Transition metal clusters—continued atoms

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]. 

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. 

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