Properties of clusters

The electron rich clusters of the first series of the transition metals are generally more sensitive to oxidation than clusters of the second and third series. The most stable electron-rich clusters are those of groups 8,9, and 10. Clusters of rhodium and iridium are exceptionally stable. However, some clusters of the first series of the transition metals can be stable. This is particularly true for compounds with stable electron configurations containing n-l-2, or n- -3 skeletal electron pairs. For example, 

Co3(CO)9CR complexes contain six skeletal electron pairs [Co(CO)3 -o- CR] and some of them are not oxidized by air in the solid state or in solution. 

Metals of this group form triangular clusters of the general formula [M3Cl6(CsMe5)3], where Af = Ti or Zr (See Chapter 10). Also known is an 86e 

Recently, octahedral compounds of the type [Zv C i2 [Zr6CCli4], KZr CClis, and CsKZrgBCljs have been prepared.  

Metals of this group are characteristic for their ability to form inorganic and organometallic compounds that possess triple and quadruple metal-metal bonds. Some illustrative examples are [M2Meg] (Chapter 4) (M = Cr, Mo, W), M2(C3115)4 (M = Cr, Mo), M2(C0T)3, M2(mhp)4, M2(chp)4, M2(dmhp)4, M2(map)4 

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Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

In Section 4.5, we discuss the optical properties of clusters formed by sexi-thienyl molecules adopting the herringbone structure characteristic of their crystal structure [34]. The theoretical results arc compared to corresponding experimental measurements. 

It should be noted that the above classification system of technetium cluster compounds is not the only possible one. In section 4 another classification is described, which is based on thermal stability and the mechanism of thermal decomposition. Section 2.2 is concerned with the classification based on methods of synthesizing cluster compounds. The classifications based on specific properties of clusters do not at all belittle the advantages of the basic structural classification they broaden the field of application of the latter, because for a better understanding and explanation of any chemical, physico-chemical and physical properties it is necessary to deal directly or indirectly with the molecular and/or electronic structures of the clusters. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

The already voluminous review literature on clusters will be considered as a basis for this review. The topics treated so far are clusters in general (109, 241) and in connection with metal-metal bonding (30, 338, 380), special types of clusters like those with TT-acceptor ligands (231), hydrides (233), carbonyls (85, 86) or methinyl tricobalt enneacarbonyls (313, 317) properties of clusters like structures (56, 316), fluxionality (110), mass spectra (226), vibrational spectra (365), and redox behavior (292). Clusters have been treated in the context of metal carbonyls (3, 4), metal sulfur complexes (2, 381), and in relation to coordination polyhedra (297). Reviews... 

With metal clusters it is even harder than in other fields of inorganic chemistry to substantiate theoretical results by energy measurements. Only two such measurements have come to the attention of the author — the photoelectron spectrum of [CpFe(C0)]4 370) andbond energy determinations in 03(00)9CX-compounds 187). However, a considerable number of papers deal with metal-metal bonding in, and the symmetry properties of, clusters as related to their stoichiometry and their electron count. These studies have confirmed the wide apphcability of the simple 18-electron rule in predicting metal-metal bonds and structures, but they have also led to an understanding of the limits of this rule for clusters with more than four metal atoms. 

Considerable advances in the field of transition metal cluster chemistry have been made during the last five years. They have confirmed that in many respects a cluster complex is comparable to a metallic surface. They have also shown that clusters allow reactions which are not observed with simple metal complexes. And they have finally demonstrated that structural and bonding properties of clusters require new concepts for their description. 

Optoelectronic properties of clusters and small supported particles... 

The study of small, homonuclear clusters of atoms Is Important In understanding nucleatlon because such clusters are Intermediates In the formation of bulk condensed phases. The dynamic process of condensation from a gas must Initially Involve the formation of tiny aggregates of the new phase. This can be Illustrated by the reaction sequence A(g)—A2(g)— A3(g)— . . . — A(1). One of the major weak points In the present day understanding of such nucleatlon phenomena Is the unknown thermodynamic properties of clusters. Certainly, the common practice of treating a 2-200 atom cluster as a tiny piece of the bulk with a large surface Is Inexact. There Is a need for precise thermodynamic data on atomic and molecular clusters to better define nucleatlon kinetics. 

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