Metal-ligand complexes geometries

Various X-ray crystal structures of metal-ligand complexes provided evidence of the geometry of the complexes in the solid state, even though the structure of these complexes may differ in solution. The hrst crystal structure of a bis(oxazoline)-metal complex was determined in 1994 by Brown and co-workers. " This group crystallized and elucidated the structure of V,V-bis-[2-((45)-(methyl)-l,3-oxazoli-nyl)]methane-bi(ri ethene)rhodium(I), 18a, as depicted in Figure 9.3. The key features of this crystal structure include the C2-axis of symmetry, the axial positions of the methyl groups and the orientation of the ethene molecules, orthogonal to the complexation square plane. In 1995, Woodward and co-workers were able to crystallize and determine the structure of benzylbis(oxazoline) with ruthenium... 

Several model systems related to metalloenzymes such as carboxypeptidase and carbonic anhydrase have been reviewed. Breslow contributed a great deal to this field. He showed how to design precise geometries of bis- or trisimidazole derivatives as in natural enzymes. He was able to synthesize a modified cyclodextrin having both a catalytic metal ion moiety and a substrate binding cavity (26). Murakami prepared a novel macrocyclic bisimidazole compound which has also a substrate binding cavity and imidazole ligands for metal ion complexation. Yet the catalytic activities of these model systems are by no means enzymic. 

In a free metal ion without any ligands, all five d orbitals have identical energies, but what happens to the d orbitals when six ligands are placed around a metal in octahedral geometry The complex is stabilized by attractions between the positive charge of the metal ion and negative electrons of the ligands. At the same time,... 

The enantioselectivity of this catalyst, which is prepared as the iodide salt, is somewhat dependent on the anion that is present. If AgSbF6 is used as a cocatalyst, the iodide is removed by precipitation and the e.e. increases from 81 to 91%. These results indicate that the absence of a coordinating anion improved enantioselectivity. Entry 2 shows the extensively investigated f-BuBOX ligand with an A-acryloylthiazolidinone dienophile. With Cu2+ as the metal, the coordination geometry is square planar. The complex exposes the re face of the dienophile. 

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