Coupling– Polymetallic Complexes
All of the examples met so far are based on simple monomeric complexes containing just one metal centre. This might suggest that these are the sole or dominant type– not so. It is not uncommon to meet complexes that feature two or even more metal centres linked together. Complexes may be monomeric, dimeric (two metals involved) oligomeric (several metals involved), or polymeric (many metals involved). The metal centres may be linked directly, yielding a metal–metal bond, or else may be linked or bridged by a ligand or ligands. This latter class can arise in a number of different ways:
linkage via a small neutral or anionic ligand that binds simultaneously to both metals through a common donor atom (e.g. Cl ̄);
linkage via a small ligand that binds to both metals through two different donor atoms (e.g. NCS- using both the N and S atoms);
linkage via a polydentate ligand that spans across two (or more) metals and provides some donor groups for each (e.g. -OOC-CH2-COO- using the two carboxylate groups);
incorporation of two or more metal ions in a large cavity in a polydentate ligand, each metal binding to several of a large set of potential donors (a process called encapsulation).
Moreover, the complex may involve: all metals of the one type and in the same oxidation states; all metals of the same type in several oxidation states; two or more different types of metals in a common oxidation state; or two or more different types of metals in a range of oxidation states - that is, all combinations are feasible!
Complexes that are oligomeric and roughly spherical are often called clusters. Clusters may involve several layers of metal ions from a core outwards and perhaps not surprisingly, metals in different environments (core or surface, for example) behave differently, even if inherently the same type and in the same oxidation state - in other words 'environmental' effects contribute to the metal's behaviour.
We shall very briefly examine some of these classes a little more carefully with some examples. One of the simplest families of related compounds is the halo-metals. Many metal ions can exist with a set of halogen anions as common donors in monomers of general formula MXnq-; an example is the [CuCl4]2-ion. In some cases, oligomers can form, of which the first is the dimer MXnq-; an example is (Fe2Cl6(2- distinguished by having two bridging halides in addition to four terminal halides (Figure 3.26). Higher oligomeric clusters are also known; trimers, such as [Re3Cl12] have the three metal centres disposed in an equilateral triangle arrangement, with both bridging and terminal halides
Figure 3.26
Examples of oligomeric complexes of metal ions with halides. Both bridging and terminal halide can be identified in the figures; terminal halides are shown in grey.
whereas tetramers, such as [Cu4Cl16]4- adopt a slightly distorted box-like shape. These may also involve metal-metal bonding; for example, [Re3Cl12] has bonds between the Re atoms additional to those formed by bridging chloride ions.
The above examples are relatively simple since they contain just two types of atom and adopt symmetrical shapes. It should be anticipated that many examples will prove much more complicated in structure. Although polymeric coordination compounds are growing in number and importance, we shall limit our exploration of these, leaving this to more advanced textbooks.
