Double-ligand bridging defined

Double-bridging of two cyclopentadienyl units inevitably reduces the conformational flexibility and allows the formation of rather rigid systems. These are excellently suited for assembling metal centres in a stereochemically well-defined manner, and therefore are attractive building blocks for many purposes. The most important ligand systems of this type, which contain the dimethylsilyl(dimethylsilandiyl), the tetramethyldisilanyl and the tetramethyldisiloxanyl bridges, are listed in Scheme 16. 

Besides hydrogen bonds and salt bridges, metal-ligand coordination is another well-known approach to constrnct well-defined supramolecular entities such as double, triple, and quadruple helical structures such as helicates, from otherwise flexible organic molecular strands. For a presentation of these metal-based architectures, the reader is referred to Self-Assembly of Coordination Chains and Helices, Self-Processes. 

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