Clusters rare earth metals

Oxidized, metastable clusters are obtained for both families via solution processes near room temperature and, in parallel, these contain 14-e niobium and tantalum members, but 12-e (and 13-e) zirconium chloride clusters. Rare-earth-metal clusters don t provide such persuasive evidence small main-group Z examples are rather scarce but similar (12-14-e for R7X-12Z), while iodides centered by large metals cover a sizable range (16-20), as was rationalized earlier (see 5). 

Exploratory solid state synthesis seems to be the only workable route to new phases because of a general inability to predict relative phase stabilities and thence structures or compositions , published in K4La6li40s A new Structure Type for Rare-Earth-Metal Cluster Compounds that Contain Discrete Tetrahedral K4l Units. S. Uma, J.D. Corbett, Inorg. Chem. 1999, 38, 3831-3835. 

Of course, valence electron concentration is not only related to the metal atoms but also to the number and valence of the ligands. Ligand deficiency creates vacant coordination sites at metal atoms and results in cluster condensation, which is the fusion of clusters via short M-M contacts into larger units ranging from zero- to three-dimensional. The chemistry of metal-rich halides of rare earth metals comprises both principles, incorporation of interstitial atoms and cluster condensation, with a vast number of examples [22, 23]. 

With Ceo, as well as the larger analogs, atoms can be introduced into the internal cavities to form main-group versions of transition-metal clusters containing interstitial atoms. Entities such as main-group atoms like N or a rare gas, molecules tike H2, rare-earth metals and others can be encapsulated. As with external metals, the maximum conductivity occurs for internal metals which are able to transfer three electrons to the radial tiu band of solid C6o-... 

Shima, T. and Hou, Z.M. (2006) Hydrogenation of carbon monoxide by tetranuclear rare earth metal poly-hydrido complexes. Selective formation of ethylene and isolation of well-defined polyoxo rare earth metal clusters. Journal of the American Chemical Society, 128, 8124. 

Many reduced (metal-rich) halides of group 4 (especially Zr) and the rare earth metals have been prepared. Most of these compounds are stabilized, by the metals forming Mg octahedral or other clusters having strong metal-metal bonds. The reactions to form these clusters are slow. Other nonmetals, especially oxygen, are undesirable impurities that may form more stable phases. Therefore the reactions are carried out with stoichiometric mixtures of pure halide and metal in degassed Ta or Nb tubes that have been loaded in an inert atmosphere and arc-welded shut. The welded ampule is then sealed in a protective quartz tube and heated to a temperature adequate to achieve a reaction in a week or more ( >600°C) . Yields may be small in some cases individual single crystals are produced as evidence of synthesis of a new material with metal-metal bonds. 

Many heterogeneously catalyzed reactions are performed using metal particles dispersed on oxide supports, commonly on alumina or silica but also many others, including transition metal and rare earth metal oxides. Metal clusters have interesting reactions with or at the surfaces of such oxide materials. It has recently been shown that the oxide surface is actually a good medium for the synthesis of... 

Recently, Hoffmann et al. (1989) reported a series of the ErgRhjCij-type compounds present in the R-Rh-C system. These compounds RgRhjCjj (R=Y, Gd-Tm) are thermodynamically stable at 900°C and have a monoclinic structure, space group C2/m, with Z — 2 formula units per unit cell. The lattice parameters of these compounds have been measured by single-crystal X-ray diffraction. The structure contains a finite chain-like centrosymmetric polyanion [Rh5Ci2] with two Rh-Rh bonds (2.708 A) and six pairs of carbon atoms. The shortest distances between adjacent Rh5Ci2 clusters are the Rh-C distances of 2.94 A and the Rh-Rh distances of 3.27 A, and thus the Rh5Ci2 units may be treated as isolated from each other. This structure is characterized by three different kinds of C2 pairs. The C-C bond distances of 1.27, 1.32 and 1.33 A are between those of a triple bond (1.20 A) and a double bond (1.34 A) in hydrocarbons, and are the shortest found so far in ternary carbides of the rare earth metals with transition metals. 

Crystallographic data for rare earth metal compounds with discrete clusters. 

It has been mentioned in the introduction that the condensed cluster halides of the rare earth metals based on the MgXi2-type cluster with an interstitial atom (or molecular unit) generally exhibit a defect rocksalt structure. Figure 10 provides clear evidence for this remark. The NaCl subcell in the structure of GdijInCg, marked by strong streaks is only weakly distorted (a = 6.07, b = 6.10, c = 5.92 A, a = y = 90°, p = 91°) by the ordering of I atoms and Cj units and occupation of all voids around the C2 units by Gd atoms. 

The structure of Gd2Cl3 contains linear chains of trans-edge-sharing metal octahedra. However, there are distinct differences which require a separate discussion of this structure. First, the compounds R2X3 (together with SC7CI10) seem to be the only binary metal-rich (X/R < 2) halides of the rare earth metals. Second, the chains in the structure are formally derived from the Mg Xg-type cluster. Last, but not least, incorporation of interstitial atoms leads to a number of phases, whose structures are closely related to that of Gd2 CI3. The structural family is summarized in table 4. 

As illustrated in the previous section, the metal-rich rare earth metal halides and their interstitial derivatives provide a vast collection of compounds that transcends the structural chemistry of both molecules and extended solids. On the one hand, these substances can be considered as connected or condensed clusters of the MgXi2-or MgXg-type, which may contain interstitial species. On the other hand, many of them can be derived from the structures of simple salts, e.g. NaCl or La20jS. 

The first examples of zirconium and rare-earth-metal (R) clusters were largely synthetic mysteries until it was established that the clusters in these scarce but well-formed crystals were all centered and stabilized by foreign adventitious atoms, largely the ubiquitous C, N, or H. Inclusion of the correct ingredient then gave high yields of the same phases. Subsequent exploration of Z prospects over much of the... 

The change to zirconium iodide clusters expands the possible interstitials with Al, Si, P and Ge. Surprisingly, most of the 3d elements Cr - Ni can be incorporated into either chloride or iodide clusters. Extension of the studies to rare-earth-metal... 

A remarkable stability and interstitial diversity occurs in the rare-earth-metal cluster iodides which encapsulate not just 3d but 4d and 5d metals as well. As summarized by Payne Corbett (1990), any of the elements... 

Hughbanks, T. Corbett, J. D. (1988). Rare-Earth-Metal Iodide Clusters Centered by Transition Metals Synthesis, Structure, and Bonding of R7I12M Compounds (R = Sc, Y, Pr, Gd M = Mn, Fe, Co, Ni), Inorg. Chem. 27, 2022-2026. 

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