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View Article Online / Journal Homepage / Table of Contents for this issue FRONTIER Dalton H. Schnöckel University of Karlsruhe, Institute for Inorganic Chemistry, Engesserstrastr. 15, Karlsruhe, D-76128, Germany. E-mail: [email protected] www.rsc.org/dalton Metalloid Al- and Ga-clusters: a novel dimension in organometallic chemistry linking the molecular and the solid-state areas?† Received 17th May 2005, Accepted 9th August 2005 First published as an Advance Article on the web 31st August 2005 Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N Formation and fragmentation of metal–metal bonds on the way between stable metal compounds in which the metal atoms are oxidised (e.g. isolated species in solution or metal salts in bulk) and the bulk metal are the fundamental steps to understand this process in which formation and chemical behaviour of metalloid Al and Ga clusters as intermediates are essential. Many examples of metalloid Al and Ga clusters show that their formation reflects a high degree of complexity like that of the simple seeming formation of the bulk metal itself: starting from metastable Al(I) and Ga(I) solutions containing small molecular entities, metalloid clusters grow during many self-organization steps including aggregation as well as irreversible redox cascades. This novel class of clusters seems to open a new dimension in chemistry between the molecular and the solid-state area, because, for the first time, it is shown that under well selected conditions definite molecular species, i.e. metalloid clusters, grow via the formation of additional metal–metal bonds and that the solid metal represents the final step. Introduction Most elements of the periodic system are metals. Their chemistry and especially their formation and dissolution belong to the oldest technical chemical processes. In general, however, only the bulk metals themselves, on the one hand, and their stable compounds (e.g. salts, oxides, sulfides, in solution or in bulk), on the other, are well known. It seems strange that intermediates in † Electronic supplementary information (ESI) available: Fig. 1 : The Aln core of the metalloid clusters [Al7 R6 ]− , [Al12 R8 ]− , [Al14 R6 I6 ]2− (R = N(SiMe3 )2 ). Fig. 2 : The Aln core of the metalloid clusters [Al69 R18 ]3− and [Al77 R20 ]2− (R = N(SiMe2 )2 ) and the different arrangements of the central Al13 units. Fig. 3 : The Al22 core of the halides Al22 X20 ·12D (X = Br, Cl; D = THF). Fig. 4 : Section of the normal-pressure (a, b, c, d) and the highpressure modifications (GaII, GaIII) of solid Ga. The fcc high-pressure modification GaIV is not shown. Fig. 5 : Gan core of the clusters Ga18 R8 and Ga22 R8 (R = SitBu3 ). See http://dx.doi.org/10.1039/b507002n DOI: 10.1039/b507002n Hansgeorg Schnöckel studied chemistry at the University of Münster, where he gained his PhD under H. J. Becher in 1970. Subsequently he started spectroscopic matrix investigations of reactive high-temperature molecules which were the basis for later synthetic work. In 1987 he became professor and in 1989 he changed to the University of Munich. Since 1993 he has held the chair for analytical chemistry at the University of Karlsruhe. H. Schnöckel This journal is © this process of the formation and breaking of metal–metal bonds (MM) are mostly unknown. This fundamental process and molecular intermediates exhibiting MM bonding are therefore central to this contribution. These molecular intermediates are mostly addressed as metal atom clusters.1 However, since this definition of F. A. Cotton is not restricted to species containing mainly MM bonding, we have introduced the term metalloid clusters.2 Such clusters contain more MM contacts than ML (metal–ligand) bonds and mostly show similarities with respect to the topology to the arrangements of atoms in the elements themselves. Though most investigations on metalloid cluster species have been, and are still, performed in the field of precious metals (e.g. Au, Pd)3 because of their relatively straightforward