Mixed gold-silver clusters

Taking advantage of both aurophilicity and argentophilicity, tetranuclear mixed-metal complexes which contain pairs of Au and Ag atoms have been prepared, as shown in Fig. 19.6.3. In the preparation of mixed gold/silver polynuclear complexes, aurophilicity and argentophilicity have been utilized to promote cluster formation. Figure 19.6.4 shows the cores of several mixed gold-silver clusters. 

AgPh is a colourless solid [144] that is rather insoluble in non-donor solvents and appears to be polymeric (AgPh) (n > 10) in addition mixed compounds (AgPh) .AgN03 (n = 2,5) can also be obtained that involve silver clusters. Mesitylsilver is a thermally stable (but light-sensitive) white crystalline solid in the solid state it is tetrameric (in contrast to the pentameric copper and gold analogues) ... 

For example, when the mixed solution of Ag(CN)2 and Au(CN)2 is irradiated by y-radiolysis at increasing dose, the spectrum of pure silver clusters is observed first at 400 nm, because Ag is more noble than Au due to the CN ligand. Then, the spectrum is red-shifted to 500 nm when gold is reduced at the surface of silver clusters in a bilayered structure [102], as when the cluster is formed in a two-step operation [168] (Table 5). However, when the same system is irradiated at a high dose rate with an electron beam, allowing the sudden (out of redox thermodynamics equilibrium) and complete reduction of all the ions prior to the metal displacement, the band maximum of the alloyed clusters is at 420 nm [102]. 

The spectra of silver and gold nanoclusters are intense and distinct (Table 4). They are thus particularly suitable to detect the evolution of a cluster composition during the construction of a bimetallic cluster in mixed solution. The system studied by pulse radiolysis was the radiolytic reduction of a mixed solution of two monovalent ions, the cyano-silver and the cyano-gold ions Ag(CN)2 and Au(CN)2 (Fig- 7) [66]. Actually, the time-resolved observation demonstrated a two-step process. First, the atoms Ag and Au are readily formed after the pulse and coalesce into an alloyed oligomer. However, due to... 

Melnik and Parish (10) correlated 197Au Mossbauer and X-ray structural data for gold clusters in 1986, and Salter (11) has recently published a review of the stereochemical nonrigidity exhibited in solution by the metal skeletons of some mixed-metal clusters containing copper, silver, and gold. 

Other metallic clusters that have been demonstrated to show the QDL effect are palladium [116, 117], silver [118] and copper [119]. Palladium MFCs capped with mixed monolayers of hexanethiolate/dodecanethiolate and ferrocene thiolate ligands are prepared in a manner similar to that employed for gold MFCs. The DPV studies exhibit a quantized charging effect but the current peaks are not as well defined as those observed for Au-MPCs. Capacitance values of the order of 0.35 aF are obtained, indicating smaller core sizes or thicker monolayer dielectrics [116]. 

Pulse radiolytic studies of the kinetics of formation of clusters containing two different metals are more readily accessible, for the reasons given above, when both ions may be reduced by a monoelectronic process. This can be achieved with mixed solutions of the monovalent ions Ag and Au, in the form of KAg(CN)2 and KAu(CN)2. " The evolution of the optical absorption spectrum with time was followed specifically at 400 and 520 nm, which correspond to the maxima of the surface plasmon bands of the monometallic silver and gold clusters, respectively. The early steps of the mechanism are rapid reductions of Ag and Au into atoms... 

---