Coordination Number and Geometry

The number of donor atoms linked to a metal ion is the coordination number of the complex. The most common coordination number is 6, with the donor atoms lying at the corners of a regular or distorted octahedron as illustrated in Figure 9-16. A 4-coordinate, tetrahedral structure also is common, and a variety of other structures are found in specific examples.

The coordination geometry (i.e., the arrangement of the donor atoms around the central metal ion), favored by a given element may vary with its oxidation state, with the nature of the donor atoms, and with other factors.

FIGURE 9-16 (a) An octahedral complex, (b) A common distortion of an octahedron: two bonds are longer than the other four,

FIGURE 9-16 (a) An octahedral complex, (b) A common distortion of an octahedron: two bonds are longer than the other four,

Comparatively few metal ions form complexes exclusively of a single geometry, although most have a limited number of preferences. The chemical behavior of metal ions in systems in which geometry is critical (e.g., biological activity) may be related to this property. As has been stated, an octahedral, 6-coordinate geometry is the one most commonly encountered, and most of the aqua complexes can be expected to exhibit this. Not all octahedral complexes are regular, even if all ligands are identical. Hexacoordinate complexes of Cu(II) are somewhat extreme examples of this, since the usual configuration has four "normal" bonds in a plane, with the remaining two ligands attached by long, weak bonds above and below this plane. Indeed, many Cu(II) complexes are often referred to as square planar, although in fact the additional two weak interactions are usually present, especially in solution, where solvent molecules can take up these positions. The striking deep-blue complex formed when ammonia is added to a solution of a Cu(II) salt has this highly distorted octahedral shape, with four NH3 ligands in a plane, but two H2O molecules still weakly attached (Figure 9-16b).

With ligands more highly polarizable than water or ammonia (e.g., halide ions), the ions of the first transition series metals tend to form tetrahedral, 4-coordinate complexes. Truly 4-coordinate square-planar compounds are found for some ions, chiefly those that have eight d electrons [e.g., Pt(II), Au(III)]. This is related to the particular stabilization possible with this unique number of d electrons for this geometry in favorable cases. A few other ions [e.g., Ag(I), Hg(II)] are encountered commonly, but not exclusively, in linear, 2-coordinate complexes. Again, this is related to a particular electronic configuration: 10 d electrons. Other coordination numbers and geometries are less common in environmental systems.

A donor atom with more than one available nonbonding pair of electrons may donate these simultaneously to two or more metal ions, acting to bridge them together. Water molecules can do this, although it is more common with OH— ligands. A ligand with two or more separate donor atoms may bridge, but often these ligands are found attached to one metal ion through both donor atoms. This behavior is called chelation, and leads to more stable complexes. An example of a chelating ligand with two donor atoms (bidentate) is ethyle-nediamine, H2NCH2CH2NH2. Each N possesses one nonbonding electron pair, as in NH3. The molecule is flexible and can bend to fit two adjacent coordination sites in most geometries. Ligands also are known with 3 (triden-tate), 4 (tetradentate), 5 (pentadentate), and 6 (hexadentate) donor groups. Some are illustrated later in this chapter. A ligand with a single donor atom is called unidentate. Chelation obviously results in a ring structure.

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