synthesis, their stability and their inertness even in air and sometimes in water, it seems to be extremely difficult to obtain crystalline materials in order to characterize them via crystal structure analysis.4 The largest cluster of this type is a Pd145 cluster for which—although only two crystals have been obtained so far—the structure could be solved by L. F. Dahl et al.:5 There is a centre of 55 naked Pd atoms surrounded by 90 ligand-bearing Pd atoms. The many synthetic results for these clusters of precious metals and the few detailed structure determinations on the one hand, and the failure of analysis for clusters of base metals until 1997 on the other, may give an idea of the great surprise when 8 years ago a metalloid cluster was presented containing 77 Al atoms, of which 57 were naked and only 20 were ligand-bearing (Fig. 1).6 However, this result was assumed to be a singularity or a curiosity.7 We will show here that an exciting story has been developed from this starting point and one which will also have impact on the development of chemistry in general. Though the organometallic chemistry of aluminium became technically important in the second half of the last century—e.g. through the work of Ziegler8 —the chemistry of the group 13 elements was assumed in general to be completely discovered by the end of the last century. The exception seemed to be the chemistry of boron. In retrospect, the “curiosities” in boron chemistry have been a stroke of luck for the whole of chemistry, for these results were the basis for novel ideas of bonding which have influenced many areas of chemistry.9 In order to develop a new chemistry, possibly similar to that of boron, for the heavier elements Al and Ga, spectroscopic and synthetic work was started in the 1980s. The spectroscopic work concentrated on Al and Ga monohalides (AlX, GaX) The Royal Society of Chemistry 2005 Dalton Trans., 2005, 3131–3136 3131 Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N View Article Online Fig. 1 The arrangement of 77 Al atoms in the metalloid cluster anion [Al77 {N(SiMe3 )2 }20 ]2− . The 20 ligand-bearing Al atoms are blue and the 44 Al atoms of the second “shell” are metallic-grey. The centre of the cluster exhibits a single Al atom surrounded by 12 Al atoms in a distorted cuboctahedral/icosahedral arrangement. generated at high temperatures and trapped in solid Ar and then allowed to aggregate to form e.g. dimers or to react with other molecules such as O2 or HCl.10 In the synthetic area the beginning of a new chemistry came with the detection of the first molecular compound containing a direct Al–Al bond via the careful reduction of R2 AlX species (R = organic ligand such as [HC(SiMe3 )2 ].11 From the matrix work on AlX and GaX species, a synthetic upscaling led to metastable AlX and GaX solutions and through substitution reactions to the first Al(I) organic species AlCp*.12 Simultaneously, synthetic routes via reduction of RAlX2 or R2 AlX compounds have been developed.13 Both strategies have been successfully applied in the last decade, with the technique of trapping AlX or GaX species and the reduction route resulting in Aln Rm and Gan Rm species (n ≈ m), as described in several reviews. Most contributions involving the latter technique came from a few groups worldwide, e.g. those of W. Uhl,13 G. H. Robinson,14 G. Linti,15 P. P. Power,16 P. Jutzi,17 H. W. Roesky,18 R. A. Fischer,19 and N. Wiberg.20 This chemistry of Al(I) and Al(II) species and their Ga analogues should be called “modern classical chemistry”; reference has been made elsewhere to as a renaissance of Al chemistry,18 since spectacular analogies to the molecular chemistry of boron, e.g. in the case of the icosahedral Al12 R12 2− , cannot be overlooked.21 From this platform of Al(I) species, e.g. [AlCp*]4 , as the first organo-Al(I) compound containing an Al4 tetrahedral core,22 a new dimension of chemistry is now visible, as convincingly illustrated by the recently published Al50 Cp*12 compound23 (Fig. 2). This largest structurally characterised