Coordination stereochemical features

Studies of the stereochemical features in the reactions of metal ions with optically active ligands have aroused great interest during the last years but are still far from being complete. A systematic discussion is not yet possible. Despite this, the known examples furnish sufficient indications as to the stereochemical requirements of ligands with a definite geometry in the coordination entity. The parameters which have to be considered in this context are ... 

Stereochemical Features of Benzene and Toluene Coordination Chemistry. Benzene forms an ordered chemisorption state on the flat Ni(lll) and Ni(100) surfaces at 20°C with unit cells of (2/3x2/3)R 30° and c(4x4), respectively (13). The symmetry data do not fix the registry of the benzene with respect to the metal atoms nor the orientation of the ring plane to the surface plane. However, basic coordination principles would suggest that the benzene ring plane should be parallel to the surface plane. In Figures 8 and 9, possible registries of the benzene with respect... 

The presence of a coordinating ether can effect a- or -chelation, as shown for imine 188. The stereochemical features of the stable six-membered metallocycle from 188 provide for diastereofacial selectivity, as illustrated in the formation of 189 (Scheme 5.2.41).58... 

The crystal structures of proteins represent an extreme in the number of atom positions needed to describe them compared to those structures more commonly studied by powder diffraction. For example, the well-known tetragonal crystal structure of hen egg white lysozyme has 1001 nonhydrogen atoms within the protein molecule another 100 or so water molecules and salt ions are also present. This gives over 3000 atomic x,j,z coordinates. Nonetheless, a Rietveld refinement of these structures from powder diffraction data can be performed by extending the suite of restraints to include all stereochemical features that show characteristic values.The suite of restraints given in Equation (24) is then ... 

The stereochemical features of the polymerization were satisfactorily accounted for by considering the nonbonded interactions between the monomer molecule undergoing insertion and the ligands on the vanadium atom. Coordination number five for vanadium was chosen to justify the more relevant data concerning the propylene polymerization and ethylene, 1-butene, and cis-2-butene copolymerization. 

The ease with which a molecular model can be fitted to the observed density depends on the resolution at which the map is calculated (compare figures 2.1(c), 2.1 (d), 10.4 and 10.5) and its quality. The resolution limit of the calculation is set by the phase determination method. For the method of isomorphous replacement, phasing is successful usually to =2.5-3.0 A. In the case where a related structure is already known the method of molecular replacement (Rossmann (1972)) can be used whereby rotation and translation matrices are determined and then calculated phases used. Clearly, these two procedures are both approximate methods. The model is usually improved by using least-squares methods of refinement (for a collection of papers see Machin, Campbell and Elder (1980)) and higher resolution data (better than 2 A or so). Refinement methods involve the determination of shifts to the atomic parameters (coordinates and thermal parameters) so as to agree better with the observed diffraction data whilst preserving the known stereochemical features of proteins and nucleic acids. This is achieved by minimising a composite observational function ... 

Planar chiral compounds usually (and for the purpose of this review, always) contain unsymmetrically substituted aromatic systems. Chirality arises because the otherwise enantiotopic faces of the aromatic ring are differentiated by the coordination to a metal atom - commonly iron (in the ferrocenes) or chromium (in the arenechromium tricarbonyl complexes). Withdrawal of electrons by the metal centre means that arene-metal complexes and metallocenes are more readily lithiated than their parent aromatic systems, and the stereochemical features associated with the planar chirality allow lithiation to be diastereoselective (if the starting material is chiral) or enantioselective (if only the product is chiral). 

CXF (chemical exchange format) is CAS external structure and non-structural data format. The stereochemical features include (among others) CIP labels, template based labels, optical rotation, and coordination geometries with stereovertex lists. The format is designed to cover all details of CAS internal representation needed for the Registry service. ... 

Perception of symbolic stereochemical features from structure diagrams is also straight forward for the common double bond and tetrahedral cases and planar arrangements are also easily converted. Other coordination polyhedra are less well-studied. 

Magnesium(II) is a milder Lewis acid than traditionally used ones such as boron(III), alumi-num(III), or titanium(IV). A characteristic feature of Mg11 is the presence of coordination sites which are occupied by Lewis bases other than counter anions. By using bidentate Lewis-basic ligands, it is therefore possible to form rigid stereochemical environments. 

General Structural Features. The general structure of halfsandwich ruthenium(II)-arene complexes is shown in Fig. 12. The structural, stereochemical and electronic features of metal-arene complexes have been discussed (63). A typical piano-stool geometry consists of an rj6-arene occupying three coordination sites of the pseudo-octahedral complex, leaving the three legs X, Y, and Z available for coordination. The sites X and Y can be taken up by two monodentate ligands, but are more commonly... 

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