Transition metal clusters—continued

For these and other reasons it is no wonder that metal cluster chemistry has attracted the attention of some of the brightest minds in science over the past several decades and continues to flourish in so many ways. In this volume some of the leading cluster chemists provide up-to-date reviews of their fields of research. The topics covered include virtually every aspect of cluster chemistry with the notable exception of early transition metal clusters with r-donor ligands, which was the subject of a recent book.FJ... 

Hexacapped Cubic Transition Metal Clusters and Derivatives 1645 Table 1 (continued)... 

The Royal Society of Chemistry continues to publish its Specialist Periodical Reports on organometallic chemistry, containing sections on organo-transition metal cluster chemistry ... 

In recent years, studies of the organometallic chemistry of bare metal ions, metal-heteroatom ions (e.g., MO " and MH ), and metal clusters continue to focus on the themes of reactivity and thermochemistry. The following survey of the literature since the more recent reviews shows that studies of transition metal ions continue to dominate. 

Fig. 1 The effect of size on metals. Whereas bulk metal and metal nanoparticles have a continuous band of energy levels, the limited number of atoms in metal clusters results in discrete energy levels, allowing interaction with light by electronic transitions between energy levels. Metal clusters bridge the gap between single atoms and nanoparticles. Even though in the figure the energy levels are denoted as singlets, we must remark that the spin state of the silver clusters is not yet firmly established...

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

The subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

Concluding Statement. Over the past years there has arisen a whole new transition metal chemistry, that of metal atom clusters and metal-metal multiple bonds. This is conceptually as well as historically a new and revolutionary departure from the preexisting Werner type chemistry, and has one aspect, namely, the quadruple bond, that is a totally new concept in chemistry as a whole. The activity and progress in this field over the past 17 years has been phenomenal and shows every sign of continuing unabated for some time. Very probably important technical applications of compounds with M-M bonds will soon become more numerous. 

Unlike aqueous clusters with predominating electrostatic interaction, the interactions in metallic clusters can be classified as either continuous transitions or metallic [95,96]. In the former interaction, there is a continuous shift from one type of bonding to another type as function of a chemical or physical variable. These types of bondings include covalent, ionic, metallic, and van der Waals interactions. Thus, the accuracy of the results on metallic clusters to a large extent depends on the ability of the theoretical method to describe several types of interactions. In the case of the clusters of higher transition metals, the theoretical method should also be able to describe relativistic effects accurately. 

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