homoleptic organometallic compound containing 50 Al, 120 C and 180 H atoms may be a milestone in the metal-rich chemistry with respect to fundamental questions and possibly also applications. Therefore, this example may be an ideal starting point to introduce and explain the fundamental aspects of metalloid Al and Ga clusters through the medium of the following sections: 1. Synthetic concepts 2. Variety of structural motifs within the crystalline compounds 3. Properties of the crystalline compounds 4. Bonding of metalloid clusters via experimental and theoretical investigations 1. Synthetic concepts The synthetic concept for metalloid clusters is based on a technically sophisticated preparation of metastable solutions formed via trapping of the high-temperature subhalide molecules AlX and GaX and a solvent mixture (mostly toluene and a donor component such as Et2 O), for which experimental details have been described before.12,20b,24,25 If these solutions are heated from −80 ◦ C to about +80 ◦ C, disproportionation to the bulk 3132 Dalton Trans., 2005, 3131–3136 Fig. 2 The arrangement of 50 Al atoms and 12 C5 (CH3 )5 moieties of the Al50 Cp*12 cluster. The 12 ligand-bearing Al atoms are blue; 30 of the remaining 38 Al atoms are metallic-grey, 8 Al atoms in the centre are pink. The topology of the 60 CH3 -groups is similar to that of the 60 C atoms of fullerene. metal (Al/Ga) and the trihalide is observed: e.g. 3 AlCl → 2 Almetal + AlCl3 . During this process, many steps of aggregation and elimination (e.g. redox chemistry) are necessary, so that the overall change exhibits a very high degree of complexity. Nevertheless, the process is fast and therefore intermediates cannot usually be trapped. Such trapping becomes possible generally only if bulky substituents R− (R− substitutes e.g. Cl− ) are added during the disproportionation. Fortunately, some of these substituted species can be isolated and structurally characterised as snapshots on the way to the bulk metal. By varying the reaction conditions (e.g. temperature, solvent, amount of the donor compound in the solvent and the amount of the substitution reagent, e.g. LiR), the size of the metalloid clusters, i.e. the degree of proceeding to the formation of the bulk metal, can be influenced. To sum up, the reproducible detection of definite intermediate clusters demonstrates that this process leading to the formation of metals follows definite rules which are completely unknown so far, and for which even the few snapshots illustrate the high complexity. 2. Variety of structural motifs within the crystalline compounds The variety of metal atom topology in the crystalline compound formed by a metalloid cluster often reflects the topology of the metal itself. This principle is shown for [N(SiMe3 )2 ]− ligandstabilized Al7 , Al12 , Al14 , Al69 and Al77 cluster compounds20b,24,25 in which distorted arrangements of the Al atoms similar to those in the close-packed structure of the metal are visible (ESI,† Fig. 1 ). Nevertheless, each of these clusters exhibits topologies of the metal atoms that are singular in cluster chemistry.26 There are small but significant differences even where the clusters are quite similar, e.g. in the largest clusters [Al69 R18 ]3− and [Al77 R20 ]2− ;27 the central Al atoms have a decahedral shell (Al69 ) and a distorted cuboctahedral coordination (Al77 ) of 12 Al atoms surrounding the unique central Al atom (ESI,† Fig. 2 ). This means that small changes in the cluster shell are reflected also in the cluster centre. Consequently it must be assumed that changes on any metal surface e.g. by oxidation will have topological and/or electrical influence on the metal atoms in the bulk below the surface. This influence may be of special importance for the properties of any nanoscaled metallic entities and devices. Besides clusters exhibiting the atomic arrangement of the known structure of the bulk metal, it is possible in principle also to form metastable, hitherto unknown modifications of Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N View