Cluster compounds introductory remarks

Considering now the authoritative treatise by Cotton and Wilkinson (1988), the following assertions can be stated  

As underlined by Mingos and Wales (1990), according to Cotton and Walton (1982) the following statement can be considered as a first-approach working definition a metal cluster compound may be defined as a group of two or more metal atoms where direct and substantial metal—metal bonding is present. 

It has been noticed moreover that in metal cluster chemistry a difference in quality is observed when the sizes of the clusters being progressively characterized are 

Having particularly in view intermetallic substances (crystals, quasicrystals) a critical discussion by Steurer (2005) on several questions concerning clusters (definition, stability, etc.) is especially interesting (see also 4.4.4.8). 

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