Formation, structure and bonding of metalloid Al and Ga clusters. A challenge for chemical efforts in nanosciences

The renaissance of Al and Ga cluster chemistry is presented in three steps: on the grounds of boron hydride chemistry and the Wade concept, the first step starts in the early nineties of the last century with the formation of single Al - Al and Ga - Ga bonds in molecular entities, obtained by different synthetic approaches. The special method via reaction of high- temperature molecules like AlCl and its disproportionation to Al metal and AlX3 leads to the second step which started about 10 years ago: the formation of nanoscaledmetalloid Al and Ga clusters as intermediates on the way to the metal. Based on the structure of several recent examples, bonding is discussed with respect to the structure of the elements and the generation of naked metal atom clusters. After discussion of the individual metalloid clusters including experiments of the gaseous species and discussion about the jellium model, the third step and main part of this review starts only a few years ago. This latest period hardly can be called a renaissance period as, so far, interactions of nanoscaled metal atom clusters in a perfect 1-, 2- or 3- dimensional arrangement of a crystal have never been investigated before. The most remarkable result in this perspective is the superconducting behaviour of a Ga-84 cluster compound in the crystalline state which had never been observed in metal atom clusters before. However, these experiments show that superconductivity is only observed if the clusters in the crystal are perfectly orientated: as a cluster arrangement of this type can hardly be fabricated by physical methods, these results, which have been predicted by theory, may be called a disillusionment for nanosciences; for chemistry, however, these conclusions pose a challenge.