Article Online Fig. 3 The [Ga51 (PtBu2 )14 Br6 ]3− cluster anion with its highly symmetrical arrangement of the surrounding ligands (6 Br atoms (green) and 6 + 2 terminally bonded and 6 bridging PtBu2 entities (purple)). The centred Ga13 unit with a volume similar to that of a Ga13 section of the high pressure fcc-modification of the metal is focussed. the metals under these mild reaction conditions, as revealed by the existence of cluster analogues. For Al a non-metallic modification analogous to that of a- or b-boron is conceivable because we were able to isolate a potential precursor species Al22 X20 in which naked Al12 icosahedra are directly bonded to 10 AlX2 moieties [Al12 (AlX2 )10 ]28,29 (ESI,† Fig. 3 ). On the basis of quantum chemical calculations, it seems possible in principle to connect the Al12 icosahedra to form a hypothetical metastable b-Al. This is a challenge for future work to prepare metastable modifications of elements, and not only of Al, as new and unusual materials. Especially for gallium with its already known seven modifications (ESI,† Fig. 4 ), there are realistic chances of producing new modifications like that of a-boron deduced from quantum chemical calculations and recent experimental observations.30 Obviously the metal atoms of Ga are very “flexible” with respect to different topologies, and consequently this flexibility allowing different connectivities is also reflected in the variety of metalloid Ga clusters prepared as nanoscaled element modifications: thus, [Ga10 (GaR)8 ] and [Ga14 (GaR)8 ] (R = supersilyl = SitBu3 ) are convincing examples that within a similar “box-volume” of 8 Ga ligand atoms arranged at the vertices of a cube there are 10 Ga atoms arranged in the topology of the normal pressure modification of b-Ga or 14 Ga atoms arranged as in the high-pressure modification Ga(III)31 (ESI,† Fig. 5 ). In order to investigate the influence of ligands on the arrangement of metal atoms in the cluster core, the [SitBu3 ]− ligand was changed for the bridging ligand [P(tBu)2 ]− . This substitution leads to a compression of the central core of the naked Gan atoms and therefore, as in the Ga51 cluster, the central cuboctahedral Ga13 unit is very close to the unit of 13 Ga atoms in the high-pressure modification Ga(IV)32 (Fig. 3). Furthermore, the easy variation of connectivities of the Ga atoms could be demonstrated recently by the preparation of four metalloid Ga clusters with four different arrangements of 22 Ga atoms (Fig. 4).33 The largest structurally characterized metalloid cluster containing 84 Ga atoms is protected by the same kind and number of ligands [N(SiMe3 )2 ] as the Al77 cluster compound34 (Fig. 5). This Ga84 cluster exhibits many singularities, e.g. its high pseudosymmetry (D5h ) which may have been the basis for addressing Fig. 4 Four different arrangements of 22 Ga atoms in the cluster species (a)–(d): (a) Ga22 R8 (R = Si(SiMe3 )3 , Ge(SiMe3 )3 , SitBu3 ), the central Ga atom has a coordination number of 13 (3, 6, 4). The directly bonded ligand atoms are grey. (b) [Ga22 R10 ]2− (R = N(SiMe3 )2 , the N atoms are red), the central Ga atom has a coordination number of 11. In contrast to (a) and (b) there is no central Ga atom in the distorted icosahedral species presented in (c) and (d): (c) [Ga22 {N(SiMe3 )2 }10 Br10 ]2− (projection along the top and bottom Ga atoms), Br atoms (green), N atoms (red); (d) via the bridging ligands PtBu2 (P atoms purple) this cluster Ga22 R12 has a low average oxidation number for the Ga atoms, i.e. this cluster, more so than (c) presents a snapshot on the way to one of the modifications of bulk Ga. it as a fragment of a quasi-crystal.35 Here we want to draw attention only to the Ga2 unit in the centre of the cluster with a short Ga–Ga distance not much longer than the so-called Ga–Ga triple bond.36,37 This topology of a Ga2 unit (as in aGa) surrounded by a cage of about 32 other naked Ga atoms is unique and noteworthy, especially with respect to possible rotations of this moiety. The unusual topology of the Ga84 cluster itself and especially the arrangement of the clusters in the crystal (Fig. 5) are responsible for the remarkable properties of this compound shortly to be discussed in the next section. 