As known, the concept of “cluster” is collective and includes substances that are quite diverse in composition and chemical structure. According to the classic definition of this term, the author of which was one of the world’s largest experts in the field of coordination chemistry, Prof. F.A. Cotton, clusters are chemical compounds containing such metal atoms, which are fully or largely covalently bonded to each other, even if the compound contains additional nonmetal atoms that are part of any structural groups associated with metal atoms. Initially, this term referred almost exclusively to the coordination compounds of p-, d-, and f-elements, but over time, more and more new compounds were included in it, although it is worth noting that, in accordance with this definition, under this concept fall primarily those chemical compounds, which include only the atoms of metal elements. This term began to denote, in particular, such compounds as fullerenes and polynuclear compounds formed by boron atoms. Although the term “cluster” has become widely used relatively recently, the very concept of a small group of atoms, ions, or molecules is natural in chemistry, since it is connected with the formation of nuclei during crystallization or associates in a liquid. Owing to this circumstance, later, agglomerations of molecules of the same type were also attributed to clusters–this is how, e.g., “clusters of water molecules” appeared. At the present, the concept of “cluster” includes all chemical compounds that are intermediate between a molecule and a bulk solid with the most varied stoichiometric composition and geometric structure. In modern chemistry, clusters also include nanoparticles of an ordered structure with a given packing of atoms and a regular geometric shape. Moreover, the concept of “cluster” has now gone beyond the framework of chemical science and has found its application in completely different fields of science and technology, which are practically not connected with chemistry. Currently, an “extended” interpretation of this term meaning the combination of several homogeneous elements, which can be considered as an independent unit with certain properties, has become widespread. Examples of such a “synthetic” approach are the word combinations “star cluster” in astronomy, “computer cluster” in computer science, “tone cluster” in music, “cumulonimbus cluster” in meteorology, etc. In addition, although the author of these lines and concurrently Guest Editor of the Special Issue indicated above (see photo right) does not feel any joy about such an “extended” interpretation of the term “cluster,” it will hardly be possible to exclude it from general use. That is why, in the future, for greater certainty and accuracy, instead of the word “cluster,” the concept “metal cluster” will be used, which means any chemical substance where there is precisely a chemical metal–metal bond, regardless of what the nature of these metals is, whether they are the same or different, whether they are connected or not by chemical bonds with non-metals, etc. In addition, be that as it may, chemical compounds that fall under the abovementioned classical definition of “cluster” (i.e., containing at least one metal-metal chemical bond) are still of very significant interest, and this interest increases from year to year.
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