Clusters interstitial atoms

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

Other interstitial atoms stabilizing such clusters are B, C, N and Examples... 

Hoffmann DK, Ruedenberg K, Verkade JG (1977) Molecuar Orbital Bonding Concepts in Polyatomic Molecules - A Novel Pictorial Approach. 33 57-96 Hogenkamp HPC, Sando GN (1974) The Enzymatic Reduction of Ribonucleotides. 20 23-58 Housecroft CE (1997) Clusters with Interstitial Atoms from the p-Block How Do Wade s Rules Handle Them 87 137-156 Huber R, see Romao MJ (1998) 90 69-96... 

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... 

This structure type evidently requires the simultaneous existence of two differently sized halide types, and thus exists only in mixed hahde systems. Recent investigations revealed this cubic stracture also for cluster phases with other interstitial atoms, i.e., the cation-free Si-centered [(Zr6Si)Cli2-xl2+x] [28]. 

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. 

Cluster unit with an interstitial atom in compounds such as ZrgCCl14 and ThgFeBr15... 

Caesium chloride is not body-centered cubic, but cubic primitive. A structure is body centered only if for every atom in the position x, y, z there is another symmetry-equivalent atom in the position x+ j,y+ j,z+ j in the unit cell. The atoms therefore must be of the same kind. It is unfortunate to call a cluster with an interstitial atom a centered cluster because this causes a confusion of the well-defined term centered with a rather blurred term. Do not say, the 04 tetrahedron of the sulfate ion is centered by the sulfur atom. 

Considerable attention has also been paid to modelling the thermodynamics of defects. This includes, for example, studies of the enthalpies of formation of vacancies or interstitial atoms and the association energies associated with the clustering of such defects. It is usually crucial to allow for the relaxation of the... 

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. 

Typical examples are carbon interstitial carbonyl clusters such as the octahedral Co6C(CO) -2, the trigonal prismatic Co6C(CO)i52- and its isoelectronic (mononegative) nitrogen interstitial Co6N(CO)15 or the icosahedral clusterNi12Ge(CO)222 with the interstitial atom (Ge or Sn) in the centre of aNi icosahedron. 

Figure 4.28. Schemes of octahedral cluster condensation, (a) Vertex-sharing, (b) edge-sharing and (c) face-sharing. In these examples octahedra centred by interstitial atoms are shown.

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. 

The last example of a cluster with interstitial atoms we will deal with is the hexaanion [Ni32(CO)36(C)6]6, whose molecular structure is shown in Figure 54.24... 

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. 

Clusters with Interstitial Atoms Available Internal Volume... 

The third DFT study [96] was motivated by the existence of the stable pentagonal antiprismatic cluster [93] Pd Biio" found in Bii4PdBri6. This cluster is an example of a cluster with an interstitial atom that obeys the Wade-Mingos rules [13-16]. Thus, the pentagonal antiprism is derived from an icosahedron by removal... 

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. 

In addition to the possibility of a cluster containing a single interstitial atom as discussed above, there is the possibility of having a smaller cluster inside a larger cluster. Such a cluster has a structure resembling a Russian matryoshka nesting doll. 

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

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