3. Properties of the crystalline compounds In order to measure reproducible physical properties of a metalloid cluster compound as an intermediate on the way to the bulk metal, well-defined crystalline material is required. Since, even for large clusters of precious metals, no suitable crystals were available so far, no reproducible data exist. For Al and Ga clusters these measurements are especially difficult since many of them spontaneously ignite in air. Nevertheless, after three years of hard work (involving measurements on crystalline materials, e.g. solid-state Ga-NMR, EPR, SQUID and conductivity), we have succeeded in showing that crystals of the Ga84 compound with its special arrangement of the Ga84 entities (Fig. 5) exhibit electrical conductivity and below 7 K even superconductivity.38,39 These unexpected and yet to be conclusively interpreted results have been investigated intensively in the last years.40 One point may be of fundamental importance: e.g. besides the Ga84 cluster compound containing the [Ga84 R20 ]4− anions, we were able to crystallize a compound containing the analogous [Ga84 R20 ]3− species with a slightly different arrangement of the cluster entities.41 This observation may be significant for the entire cluster chemistry because cluster doping (e.g. a small amount of [Ga84 R20 ]3− cluster may replace the [Ga84 R20 ]4− anions in the lattice of the latter cluster compound or vice-versa), which may occur during the formation of crystals depending on the redox potential of the mother-liquor, may be responsible for the resulting properties and especially the conductivity Dalton Trans., 2005, 3131–3136 3133 Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N View Article Online 3134 Fig. 5 The single [Ga84 R20 ]4− cluster (R = N(SiMe3 )2 , N atoms red) (left) with its 20 ligand-bearing Ga atoms (blue) and the arrangement of these clusters in the crystal (right) via the “naked” top and bottom Ga atoms (dark-grey, left). The space between these Ga atoms (1.4 nm) is filled with two toluene molecules (right). mentioned above. In the crystalline state in particular, metalloid clusters seem to exhibit unpredictable properties which may be important ultimately for understanding fundamental processes, e.g. conductivity or superconductivity,42 and consequently they may also be suitable for special applications. 4. Bonding of metalloid clusters via experimental and theoretical investigations The description of bonding within the large variety of different topologies of metalloid Al and especially Ga clusters seems to be a real problem. This is not unexpected since the group 13 elements boron, aluminium and gallium, though containing three valence electrons each, show in their elemental forms that, depending on small changes of conditions (p, T), different connectivities of the metal atoms are achieved in the different modifications. Consequently, the metalloid cluster species reflecting this variety cannot be understood via a single electron-counting rule.43 In order to understand the bonding, therefore, the electronic situation of every metalloid Al and Ga cluster species has to be discussed thoroughly and individually with the help of quantum chemical calculations. However, a qualitative description of the topology can often be obtained via comparison with modifications of the elements themselves (cf . above). Gallium with its seven known modifications is a nearly ideal case in order to make plausible this concept that metalloid clusters are intermediates during the formation of the element, i.e. that metalloid clusters are snapshots taken during these highly complex processes of self-organisation. Such a hypothetical description is visualized in Fig. 6 demonstrating via a cartoon that starting from GaX species via disproportionation different routes to the 7 modifications of gallium can be expected to proceed via