**4. Conclusions and prospects**

Therefore, it is difficult to form an alloy cluster using this method when there is a large difference in redox potential between the precursor metal ions. As a result, alloy clusters synthesized

H4

Metal clusters can exchange metal atoms with metal complexes (**Figure 8(b)**). This reaction enables heteroelements to be introduced into metal clusters to synthesize alloy clusters [61]. Although there are some exceptions [62], the number of constituent atoms of the metal core generally does not change during this exchange [63–71]. Therefore, this reaction enables some of the atoms in a cluster to be replaced with other elements while maintaining the original number of constituent atoms and geometry. In addition, this reaction allows heteroelements to be mixed more easily than the co-reduction method. The metal exchange reaction enables the synthesis of alloy clusters composed of metal elements with very different redox potentials, and a larger number of heteroatoms can be replaced than that achieved by the co-

by this method are presently limited to those containing Au, Ag, Cu, Pt, and Pd.

**Figure 9.** Thiolate-protected alloy clusters synthesized by the co-reduction method: (a) Au25−*<sup>x</sup>*

Ph)18, and (d) Au24Pd(SC<sup>2</sup>

reduction method. Using this type of exchange reaction, alloy clusters such as Au25−*<sup>x</sup>*

**Figure 10.** Thiolate-protected alloy clusters synthesized by the metal exchange method: (a) Au24Cd(SC<sup>2</sup>

Ph)18, and (d) Au22AgCuPd(SC12H25)

Ag*<sup>x</sup>*

(SR)24 (*x* = 1–11) have been synthesized to date [63–71].

Au*<sup>x</sup>*

(SR)18 (*x* = 1–9), Au24Cd(SR)18 (**Figure 10(a)**), Au24Hg(SR)18 (**Figure 10(b)**),

(SR)18 (*x* = 1, 2), Ag24−*<sup>x</sup>*

Hg(SR)18 (*x* = 1–8; **Figure 10(c)**), Au24−*x*−*<sup>y</sup>*

Au*<sup>x</sup>*

Ag*<sup>x</sup>* (SR)18

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<sup>18</sup>. Hg/Au indicates Hg or Au. R groups

Ph)18, (b)

Pd(SR)18

Ag*<sup>x</sup>* Cu*<sup>y</sup>*

Ag*<sup>x</sup>* (SC<sup>2</sup> H4

Ph)18. Ag/Au indicates Ag or Au. R groups are omitted

Ph)18, (b)

Pt(SR)18 (*x* = 1, 2, 4–9), and

**3.2. Metal exchange with metal complexes**

Ph)18, (c) Au24Pt(SC<sup>2</sup>

for clarity. Figures were adapted from Refs. [50, 51, 54].

56 Descriptive Inorganic Chemistry Researches of Metal Compounds

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(*x* = 1–8), Au25−*<sup>x</sup>*

Au24−*<sup>x</sup>* Ag*<sup>x</sup>*

Au25−*<sup>x</sup>* Cu*<sup>x</sup>* (SC<sup>2</sup> H4

Au38−*<sup>x</sup>* Ag*<sup>x</sup>*

Au24Hg(SC<sup>2</sup>

H4

Ph)18, (c) Au24−*<sup>x</sup>*

Ag*<sup>x</sup>* Hg(SC<sup>2</sup> H4

are omitted for clarity. Figures were adapted from Refs. [64, 66, 68, 71].

Cu*<sup>x</sup>*

(*x* = 1–3, *y* = 1, 2; **Figure 10(d)**), Ag25−*<sup>x</sup>*

Cd(SR)18 (*x* = 2–6), Au24−*<sup>x</sup>*

In this chapter, we focused on Au*<sup>n</sup>* (SR)*m* and related alloy clusters as examples of metal nanoclusters and described the latest techniques and knowledge regarding their precise synthesis. The study of Au*<sup>n</sup>* (SR)*m* clusters has progressed with spectacular speed in recent years. Consequently, the associated synthetic techniques have also advanced dramatically, and a greater understanding of their characteristics has been obtained [74–76]. These clusters are now expected to be applied in various fields such as sensing, imaging, cancer radiation therapy, catalysis, photocatalysis, solar cells, fuel cells, photosensitizers, and single-electron devices. If these Au*<sup>n</sup>* (SR)*m* clusters can be regularly assembled [77], further new functions could be induced and their fields of application might be further expanded. In the future, it is expected that intensive investigations will be conducted regarding the formation of various nanoarchitectures using Au*<sup>n</sup>* (SR)*m* clusters as structural units in addition to research on the Au*<sup>n</sup>* (SR)*m* clusters themselves.
