Higher Coordination Numbers (ML7 to ML9)

BeyondML6 lies a range of higher coordination numbers that reach as high as ML14. While we will not dwell on these at any great length at this level, it is appropriate to be aware of some of these, and how they may arise. Thus we will briefly examine ML7 through to ML9. For these, a point-charge model of the distributions predicted for charges dispersed on a spherical surface can still be applied, and predicted shapes are found experimentally. Rather than expand on this aspect, however, we will examine another approach to understanding how some of the structures arise by relating the mtoourlower coordination numbers already described. If we examine selectively some shapes for two- to six-coordination already discussed, we can arrange them in such a manner where they are related by an increase in the number of groups dispersed in a symmetrical fashion around a plane including the metal along with an additional group in each of the two axial sites (Figure 4.19). Following this trend beyond six-coordinate octahedral, where there are four donor groups around the centre in a square arrangement, it is possible to suggest that one seven-coordinate shape could arise through placing five groups around the centre (the symmetrical arrangement for which is a pentagon) or else, for eight-coordination six groups (in a hexagonal arrangement). These would lead to a pentagonal bipyramid and a hexagonal bipyramid respectively (Figure4.19). As it happens, both these are known shapes for seven- and eight-coordination respectively.

Figure 4.19

Shapes for coordination numbers from 2 to 8 which reflect a trend involving a stepwise addition of groups arranged around the central plane that also includes the metal.

How far this approach can be extended depends on the size of the metal ion and the size of the donor groups, as clearly steric factors will become important as more and more groups are packed into the plane around a metal ion. This also introduces one of the general observations regarding complexes with high coordination numbers– they are found with larger metal ions, or else with metal ions that exhibit long metal–donor distances, as both these aspects contribute to a reduction in steric ‘crowding’ of the metal centre. Another approach to expansion of the coordination number is through expanding the layers (or planes) of donors around a metal ion. This concept is best explained with some illustrations. If we consider the six-coordinate trigonal prismatic shape, we can visualize this as a metal ion in a central layer and two layers of donors above and below the central layer, as illustrated (Figure 4.20). We can then expand thecoordination numberintwoways: addition of extra donors to the upper and lower planes of donors; or addition of donors to the central plane containing the metal, analogous to that already described in Figure 4.19 except here we are starting with more than one group in ‘axial’ locations. Converting the two donor layers from three to four donors each, effectively converts each trigonal plane of donors to a square plane of donors, leading to eight-coordination cubic. In reality steric clashing between the layers is significantly reduced by a rotation of the top square through 45◦ to produce a square antiprismatic (or Archimedean antiprismatic) shape (Figure 4.20). This is analogous to twisting the six-coordinate trigonal prismatic shape through 60◦ to produce the preferred octahedral shape, discussed earlier. In line with the expectations for relative stability, the square antiprismatic shape is experimentally much more common than cubic, although both are known; for example, the [MF8]5- complex ion for M = Pr (III) is cubic but for M = Ta (III) is square antiprismatic.

Alternatively, adding additional donors into the central plane including the metal, best achieved by making a new bond through the centre of a square face of the six-coordinate trigonal prism, leads to different geometries (Figure 4.20). With one insertion, a seven coordinate one face-centred (or mono-capped) trigonal prismatic structure results where as if an addition is made to all three-faces the nine-coordinate three-face centred (or tricapped) trigonal prismatic form is obtained. There are additional shapes for these coordination num bers such as a one face-centred octahedral (seven-coordinate) and dodecahedral (eight coordinate) but we shall not extend the story. What has been established is that higher

Figure 4.20 Methods of converting the six-coordinate trigonal prismatic shape where the metal is ‘sandwiched’ between two layers of donors, into various seven-, eight- and nine-coordinate shapes through addition of other groups either in the plane of the metal to form another layer of donors or else in the two existing layers of donors to expand the set of donors in those layers.

coordination numbers can be evolved or understood through extrapolation from the known lower coordination shapes. Again, distortions from these limiting shapes are common in actual complexes.

Becausethereisa‘crowding’ofligandsaroundthemetalioninthesehighercoordination number species, repulsive interactions between adjacent ligands become more important than in lower coordination number complexes. Thus it is smaller ligands that tend to occupy sites in the coordination sphere of such d-block metal ions. For example, the small f luoride ion forms with zirconium(IV) the [ZrF7]³⁻ complex ion, which has the pentagonal bipyramidal geometry, whereas the tiny hydride ion forms with rhenium (VII) the tricapped trigonal prismatic [ReH₉]²⁻. Further, larger metal ions, with longer metal–ligand bond lengths, also tend to support higher coordination numbers. Thus molybdenum(V) exists as either square antiprismatic or dodecahedral [Mo(CN)₈]³⁻ ion, the shape dependant on the cation involved. However, it is with the f block that the higher coordination numbers are dominant, with examples of complexes with coordination numbers below seven much less common than those with seven or higher coordination. For example, for the aqua ions   [M(OH2)n]3+ n ≤ 6 for the d-block metal ions whereas n ≥ 7 for the f-block metal ions.

Idealized structures up to nine-coordination are summarized in Figure 4.21. These do not represent all of the shapes met, since, apart from all these idealized structures, it is necessary to remember that bond angle and bondlength distortions of these structure scanoccur; some of the shapes resulting from these effects are themselves common enough to be represented as named shapes, and we have discussed some examples of these earlier. Further, beyond nine-coordination, an array of additional shapes can be found, of which perhaps the best known are the bicapped square antiprism (for ten-coordination), the octa decahedron (for eleven-coordination) and the icosahedron (for twelve-coordination). Clearly, the options are extensive, so it may be time to find out what directs a complex to take a particular shape.

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مواضيع ذات صلة

Constitutional (Structural) Isomerism

Isomerism– Real 3D Effects

Chameleon Complexes

Moulding a Relationship - Ligand Influences

Metallic Genetics– Metal Ion Influences

Six Coordination (ML6)

Five Coordination (ML5)

Four Coordination (ML4)

Three Coordination (ML3)

Two Coordination (ML2)

Complex Lifetimes– Together Forever

?Preferences– Do You Like What I Like