Heteronuclear clusters transition metals

As is the case for transition metal cluster compounds in general, singlecrystal X-ray diffraction is normally the only technique available for the unambiguous structural characterization of heteronuclear Group IB metal clusters. Tables I, II, and IV-XIV indicate the mixed-metal clusters containing one or more ML (M = Cu, Ag, or Au L = two-electron donor ligand) units which have been studied by X-ray crystallography. Other... 

As stated in the previous section, dynamic behaviour involving intramolecular rearrangements of the metal skeletons of Group 11 metal heteronuclear clusters is relatively common, in marked contrast to the situation observed for almost all other transition metal clusters, which have metal frameworks that are stereochemically rigid in solution. The mechanisms of these metal core rearrangements are, therefore, of considerable interest. 

A great variety of heteronuclear gold cluster compounds are known. In many of these clusters the AUPR3 moiety behaves like an isolobal proton" " and bridges an edge of a transition metal cluster. While another type of heteronuclear clusters is built up by one central transition metal atom and up to twelve gold atoms, as described in several reviews. " ... 

For the benefit of clarity, this Chapter has been restricted fundamentally to the discussion of the chemistry of molecular transition metal clusters no dinuclear compounds, which were analyzed to some extent in Chapter 1, nor microcrystalline metal particles are considered. For the same reason the main emphasis is given to homonuclear metal compounds. However, heteronuclear species with different transition metals or containing main group atoms are taken into account whenever they are useful for a better understanding of cluster chemistry. 

In initial studies on transition metal cluster chemistry synthetic work was frequently based on chance discoveries. However the explosive development of this branch of chemistry in the last fifteen years has also produced an accelerated evolution of cluster synthetical methods. On going from casual synthesis to tailored cluster synthesis, the systematic preparation of a great variety of homo and heteronuclear cluster compounds is already possible. 

The structures of some of the homo and heteronuclear anion clusters mentioned in Table 4.14 are illustrated in Figs. 4.66 and 4.67. Almost all these structures are cluster species fundamentally similar to those of the transition metal clusters discussed in Chapter 2. They belong however to the so-called naked clusters because they do not need to be stabilized by ligands. 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Mercury(II) forms a series of compounds in which it is bonded to a transition metal to form heteronuclear Hg—M bonds.540,541 The most widely used synthetic routes have been reviewed elsewhere.542-549 Besides heteronuclear bonds M—Hg there are structural elements M—Hg— M, 550 e.g. (31), or cyclic arrangements as in Os3(CO)uHg 3415 or (1/5-MeC5H4)Mn(CO)2Hg 4.416 A trigonal prismatic coordination of mercury has been reported in the green zerovalent mixed metal cluster [Hg Pt(2,6-Me2C6H3NC)fi ]. a... 

The subject of heteronuclear cluster compounds of the transition metals remains an active area of research interest, and was reviewed in the early 1980s by Geoffroy el al. (1,2). Clusters with novel architectures, exemplified by the star clusters of Stone and co-workers (5), continue to be synthesized. Whereas there is undoubtedly strong academic interest in the structure, bonding, and chemical reactivity of heteronuclear clusters in their own right, additional impetus to this field is given by the important relationship between heteronuclear clusters and bimetallic alloy catalysts. This relationship was the subject of a published symposium (4). 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

Fig. 1. Examples of the synthesis of heteronuclear gold cluster complexes by the reaction of Au(PPh3)Cl with transition metal anions.

The addition of gold phosphine fragments to transition metal compounds is readily extended to include reactions of heteronuclear gold cluster compounds and has been used to build up clusters of increasing nuclearity, as the following examples illustrate ... 

Heteronuclear M—M Bonds. The transition metals, especially in their carbonyl-type compounds, form many bonds to the non-transitional metal or metalloidal atoms. This is particularly true for the elements Zn, Cd, Hg, Cu, Ag, Au, Tl, Ge, Sn and Pb. These are nearly always 2c-2e bonds which require no special comment. However, there are cases where mixed metal clusters of a more complex nature are formed. Examples of these are trigonal-bipyramidal Sn2Pt3 species and tetrahedral Ge2Co2 species. 

Hieber and his school have carried out a systematic investigation of the transition metal carhonyl anions and derivative hydrides 169), and have synthesized many carbonyl anion and hydride clusters, particularly of the first row transition metals. The preparations of these clusters were all based on the reduction of a carbonyl compound. More recently a number of heteronuclear carbonyl anion and hydride clusters have been produced by condensation reactions. 

M = Cu, Ag or Au R = alkyl or aryl) units in heteronuclear clusters. M is a transition metal other than a Group 11 metal. 

Figure 13. Two possible arrangements for the two Au atoms in a digold heteronuclear cluster. The trigonal planar Mj fragment can either be a discrete three-metal unit or one face of a larger polyhedron of transition metals M other than Group 11 metals, (a) Au arrangement A. (b) Au arrangement B (reprinted by permission of the Royal Society of Chemistry from ref. 23).

Heteronuclear Clusters Having Transition Metals and Metals of Group 14... 

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