Ligands tetrahedral centers

The remaining cases reclassified by Prolog and Helmchen are tetrahedral centers with four distinct ligands, of which two and only two are enantiomorphic ligands in diastereotopic positions (as in 16a, Fig. S). These centers were originally regarded as pseudoasymmetric (39), and had received descriptors that did not change on reflection. Therefore, no anomaly had to be corrected. When these centers were reclassified as chiral (5), the Sequence Rule was also modified ... 

The preparation, and even more the resolution, of an asymmetric tetrahedral center in Werner-type complexes has been thwarted by the configurational instability of tetrahedral complexes. However the use of ligands of the strongly a, 7t bonding type imposes stability and the forma-... 

Figure 7 Crystal structures of prismatic metallo-supramolecular assemblies, (a) Tetrahedral [Ag4(33)4]" + with included CH3CN shown in space-filling mode (b) the topologically complicated complex [Pd4(34)4(N03)2(H20)2] + (c) tetrahedral [(VO)6(CTC-6H)4(Mg(H20)4)3] anion with chelating deprotonated CTC ligands Mg centers shown in green, V in purple and (d) the stella octangula structure of [Pdg(35)8] + shown in space-filling mode with each ligand in a different color.

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

A model for the intermediate consisting of substrates 36 and 8a coordinated to catalyst 37a was proposed as shown in Scheme 6.30 [74]. In the model 39 the two triflate ligands are dissociated from copper. The ligands are arranged around copper as a trigonal bipyramid and it should be noted that in this model the oxygen atom of the vinyl ether 8a also coordinates to the metal center. However, another tetrahedral intermediate consisting of only the catalyst and the nitrone could also account for the absolute selectivity of the reaction. 

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

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