Bridging ligands polynuclear species

An example of this type of dendrimer is the polynuclear species that can be prepared from 2,3-bis(2-pyridyl)pyrazine as bridging ligands, bipyridines as terminal bgands, and either Ru(II) or Os(II) as the metal centre. 

While the structures so far described in Figure 4.1-4.7 involve distinct molecular and polynuclear copper(I) species, they are characterized by the relatively low coordination numbers of two, three and four. Such low coordination numbers are conducive10 to bridging ligand functions and hence to infinite lattices involving chains (or ribbons), sheets and three-dimensional lattices.199 Figure 4.8 summarizes the types of chain structures characteristic of copper(I). 

Some of these ligands have been shown to coordinate to three different copper atoms in polynuclear species, acting both as a chelate and as a bridging ligand.96... 

Finally, it is important to note that, while H2O is a relatively poor bridging ligand, both hydroxo and 0x0 ligands readily act as bridges between metal atoms, giving rise to polynuclear species, which are not discussed in detail here see Polyoxometalates). 

Pyrazine, piperazine, and pyrazole are, like 4,4 -bipyridyl, suitable ligands for bridging pairs of Zn + ions to give a variety of polynuclear species, while multifunctional (bi)pyridine-pyrazine ligands give scope for the assembly... 

There are few monodentate ligands that interact with beryllium at higher pH in aqneons conditions acetates show some affinity for beryllium and have been the most studied. Similar to the hydroxide speciation, there is a tendency for the formation of polynuclear species such as Bc4(OAc)6, where the six acetates act as bridging ligands for the four beryllium atoms to form a tetrahedral Bc40 core (Figure 7). 

Bridging ligands. In polynuclear species it is necessary to distinguish yet another ligand behavior, the action of the ligand as a bridging group. 

In polynuclear species a ligand can also act as a bridging group, by forming bonds to two or more central atoms simultaneously. Bridging is indicated in names and formulae by adding the symbol p as a prefix to the ligand formula or name (see Section IR-9.2.5.2). 

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