Ligands trigonal bipyramidal centers

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. 

The symmetry properties of the pentacoordinate stereoisomerizations have been investigated on the Berry processes. They have been analyzed by defining two operators Q and The operator / is the geometrical inversion about the center of the trigonal bipyramid. Since this skeleton has no inversion symmetry, / moves the skeleton into another position. Moreover, if the five ligands are different, it transforms any isomer into its enantiomer, as shown in Fig. 3. 

Methyl-5-amino-l-formylisoquinoline thiosemicarbazone, 22, also yields cobalt(II) complexes from unheated methanol solution [202]. However, due to this ligand s added steric requirements, a complex, [Co(22)Cl2], with one ligand per metal ion center is formed. This brown solid has a magnetic moment of 4.42 B.M., is a non-electrolyte, has coordination of a neutral NNS ligand, and the electronic spectrum indicates approximate trigonal bipyramidal stereochemistry. 

The X-ray structure of 4rCl(CO)(triphos)] confirms the trigonal-bipyramidal geometry of the Ir center, with the three phosphorus atoms occupying one axial and two equatorial positions and the Cl ligand in the remaining axial position.608... 

The l-(7V-methyl)benzimidazolyl-3-benzimidazolyl-2-oxapropane ligand (107) shows coordination through the N20 donor set of the ligand, a water molecule and an acetonitrile to give a five-coordinate distorted trigonal-bipyramidal metal center.123 This contrasts with the behavior of the related thiapropane ligand that forms bidentate complexes with zinc. 

Chelation of the nitrone to a square-planar metal center predicts the wrong absolute stereochemistry, as determined for the exo adduct. Jprgerisen proposes that a trigonal bipyramidal metal geometry may be operative, involving chelation of the nitrone, the ligand and vinyl ether in the transition state, 403 in Fig. 32. Alternately, a tetrahedral geometry may also account for the observed sense of induction. The absolute stereochemistry of the endo adduct was not determined. 

The ylides and imides are present as monomers, and the bismuth center adopts a distorted tetrahedral geometry. In contrast, the structural properties of the bismuth oxides vary widely depending on the aryl ligands attached to the bismuth center the oxides exist as hydrates, dimers, or polymers in solution and in the solid state. X-ray structural analysis of an oxide dimer revealed that the bismuth center has a distorted, trigonal bipyramidal geometry with the two oxygen atoms at the apical and equatorial positions [47, 48]. 

---