry planes perpendicular to the plane of the double bond and an optically active ligand such as a-phenylethylamine are coordinated to plati-num(II). When a double bond is coordinated to the metal atom, each of the trigonal carbon atoms, if already linked to two different substituent groups, becomes an asymmetric center. One would thus expect to obtain two diastereoisomers from propylene, styrene, or meso compound would be expected since the two asymmetric carbon atoms have opposite configuration. 

The substituents at the asymmetric carbon atoms of the C2 linker between the oxygen atom and the fluorenyl group are either in a trans (29a—c) or in a cis (29d) arrangement. For all compounds NMR spectroscopic data indicate dynamic coordination behavior of the THF ligands in solution similar to... 

Unsaturated organic molecules, such as ethylene, can be chemisorbed on transition metal surfaces in two ways, namely in -coordination or di-o coordination. As shown in Fig. 2.24, the n type of bonding of ethylene involves donation of electron density from the doubly occupied n orbital (which is o-symmetric with respect to the normal to the surface) to the metal ds-hybrid orbitals. Electron density is also backdonated from the px and dM metal orbitals into the lowest unoccupied molecular orbital (LUMO) of the ethylene molecule, which is the empty asymmetric 71 orbital. The corresponding overall interaction is relatively weak, thus the sp2 hybridization of the carbon atoms involved in the ethylene double bond is retained. 

The great majority of known chiral compounds are naturally occurring organic substances, their molecules having one or more asymmetrically substituted carbon atoms (stereogenic atoms). Chirality is present when a tetrahedrally coordinated atom has... 

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