Clusters Containing Interstitial Atoms

Interstitial atoms within [MgXg] type clusters have only been observed with NbgliiH, [33] CsNbglnH, [34] and more recently, Nbgl9SH. [36] There is no indication for interstitial atoms in the clusters of the Chevrel phases, despite the 

Ihble 5-3. [MgMg] and [MgXu] halide clusters having interstitial atoms, their interconnection patterns and their corresponding empty cluster structure types. Thken from [6, 10, 13] and additional references in the Ihble. 

5 Discrete and Condensed Transition Metal Clusters in Solids 

NbfilnH and CsNbelnH are formed by heating the binary and ternary compounds respectively, in the presence of H2. Hence, the [Nb Ig] cluster is known both with and without an interstitial atom. The hydrogen is only lost under vacuum at such a high temperature that further decomposition of the residue occurs through the formation of gaseous Nbl. If this decomposition reaction is suppressed by the inclusion of the sample in an Nb ampoule, which is permeable to H2, the hydrogenation reaction can be performed reversibly. 

Moving from Nb to its more electron defident neighbor Zr creates a qualitatively new situation. The d elelctron concentration is too low to allow for the formation of the [MfiXg] duster. The number of electrons is even too low for the [M6X12] type duster. Unlike Nbjn, where the duster centers can be reversibly occujned by impurity atoms, interstitial atoms are a necessary requirement for the formation of the [ZreX,2] unit. All the [ZreQ,2] duster compounds which 

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In Figure 3.7, a selection of metal clusters containing interstitial atoms is shown. Examples with interstitial H atoms as well as transition-metal atoms are also known. Addition of an interstitial metal atom is the first step towards extended metal structures. The term interstitial derives from its use in solid-state chemistry where atoms are found in the interstices of metal lattices, e.g., the tetrahedral or octahedral... 

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-... 

Examples of nickel complexes are rare and are confined to three reported nickel carbonyl clusters containing interstitial germanium or tin atoms (90). The reaction between [Ni6(CO)l2]2 and GeCl4 affords two species, [Ni10Ge(CO)2o]2 , 79, and [Ni12Ge(CO)22]2, 80. The former structure is based on a pentagonal antiprismatic framework of nickel atoms with an... 

There is much interest in transition-metal carbonyl clusters containing interstitial (or semi-interstitial) atoms in view of the fact that insertion of the encapsulated atom inside the metallic cage increases the number of valence electrons but leaves the molecular geometry essentially unperturbed. The clusters are generally anionic, and the most common interstitial heteroatoms are carbon, nitrogen, and phosphorus. Some representative examples are displayed in Fig. 19.4.3. 

Fig. 12. Some cobalt and rhodium clusters containing interstitial C2 ligands or two C atoms (a) [CoXC XCO) ]2 (380) ... 

Clusters containing interstitial group V atoms are synthesized at high temperatures under 2-4 x 10" N/m CO/H2 ... 

A potential problem with these stabilizing bridging ligands is that they are often bulky and some catalytic reactions are then blocked because larger reagents cannot find access to the metal centers. An elegant solution is to use an interstitial atom which can bind to six or more transition metal atoms. This is an important component of Section 2.11, in which clusters containing interstitial carbide are used as examples. 

Large Metal Clusters containing Interstitial Carbide 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]

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. 

X-ray structure of the mesitylene derivative was reported shortly afterward.11 This represented the second structurally characterized cluster containing an interstitial atom [the structure of FesC(CO)i5 having already been established]12 and the first example of a cluster with a completely encapsulated carbide atom. At the time that the synthesis of 2 was first reported, another paper described the synthesis of a cluster also obtained from 3 when heated to 150°C in either benzene or cyclohexane. Based on an estimation of the mass of this compound from a differential vapor pressure measurement, the authors suggested that this compound corresponded to Ru6(CO)18.13 It was subsequently noted from a comparison of vco IR data and a structural determination that this compound was in fact 2. 

The structure of 2 was first determined in 1969 from the visual estimation of diffraction intensities obtained from Weissenburg rotational photographic data.26 This study established that the octahedral cluster contained an interstitial C atom and also showed the distribution of ligands around... 

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