certain metalloid Ga clusters. Besides topological arguments for the structure of the clusters, recent mass spectrometric results obtained via FT-ICR-MS provide an experimental basis for the bonding description of these clusters. After the structurally characterised cluster anion [Ga19 R6 ]− [R = C(SiMe2 )3 ]2 has been transferred to the gas phase without decomposition by electrospray ionisation (ESI) (this being the first time for a metalloid cluster),44 the weakest bonds could be dissociated via collision experiments (Fig. 7). Thus, the weakest bonds are those between the cuboctahedral Ga13 − core and the six neutral GaR units which have been shown previously to be stable in the gas phase. Since the stable Ga13 − anion finally results (the analogous Al13 − has been called a superhalogenide species recently because of its stable electronic configuration Dalton Trans., 2005, 3131–3136 Fig. 6 This cartoon with Mt. Bromo, East Java, Indonesia, may illustrate in a simple manner the hypothetical routes to the formation of different modifications of bulk gallium via different metalloid clusters as snapshots of these highly complex processes of self-organisation starting with the high-temperature gallium monohalide molecules GaX. (jellium model) and high electron affinity of the neutral species45 ) we have combined the high-vacuum world of physicists dealing with naked metal atom clusters as intermediates between the bulk metal and metal atoms and the world of chemists, which is usually the solution from which the crystalline compounds are grown (Fig. 8). This description of metalloid Al and Ga clusters as naked metal atom cluster cores which are oxidised on the outside to M+ or M2+ , and for which the bonds between the core and the MR or MR2 entities are weak, is also evident in the structure of the cluster species presented in Fig. 9. On the basis of the structures of so many metalloid clusters, a hypothetical mechanism for the dissolution of a bulk metal can be deduced. This is visualized in Fig. 9. Ongoing experiments with naked Aln clusters by FT-ICR mass spectrometry will confirm this mechanism in the gas phase for a fundamental process in chemistry, i.e. the dissolution of metals and, via retro-synthetic considerations, also the formation of a metal from a salt-like solution.46 Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N View Article Online Fig. 9 (a) A simplified picture of the hypothetical stepwise oxidation of a surface of bulk aluminium. The mechanism is based on some structurally characterized metalloid clusters as intermediates, i.e. as snapshots during this highly complex process. (b) The metalloid clusters containing a shell of M+ atoms (blue) are [Al7 R6 ]− [R = N(SiMe3 )2 ],23,24 [Ga13 R6 ]− [R = C(SiMe3 )3 ]2 and Al50 Cp*12 .22 (c) The metalloid clusters containing the oxidized M2+ species (orange) are: [Al5 Br8 ]− ,23 Al22 Cl20 27,28 and Ga24 Br22 .29 Conclusions and outlook Fig. 7 The relation between the metalloid cluster anions [Ga13 (GaR)6 ]− (R = C(SiMe3 )3 ) and the naked Ga13 − cluster. SORI-CAD FT-MS experiments after electrospray (ESI) show that the six weakest bonds between the Ga13 core and the GaR units can be fragmented, in a stepwise fashion or removed in a single step via LDI (laser desorption ionisation). Fig. 8 Visualization of the generation of two types of cluster containing mainly metal–metal bonds; left: metalloid clusters starting from diatomic monohalide molecules (the oxidation states of the metal atoms are given in parentheses); right: naked metal atom clusters starting from the bulk metal. It is a challenge for future work to connect these two “worlds” of physicists and chemists. Formation and fragmentation of metal–metal bonds on the way between stable metal compounds in which the metal atoms are oxidised (e.g. isolated species in solution or metal salts in bulk) and the bulk metal are the essential steps to understand the formation and chemical behaviour of metalloid Al and Ga clusters as intermediates during this process. These clusters seem to open a new