Bridging ligands polymetallic complexes

A new field of coordination chemistry is that of polymetallic cage and cluster complexes [Mm(p-X)xLJz with molecular (i.e. discrete) structure. They contain at least three metal atoms, frequently with bridging ligands X and terminal ligands L. These compounds link the classical complexes (m = 1) and the non-molecular (m - oo) binary and ternary compounds of the metals.1 Molecular polymetallic clusters (with finite radius) also provide a link with the surfaces (infinite radius) of metals and their binary compounds.2"5 Polymetallic complexes are known for almost all metals except the actinides. 

Polymetallic complexes with suitable bridging ligands can be useful models of molecular wires. In the present chapter, we shall consider mainly bimetallic systems in which two metal atoms are connected by a bridging ligand, with the general topology shown in Figure 1. 

Bridging ligands capable of binding more than two metal centers are less studied. Some examples were highlighted earlier. Polymetallic ruthenium complexes have been synthesized using the tetra-bidentate ligands (62) and (63) as multi-electron transfer agents.181,182... 

Stereochemical questions concerning the structures of previously noted polymetallic complexes, using dipyridylpyrazine bridging ligands,... 

There exist a vast number of discrete, polymetallic coordination clusters both of the metal-metal bonded type and linked by a tremendous variety of bridging ligands, notably carboxylates. Such compounds are not coordination polymers but oligomers and hence can be more soluble, more well-defined and easier to characterise than coordination polymers, for which they can serve as useful model systems. We will discuss discrete, self-assembled complexes of semi-protected metal ions that act as hosts in solution or as 3D capsules in the next chapter and we will not cover these systems in detail here except to note in passing a couple of examples that are of particular interest. One particularly prominent cluster is Muij-acetate, the mixed-valence compound... 

The structure is related to that of CdCl, with M(II) ions and P—P pairs occupying the Cd positions and sulfur atoms (close packed) occupying the chloride positions. This structure is very similar to that encountered in the wide class of the layered dichalcogenides MX. Each MPSj layer can also be described as a polymetallic complex, which consists of an array of M(II) cations coordinated to the sulfur atoms of (PjSg) - bridging ligands. This coordination chemist s view turns to be very useful to understand the reactivity of these peculiar materials. 

The chemistry of polymetallic complexes featuring bridging alkyne and alkynyl (M-G=GR) ligands has been an intense area of study in recent years, which has also been expanded to polyynes and polyynyl moieties linking the metal centers. Recent developments in the chemistry and also materials properties of these complexes have been the subject of several reviews and highlight articles. " " " " In this section, we only present complexes featuring 77 -alkyne-platinum bonding interactions. 

In the construction of polymetallic charge transfer complexes, bridging ligands (BLs) are used to chemically bond the individual components within the supramolecular assembly. A wide range of BLs have been used for this purpose. Some common BLs consist of simple ligands sueh as CN and polyazine-type BLs separated by rigid and nonrigid spacers (Fig. 8). " Polyazine type BLs... 

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