Coordination number , gold/silver

Although 6 Is most prevalent, a coordination number of 4 Is also common, and several important complexes have a coordination number of 2. In addition, a few complexes display coordination numbers of 3, 5, and 7. Examples of coordination number 2 include the silver-ammonia complex and the gold-cyanide complex, both described in Chapter 18. To minimize ligand-ligand repulsions, a complex with a coordination number of 2 is invariably linear, as Figure 20-6 shows. 

The small difference in energy between the s, p and d states leads to the efficient formation of s/d or s/p hybridizations, which are important for explaining the pronounced tendency of gold(I) to form linear two-coordinate complexes. This tendency for two coordination is much greater than for other isoelectronic centers, such as platinum(O), silver(I), or mercury(II), which normally yield compounds with higher coordination numbers. 

In the second arrangement, the spheres of the third layer lie in the dips of the second layer that do not lie directly over the atoms of the first layer (Fig. 5.25). If we call this third layer C, the resulting structure has an ABCABC. . . pattern of layers to give a cubic close-packed structure (ccp). The name comes from the fact that the atoms in a ccp structure form a cubic pattern (Fig. 5.26). The coordination number is also 12 each sphere has three nearest neighbors in the layer below, six in its own layer and three in the layer above. Aluminum, copper, silver, and gold are examples of metals that crystallize in this way. 

The most common strategy in the synthesis of heteronuclear complexes is the use of bidentate donor ligands bearing different donor centers. In that way both donor atoms can be coordinated selectively to two different metal centers in a consecutive way. If the space between the donor atoms of the ligands is short, interactions between both metals usually appear, normally intramolecular. Sometimes, albeit not very often, the bidentate units bind to one another, leading to extended structures through metallophilic interactions. As we have commented, in the case of gold-silver derivatives the number of luminescent studies of these derivatives is very scarce. 

A very large number of stable copper(I) complexes exist in a variety of stoichiometries. In few of these complexes does the formal coordination number of the metal atom exceed four. Indeed, along with silver(I) and gold (I), it is one of the few oxidation states to exhibit regularly the low coordination numbers two and three. The simple amine and halo complexes isolated from aqueous solution fortuitously contain linear copper(I) ions. As a result, coordination number two is erroneously considered to be a common coordination number for this oxidation state. In fact, two-coordinate complexes are probably outnumbered by the trigonally coordinated complexes, whereas against the vast host of tetrahedral complexes the two- and three-coordinate complexes are numerically insignificant. 

The low stability of two-coordinate complexes with respect to other possible structures is well illustrated by the cyano complexes. Althot silver((> and gold( ) form discrete bislcyano) complexes, solid KCu(CN)2 possesses a chain structure in which the coordination number of the cpppertD >s 3. 

Ito and coworkers found that chiral ferrocenylphosphine-silver(I) complexes also catalyze the asymmetric aldol reaction of isocyanoacetate with aldehydes (Sch. 26) [51]. It is essential to keep the isocyanoacetate at a low concentration to obtain a product with high optical purity. They performed IR studies on the structures of gold(I) and silver(I) complexes with chiral ferrocenylphosphine 86a in the presence of methyl isocyanoacetate (27) and found significant differences between the iso-cyanoacetate-to-metal coordination numbers of these metal complexes (Sch. 27). The gold(I) complex has the tricoordinated structure 100, which results in high ee, whereas for the silver(I) complex there is an equilibrium between the tricoordinated structure 101 and the tetracoordinated structure 102, which results in low enantioselectivity. Slow addition of isocyanoacetate 27 to a solution of the silver(I) catalyst and aldehyde is effective in reducing the undesirable tetracoordinated species and results in high enantioselectivity. 

A number of thiourea complexes of silver have shown the tendency to bind up to four ligands, in contrast to gold. Thus Agtu2X (X = Cl, NCS) have essentially 3-coordinate silver (one distant fourth atom) Agtu3C104 is a 4-coordinate dimer (Figure 4.12) [58]. 

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