dimension in chemistry between the molecular and the solid-state area, because, for the first time, definite molecular species grow only via the formation of additional metal–metal bonds. In solid-state chemistry, this process of forming larger metal-containing entities (e.g. Zintl anions) has been fruitfully expanded e.g. by Corbett, Fässler, Nesper, von Schnering, and Simon.47 However, the molecular chemistry of species containing mainly metal–metal bonds may now open new territory since unusual properties may be expected for these novel substances, as has been shown for the Ga84 cluster compound. This metal–metal chemistry would be a natural development of classical chemistry of former periods when (a) many non-metal elements (e.g. C, N, P, Si) are connected to molecular entities also by sophisticated methods, such as those of modern organic chemistry, or (b) metal atoms and non-metal atoms are combined, as in traditional inorganic chemistry. However, since even the synthesis of e.g. C60 fullerene species via a stepwise route does not seem to be possible so far, stepwise synthetic procedures are even more unlikely to succeed in the case of metalloid clusters. Therefore, the present method of trapping these clusters starting from metastable Al(I) or Ga(I) solutions on the way to the bulk metal seems to be the only way known to date for forming molecular species containing mainly metal–metal bonds. In our opinion, however, this process of forming metalloid clusters, which might seem simple at first glance, exhibits a high degree of complexity; i.e. the self-organization steps within this process–aggregation steps as in supermolecular chemistry as well as irreversible redox cascades–may not be much simpler than in many reactions in biology,48 but experimentally more difficult to detect because all the intermediates in the case of Al and Ga clusters are sensitive to attack by air and moisture. Furthermore, since most of the elements of the Periodic Table are metals, and since every metallic element may be formed via alternative routes, this novel chemistry of metalloid clusters exhibits a greater diversity than many biological options.48 In the next decades, therefore, much work in theory, spectroscopy, physics, and especially via synthetic chemistry is needed to characterise more intermediates Dalton Trans., 2005, 3131–3136 3135 View Article Online as snapshots during this highly facile process in order to gain a deeper understanding of the fundamental mechanism of metal formation and dissolution. Acknowledgements We thank the Deutsche Forschungsgemeinschaft and the Centre for Functional Nanostructures (CFN) and the Fonds der Chemischen Industrie for financial support. Special thanks to P. Hauser, M. Kayas, and S. Schneider for essential help in preparing this manuscript and the figures. 27 28 29 30 Downloaded by University of Washington on 27/03/2013 22:34:44. Published on 31 August 2005 on http://pubs.rsc.org | doi:10.1039/B507002N References 3136 1 F. A. Cotton, Q. Rev. Chem. Soc., 1966, 20, 397. 2 A. Schnepf, G. Stößer and H. Schnöckel, J. Am. Chem. Soc., 2000, 122, 9178. 3 (a) Cluster and Colloids, ed. G. Schmid, Wiley-VCH, Weinheim, 1994; (b) Metal Clusters in Chemistry, ed. P. Braunstein, L. A. Oro and P. R. Raithby, Wiley-VCH, Weinheim, 1999; (c) Nanoparticles, ed. G. Schmid, Wiley-VCH Weinheim, 2004. 4 Although suitable crystals of the Au55 cluster compounds for an X-ray structure determination are not available so far, a face-centred cubic arrangement of the Au atoms with shorter average distances than in the bulk metal could be inferred recently via EXAFS, XANES and WAXS: R. E. Benfield, D. Grandjean, M. Kroell, R. Pugin, T. Sawitowski and G. Schmid, J. Phys. Chem. B, 2001, 105, 1961. 5 N. T. Tran, D. R. Powell and L. F. Dahl, Angew. Chem., 2000, 112, 4287; N. T. Tran, D. R. Powell and L. F. Dahl, Angew. Chem., Int. 