Tetracoordinated metals

Eig. 1. Types of chelates where (1) represents a tetracoordinate metal having the bidentate chelant ethylenediamine and monodentate water (2), a hexacoordinate metal bound to two diethylenetriamines, tridentate chelants (3), a hexacoordinate metal having triethylenetetramine, a tetradentate chelant, and monodentate water and (4), a porphine chelate. The dashed lines iadicate coordinate bonds. 

Eig. 2. Stmctural representations of chelates where (8) corresponds to M(acac)2 (9) to ML2, L = S-sulfo-S-hydroxyquiaoline (10) to M-Trien (11) to a tetracoordinate metal bound to the hexadentate ligand EDTA and (12) to the Ee(III) chelate of EHPG. Stmcture (13), which emphasizes the spatial... 

Consider the equiUbria in an aqueous system composed of a bidentate ligand HA, eg, the enol form of acetylacetone, and a tetracoordinate metal, stmcture (8). The equations are... 

T. G. Takhirov X-Ray Structural Investigation of Stereochemi-cally Nonrigid Tetracoordinated Metal-Chelates of Zn(II), Cd(II), Hg(II) and Ni(II) on the Base of Derivatives of 4-... 

Corey and Ishihara29 report the synthesis of a new bis(oxazoline). This catalyst effects Diels-Alder reaction via a tetracoordinated metal complex. Ligand (.S )-8I is synthesized from (iS )-phenylglycine, as depicted in Scheme 5-25. Treatment of 81 with Mgl2 L gives a dark solution of complex 82, which can be utilized as a Diels-Alder reaction catalyst. Thus, reaction of cyclopentadiene with 71 in the presence of 82 yields product 72a with an enantiomeric ratio of over 20 1 (Scheme 5-26). 

Gamovskii, A. D. Nivorozhkin, A. L. Minkin, V. 1. Ligand environment and the structure of schiff base adducts and tetracoordinated metal-chelates. Coord. Chem. Rev. 1993,126, 1-69. 

Fig. 2. The zinc-bound water molecule of this rigid metalloamide complex exhibits a pKa of about 7 (Groves and Olson, 1985). The complex is a biomimic of the tetracoordinate metal ion in the carboxypeptidase A active site.

Figure 13.5. Valence orbitals of a tetracoordinated metal A (long gray bracket) is analogous to...

Based on our previous calculations on the stability of dimeric and trimeric structures, different ID and 2D networks were built. The stable dimers 6 and 7, as well as structure 5, lend themselves to extension to ID chains of C5 units. These polymeric chains are depicted in Figure 12 as I, n, and HI. Since the ratio of a C5 unit to the metal in a unit cell of I and III is 1 1, a divalent metal ion is needed to compensate for the —2 charge on C5. In the case of II, the 1 2 C5 to metal ratio requires a singly charged cation like Li+. If Li+ is replaced by a divalent tetracoordinate metal ion, n can be converted into a 2D network IV, as shown in Figure 12. 

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