Coupling - Polymetallic Complexes

Notably, the spectrochemical series introduced in Section 3.3.1 is also rationalized partly by the ir-donor/acceptor concept, since ligands on the left-hand side tend to be ir-donors, whose interaction with the octahedral Ug set tends to raise the energy of this set and decrease A o, whereas ligands on the right-hand side tend to be ir-acceptors, whose interaction lowers the Ug set and increases A Q. Ligands which are pure r-donors (like ammonia, which offers no additional lone pair after the r-bonding pair, or vacant orbitals) do not participate in ir overlap at all, and thus tend to lie in the middle of the spectrochemical series. 

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 

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. 

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. 

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Palladium-mediated cross-coupling reactions between ethynyl and bromo-functionalized octahedral Ru11 complexes has led to di and tri, (48), nuclear complexes.147 This represents another example of formation of the monometallic synthons prior to formation of the polymetallic complex. 

Axial symmetry according to Bleaney s approach is maintained in polymetallic lanthanide complexes when the metals lie on the molecular threefold or fourfold axes. For n magnetically non-coupled lanthanide ions packed along the symmetry axis, contact and pseudo-contact contributions can be considered as additive and the original model-free equation (eq. (47)) is transformed into eq. (61) in which the sum runs over the n paramagnetic centres, each being located at the origin of its own reference frame associated with a specific set of axial coordinates O 1 and r" (the z axis corresponds to the molecular symmetry axis, fig. 54),... 

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... 

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