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Köppe, E. Weckert and H. Schnöckel, Angew. Chem., 1999, 111, 1851; Ch. Klemp, R. Köppe, E. Weckert and H. Schnöckel, Angew. Chem., Int. Ed., 1999, 38, 1739. C. Klemp, M. Bruns, J. Gauss, U. Häussermann, G. Stößer, L. van Wüllen, M. Jansen and H. Schnöckel, J. Am. Chem. Soc., 2001, 123, 9099. T. Duan, E. Baum, R. Burgert and H. Schnöckel, Angew. Chem., 2004, 116, 3252; T. Duan, E. Baum, R. Burgert and H. Schnöckel, Angew. Chem., Int. Ed., 2004, 43, 3190; H. von Schnering and R. Nesper, Acta Chem. Scand., 1991, 45, 870; U. Häussermann, S. Simiak, I. Abrikosov and S. Lidin, Chem. Eur. J., 1997, 3, 904; X. G. Gong, G. Chiarotti, M. Parinello and E. Tasatti, Phys. Rev. B, 1991, 43, 14277; U. Häussermann, S. I. Simiak, R. Ahuja and B. Johansson, Phys. Rev. Lett., 2003, 90, 65701. A. Donchev, A. Schnepf, G. Stößer, E. Baum, H. Schnöckel, T. Blank and N. Wiberg, Chem. Eur. J., 2001, 7, 3348. J. Steiner, G. Stößer and H. Schnöckel, Angew. Chem., 2004, 116, 305; J. Steiner, G. Stößer and H. Schnöckel, Angew. Chem., Int. Ed., 2004, 43, 302. References to all Ga22 species in Fig. 4 are mentioned by: J. Steiner, G. Stößer and H. Schnöckel, Angew. Chem., 2004, 116, 6712; J. Steiner, G. Stößer and H. Schnöckel, Angew. Chem., Int. Ed., 2004, 43, 6549. A. Schnepf and H. Schnöckel, Angew. Chem., 2001, 113, 734; A. Schnepf and H. Schnöckel, Angew. Chem., Int. Ed., 2001, 40, 712. R. B. King and P. V. R. Schleger, Molecular Clusters of the Main Group Elements, ed. M. Driess and H. Nöth, Wiley-VCH, Weinheim, 2004, pp. 1–33. J. Su, X.-W. Li, R. C. Crittendon and G. H. Robinson, J. Am. Chem. Soc., 1997, 119, 5471; Y. Xie, R. S. Grev, J. Ga, H. F. Schäfer III, P. von R. Schleyer, J. Su, X.-W. Li and G. H. Robinson, J. Am. Chem. Soc., 1998, 120, 3773; N. Takagi, M. W. Schmidt and S. Nagase, Organometallics, 2001, 20, 1646; A. J. Bridgeman and L. R. Ireland, Polyhedron, 2001, 2841. R. Köppe and H. Schnöckel, Z. Anorg. Allg. Chem., 2000, 626, 1095; H.-J. Himmel and H. Schnöckel, Chem. Eur. J., 2002, 8, 2397; H.-J. Himmel and H. Schnöckel, Chem.–Eur. J., 2003, 9, 748. J. Hagel, M. T. Kelemen, G. Fischer, B. Pilawa, J. Wosnitza, E. Dormann, H. v. Löhneysen, A. Schnepf, H. Schnöckel, U. Neisel and J. Beck, J. Low Temp. Phys., 2002, 129, 133. O. N. Bakharev, N. Zelders, H. B. Brom, A. Schnepf, H. Schnöckel and L. Jos de Jongh, Eur. Phys. J. D, 2003, 24, 101. J. Frenzel, S. Gemming and G. Seifert, Phys. Rev. B, 2004, 70, 235404. A. Schnepf, E. Herrling, B. Pilawa, E. Weckert, A. Meents, B. Jee and H. Schnöckel, Inorg. Chem., 2003, 629, 2168. The results of the first NMR measurements on metal atoms within a crystalline metalloid cluster compound during the transition to the superconducting state are about to be published (O. Bakharev, H. B. Brom, L. J. de Jongh, A. Schnepf and H. Schnöckel, Nature, to be submitted). K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 1; R. W. Rudolph, Acc. Chem. Res., 1976, 9, 446; R. E. Williams, Adv. Inorg. Chem. Radiochem., 1976, 18, 67; D. M. Mingos, Nature, 1972, 336, 99. K. Weiß and H. Schnöckel, Z. Anorg. Allg. Chem., 2003, 629, 1175. D. E. Bergeron, P. J. Roach, A. W. Castleman, Jr., N. O. Jones and S. N. Khanna, Science, 2005, 307, 231. First results with Cl2 as an oxidising agent reveal different chemical behaviours for different Aln clusters (R. Burgert, M. Olzmann, K. Bowen and H. Schnöckel, Angew. Chem., submitted). The planned oxidation with O2 promises to be especially complex because even Al atoms under matrix conditions yield unexpected species (e.g. AlO6 : G. Stößer and H. Schnöckel, Angew. Chem., 2005, 117, 4334; G. Stößer and H. Schnöckel, Angew. Chem., Int. Ed., 2005, 44, 4261. Of the large number of publications in this field only two are selected which contain references to many other contributions: J. D. Corbett, Angew. Chem., 2000, 112, 682; J. D. Corbett, Angew. Chem., Int. Ed., 2000, 39, 692; T. F. Fässler and S. D. Hoffmann, Angew. Chem., 2004, 116, 6400. J.-M. Lehn, Supramolecular Chemistry, Concepts and Perspectives, Wiley-VCH, Weinheim, 1995, pp. 204–206. In this chapter (Chemistry and Biology, Creativity and Art), the author compares chemistry and biology with respect to complexity and breadth or diversity.