Trinuclear metal clusters

The number of cluster species with a triangular array of metal atoms which have been characterized by X-ray diffraction analysis is rather large. Low as well as high-valence triangular clusters are known. The former are numerous and mostly carbonyl species the later are practically restricted to rhenium halides. Selected examples of triangular trinuclear metal clusters are described in Table 2.2. In most cases the metal atoms in the cluster are equidistant, but there are also many examples in which the internuclear distances are not equivalent. 

Three-dimensional infinite structure formed by 1 cross linking of Nb3 triangles. 

M = Cr, Mo and W (in M02W). Cluster core is 6 a tetrahedral M3S unit with p-CO ligands lying below M3 plane. 

R = CH2Ph R = O-i-Pr. Triangular array of 13 W atoms. Both hydrocarbyl ligands are equivalent. 

Two dimensional polymeric sheets build by Re3 18 units held together by chloride bridges (Fig. 

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Exercise 4.8. Consider the WFe2 complex at the bottom of Figure 4.19 as a trinuclear metal cluster and show that the number of cluster valence electrons = 48. 

MPD of di- and trinuclear metal clusters under colli- 546 sionless conditions. Photodissociation mechanisms and yields of atomic fragments... 

Table 2. Structural data for H3O2 bridging ligands in trinuclear metal cluster compounds... 

Bridges between trinuclear metal clusters [of Ru, Os] Synthesis of ruthenium and osmium carbonyl clusters with unsaturated organic rings Oxyligand derivatives of triosmium dodecacarbonyl... 

I. Copper proteins containing dinuclear and trinuclear metal clusters... 

Class I terpene synthases are highly a-helical proteins containing conserved aspartate-rich motifs. These motifs bind divalent metal ions (Mg " ) via salt bridges to form a trinuclear metal cluster that complexes the polyisoprenoid for diphosphate abstraction. Like ( )-selective prenyltransferases, type I terpene synthases exhibit one conserved aspartate-rich DDXX(D,E) motif, but instead of a second DDXXD on the opposite side of the active center, a consensus sequence of (N,D)D(L,I,V)X (S,T)XXXE (also termed NSE/DTE triad) is found here. One exception is the (-l-)-5-cadinene synthase from Gossypium arboreum that like prenyltransferases contains two DDXXE motifs [195]. 

Hydrocarbons on metal surfaces provide greater challenges in spectral interpretation and we choose the example of ethene chemisorbed on different metal surfaces. Here the relevant model compounds are inorganic binuclear or trinuclear metal clusters with the hydrocarbon ligand of interest and additional... 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

All these results indicate that one is just at the beginning of understanding the function of catalysts being deposited on a semiconductor. There is still quite a confusion in many papers published in this field. Therefore the catalytic properties depend so much on the procedure of deposition . It seems to be rather difficult to produce a catalyst for 02-formation, as shown by results obtained with Ti02 (see e.g.) . Rather recently new concepts for the synthesis of new catalysts have been developed applicable for multielectron transfer reactions. Examples are transition metal cluster compounds such as M04 2RU1 gSeg and di- and trinuclear Ru-complexes . 

Au-B bonds are also present in metal clusters with intersticial or peripheral boron atoms. An example is the cluster [Fe4(CO)12BH(AuPPh3)2], which was prepared by reaction of [AuCl(PPh3)] with the carbonyl iron dihydride. With the oxonium salt the reaction proceeds to the trinuclear gold derivative [Fe4(CO)12B(AuPPh3)3] (357).2063-2070 The ruthenium analogues and complexes with other ligands have been also synthesized as, for example, (358).2071-2079... 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Penta- and hexanuclear clusters of the metals osmium and ruthenium coordinate with the same r ri ri - binding mode as the trinuclear clusters to CgQ. Such complexes are known for the clusters OsjC [75,76], RU5C [77-79], RugC [78], PtRu5C [77] and Rhg [80], In this collection of metal clusters rhenium plays a special role, because it forms a new fullerene-metal sandwich complex, where two C50 are bound to one cluster. 

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