Metal cluster interstitial atoms

The comproportionation route is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known. It led to a great variety of reduced rare earth haUdes, binary, ternary, and higher, simple and complex salts and such that incorporate metal clusters interstitially stabiUzed by a nonmetal atom or by a (transition) metal atom for example,... 

Table 25.9 Some metal carbonyl clusters with interstitial atoms...

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

As holds for other cluster systems, certain magic cluster electron counts exist, which indicates for a certain cluster-halide ratio and interstitial present the filling of all bonding molecular orbitals and therefore the thermodynamically most stable situation. For main group interstitial atoms these are 14 cluster-based electrons whereas for transition-metal interstitials the magic number is 18 [1, 10-12]. All of these phases are synthesized by high-temperature solid-state chemical methods. A remarkable variety of different structure types has been... 

Clusters derived from metals which have only a few valence electrons can relieve their electron deficit by incorporating atoms inside. This is an option especially for octahedral clusters which are able to enclose a binding electron pair anyway. The interstitial atom usually contributes all of its valence electrons to the electron balance. Nonmetal atoms such as H, B, C, N, and Si as well as metal atoms such as Be, Al, Mn, Fe, Co, and Ir have been found as interstitial atoms. 

Even the extremely electron-deficient alkali metals can form clusters when interstitial atoms contribute to their stabilization. Compounds of this kind are the alkali metal suboxides such as Rb902 it has two octahedra sharing a common face, and each is occupied by one O atom (Fig. 13.16). Flowever, the electron deficiency is so severe that metallic bonding is needed between the clusters. In a way, these compounds are metals, but not with single metal ions as in the pure metal Rb+e-, but with a constitution [Rb902]5+(e )5, essentially with ionic bonding in the cluster. 

The chemistry of octahedral metal clusters culminates in the center of the Periodic Table with the heavy transition metals Nb, Ta, Mo, W, and Re. There is a plethora of clusters where the M-M bonded core is surrounded (and shielded) by non-metal ligands. When moving to the left of the Periodic Table the decrease in valence electron concentration calls for a stabilization through incorporation of interstitial atoms into the cluster core. Actually, the stabilization of the cluster occurs... 

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]. 

Interstitial atoms in clusters. As the size of clusters increases (and also that of their central cavity) the insertion of atoms becomes easier and easier. In particular for 12-atom clusters having a cubo-octahedral structure, the insertion of an atom having the same radius as that of the peripheral atoms is possible. Notice that this arrangement can be compared with those of the metallic cubic and hexagonal, close-packed structures. 

In the field of carbonyl clusters it is not rare to find compounds in which atoms such as C, N and P become trapped in interstitial or semi-interstitial positions inside the metal cage. We will briefly consider this type of compound in order of increasing encapsulation of the interstitial atom. 

Palladium metallic clusters have been prepared at room temperature by sonochemical reduction of Pd(OAc)2 and a surfactant, myristyltrimethylammonium bromide, in THE or MeOH [160[. It is noteworthy that nanosized amorphous Pd is obtained in THE, but in a crystalline form in MeOH. In this solvent, and in higher homologous alcohols, sonolysis of tetrachloropalladate(II) leads to Pd nanoclusters in which carbon atoms, formed by complete decomposition of the solvent, can diffuse. This results in an interstitial solid having the formula PdQ (0 < x < 0.15) [161]. Noble metal nanoparticles of Au, Pd, and Ag are obtained by sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilise the naked col-... 

Abstract This chapter reviews the methods that are useful for understanding the structure and bonding in Zintl ions and related bare post-transition element clusters in approximate historical order. After briefly discussing the Zintl-Klemm model the Wade-Mingos rules and related ideas are discussed. The chapter concludes with a discussion of the jellium model and special methods pertaining to bare metal clusters with interstitial atoms. 

This consideration also applies to 8-vertex clusters with interstitial atoms. The most spherical 8-vertex deltahedron, namely the bisdisphenoid (Eig. 1), appears to have too small a cavity for an interstitial transition metal. Plowever, the square antiprism has two fewer edges and can be partially flattened to make a puckered eight-membered ring, which can accommodate a transition metal in the center (Pig. 8). Known clusters of this type include M E8" (M = Cr [98], Mo [98], Nb [99] E = As, Sb n = 2,3 for Cr and Mo = 3 for Nb). The transition metal in such structures can be considered to be eight-coordinate with flattened square antiprismatic coordination. The Eg ring (E = As, Sb) can be considered formally to be an octaanion, isoelectronic with the common form of elemental sulfur, Sg. Thus in M Eg (M = Cr, Mo E = As, Sb), the central transition metal has the formal oxidation state of +6. Similarly in Nb Eg , the central niobium atom has its d formal oxidation state of +5. 

The chemistry of bare metal clusters with interstitial atoms is clearly the new frontier as indicated by two recent serendipitous discoveries ... 

I contrast to cages like B12X12 or Cgg, clusters with similar sizes consisting of metal atoms are not stable if they are hollow the bonds at their surfaces are too weak. However, Urey can be stabilized by interstitial atoms, even if the interstitial atoms do not contribute with their electrons. Such clusters are called endohedral. Examples are the icosahedral clusters [Pt Pbi2] and [Cd Tli2] 2 with a Tl cage. The atom mentioned before the sign is the enclosed, endohedral atom. These clusters fulfill the Wade rule for closo clusters if one assumes a neutral Pt atom and a Cd ion. 

Very few examples are known, apart from solid state structures, where the sulfide acts as a bridge to more than four metal atoms. Two Rh clusters [Rh17(S)2(CO)33]3 and [Rh,0S(CO)22]2 have been reported in the literature77,78 where the sulfide is found interstitially in the center of the metal cluster and has contacts to nine and eight Rh atoms, respectively. In the former example an almost linear S—Rh—S unit (with d(Rh—S) as short as 2.16 A) is encapsulated in an Rh16 cluster, with four Rh—S contacts at about 2.33 A and four more at about 2.8 A.77 Other related systems are, for example, the [M6S,7]4 ions (13 M = Nb, Ta) which have been recently prepared by Holm and co-workers.80 Here, among other types of sulfide coordination, a /vS has been found in the base of the bell-shaped ions. 

Figure 2. Idealized structures of a trigonal prismatic ML6 complex (a) and a centered edge-bridged trigonal-prismatic M6ZX12L6 cluster (b). Black, small shaded, large shaded, and white spheres represent metal atoms M, interstitial atom Z, inner ligands X, and terminal ligands L, respectively. Each structure conforms to I) i, symmetry.

Table IV lists specific examples of compounds related through this form of dimensional reduction, By far, the majority of these are zirconium chloride and iodide phases, in which case lower main group and even transition metals have been found to incorporate as interstitial atoms. A few analogues are known with hafnium (135), and very recently it has been shown that nitrogen can be substituted for carbon in tungsten chloride clusters adopting the centered trigonal-prismatic geometry (see Fig. 2) (32). It is hoped that a variability similar to that exposed for the octahedral zirconium clusters will be attainable for such trigonal-prismatic cluster phases.

Interstitial H atoms have also been found in larger metal clusters. In a neutron diffraction study of [HNii2(CO)2i]3 and [H2Ni12(CO)21]2, Williams, Dahl, Chini and co-workers found H atoms lodged in octahedral cavities of these multi-hole... 

Finally, eight-coordinate H atoms are theoretically possible since a cubic cavity is inherently much larger than an octahedral one. Although several cubic metal clusters are known258), none of them is, as far as we know, suspected to contain interstitial H atoms. 

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