Unusual Coordination Geometries

The only other example of square-planar coordination around tin(II) atom, Sn N( Pr2PSe)2 2 (127), was reported in 2005. It seems that this coordination mode is possible only when the tin atom is coordinated to soft, highly polarizable donor atoms. ° 

Earnshaw and N. Greenwood, Chemistry of the Elements, 2nd edn. Elsevier, New York, 1997. 

Eischer, J. Baumgartner, C. Marschner and E. Uhlig, Inorg. Chim. Acta, 358, 3174 (2005). 

Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th edn, John Wiley Sons, Inc., New York, 1980, p. 982. 

Porchia, U. Casellato, F. Ossoia, G. Rossetto, P. Zanella and R. Graziani, J. Chem. Soc., Chem. Commun., 1034(1986). 

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H2L34, respectively, shows that the macrocycles do not confer unusual coordination geometries on the metal ions. 

Kleij, A.W., Kuil, M., Tooke, D.M., Spek, A.L. and Reek, J.N.H. (2005) Template-assisted ligand encapsulation the impact of an unusual coordination geometry on a supramolecular pyridylphosphine-Zn(ii) porphyrin assembly. Inorg. Chem., 44, 7696-7698. 

The calcium, strontium, barium, and lead 80) complexes of 160 and 161 have also been reported. In these two ligands the six donor atoms are essentially confined in a plane these complexes thus permit study of unusual coordination geometries in species of high coordination number. Attempts to form alkali metal complexes with 160 and 161 under the same conditions as employed for the alkaline earth metal complexes have failed. The successful syntheses of complexes of the latter type indicate that the higher charge to radius ratio is of consequence when spherically charged cations are employed. Such metal ions have no apparent coordinative discrimination as the template ion 87). 

The structure of the complex [Et2Si(C5H4)(Me4C5)Lu]2(/r,-H)(/x-C2H5) shown in Fig. 6.21 has an unusual coordination geometry of the /i-ethyl hgand which bridges asymmetrically two Lu ions. 

Nucleophilic and electrophilic additions and oligomerization (cluster formation) have been demonstrated. Unusual coordination geometries and bonding may be adopted by the coordinated ligands. These in turn may lead to new reaction types for these ligands. 

In addition to the cryptates, which are synthesized apart from metal ions and then used to form complexes, there are other types of multicyclic ligands called encapsulating ligands, which are synthesized around the metal ion and cannot release it. Complexes of this sort are sometimes called sepulchrates. Two of these are (1-XXIII) and (1-XXIV). An encapsulation complex allows studies to be carried out under extremely acidic or basic conditions since the metal ion, though it cannot be removed, can be oxidized or reduced. Such ligands also can enforce unusual coordination geometries in the examples shown the coordination is much closer to trigonal prismatic than to octahedral. 

Unusual Coordination Geometries at Carbon Stabilized by Group 4 Metallocenes 13... 

UNUSUAL COORDINATION GEOMETRIES AT CARBON STABILIZED BY GROUP 4 METALLOCENES... 

Many compounds have been synthesized, often fortuitously, in which one or more atoms have been partially or completely encapsulated within metal clusters. The most common of these cases have been the carbide clusters, with carbon exhibiting coordination numbers and geometries not found in classic organic structures. Examples of these unusual coordination geometries are shown in Figure 15-22. 

Although they have been developed primarily for use with alkali and alkaline-earth ions (see page 200), it is appropriate to mention here the polyether macrocycles. These are of two main types (1) monocyclic ones,7 such as (21-XXV1), and (2) the bicyclic molecules,8 such as (21-XXVII). These compounds are remarkably powerful complexing agents for alkali and alkaline-earth ions, giving unusual coordination geometries.9 There is as yet no detailed report of their complexes with other metal ions. 

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