Twelve-coordinate complexes

As for ten- and eleven-coordination, there are no twelve-coordinate complexes containing only unidentate ligands. The most common twelve-coordinate molecules are those of the type [M(bi-dentate)6]. The relevant stereochemistries are shown in Figure 104, and the repulsion energy coefficients listed below ... 

Fig. 1. Top and side views of a bulk atom in an fee crystalline lattice shown as a twelve coordinate complex.

Twelve-coordination is certainly rare, as liscus[Pg.809]

Recently, a similar approach has been used to study eight- to twelve-coordinate aqua and nitrato complexes of the complete series of the lanthanides12871. Again, good agreement between observed and calculated geometries was obtained. In all three of these studies, modified versions of MM2 were used and electrostatic interactions were not explicitly included. 

Idealized structures up to nine-coordination are summarized in Figure 4.21. These do not represent all of the shapes met, since, apart from all these idealized structures, it is necessary to remember that bond angle and bond length distortions of these structures can occur some of the shapes resulting from these effects are themselves common enough to be represented as named shapes, and we have discussed some examples of these earlier. Further, beyond nine-coordination, an array of additional shapes can be found, of which perhaps the best known are the bicapped square antiprism (for ten-coordination), the octadecahedron (for eleven-coordination) and the icosahedron (for twelve-coordination). Clearly, the options are extensive, so it may be time to find out what directs a complex to take a particular shape. 

Beryllium, bis(t ifluo oacetylacetone)-gas chromatography, 560 Berzelius s conjugate theory, 4 Bicapped pentagonal prisms repulsion energy coefficients, 34 twelve-coordinate compounds, 100 Bicapped square antiprisms repulsion energy coefficients, 33,34 ten-eoordinate compounds, 98 1, r-Binaphthyl, 2,2 -bis(diphenylphosphino)-complexes... 

Hay (1991) has studied the high-coordinate metal complexes containing lanthanide (III) ions using molecular mechanics calculations. He has applied the molecular mechanics technique to 58 known structures of eight- to twelve-coordinate aqua- and nitratolanthanide (III) complexes. He has also presented various required parameters for the computation of these species. He has studied complexes such as M(OH2)gX3,... 

The ligand, 1,8-naphthyridine, forms complexes with what appear to be very high coordination numbers. With perchlorate as the anion the complexes R(napy)s(C104)3 (R = La-Pr) and R(napy)s(C104)3 (R = Nd-Eu) are isolated from ethyl acetate solutions (Foster et al., 1972). The infrared evidence indicates that in all cases the naphthyridine is functioning as a bidentate ligand and that the perchlorate is present in the ionic form. This would require that the larger lanthanide ions be twelve-coordinate and that the heavier lanthanides be... 

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