Interstitially stabilized clusters

In the 1980s, reports from Corbett s and Simon s laboratories presented a then-new chemistry based on interstitially stabilized clusters of zirconium and the rare-earth elements. " Coincident with those discoveries, molecular orbital schemes were offered to aid in understanding the electronic factors that exert influence on the stability of these cluster-based compounds. Nevertheless, direct physical measurements that supported the schemes were relatively scarce, and were almost entirely restricted to magnetic susceptibilities. " " ... 

A particularly interesting development concerns the solution chemistry of [Zr6Gi2Z] clusters. [100, 101] Alkali metal salts of the kind A Zr Q,2+ Z can be dissolved (e.g. in CH3CN in the presence of a cryptand) and the clusters are then isolated as R4N or (C H5)4P salts. This solution chemistry of interstitially stabilized clusters entered via solid state preparations promises great versatility and adds new facets to the traditional cluster chemistry of the neighboring elements Nb and Ih. 

K. Daub, New Interstitially Stabilized Cluster Complexes of Dysprosium, Hohnium and Erbium, Dr. rer. nat. Dissertation, Universitat zu Koln, 2009. 

The NH3 treatment for 120 min completed the clusterization of the Re species as shown in Figure 10.9. The CN of the Re-Re bonds was 5.210.3 (0.276 0.002 nm) (Table 10.7). A desorphon peak of N2 in TPD of the NHs-treated Re/HZSM-5 catalyst appeared at around 673 K, which indicates that the Re cluster possesses N atoms supplied by the NH3 treatment [73]. The amount of N2 evolved was 1.2 N2 per Reio. DFT modeling of the structure of the Re cluster based on the structural parameters obtained by FXAFS analysis revealed the formation of an N-interstitial Reio cluster in the HZSM-5 pore (structure is illustrated in Scheme 10.4) [73]. N atoms at the edge and hollow sites of the Re cluster never stabilized the Re cluster framework with the Re-Re bonds at 0.276 nm. Adsorption of nitrogen atoms on the exterior surface of the Re cluster also did not reproduce the Re-Re bond distances. 

A survey of some of the now considerable physical evidence in support of bonding schemes offered for interstitial stabilization of clusters is presented here. This evidence derives from optical, NMR, and Mossbauer spectroscopy, and measurements of magnetic and electrochemical properties. The gathering of much of this evidence has been made possible by successful efforts to excise centered hexazirconium clusters from solid state precursors and to study them as discrete complexes, [(ZrgZjXnLg]"" (Figure 1), both in solution and as discrete species in salts. 

Owing to the ample contributions of the groups of Corbett and Simon, one must have the impression that the so-called reduced halide chemistry of the rare-earth elements is that of interstitially stabilized [R Z] clusters. It is certainly dominated by these units when condensed metal clusters are considered. This chemistry has been reviewed several times (Corbett 1992, Simon 1995, Meyer 1988). 

Octahedral clusters derived from the 6-12 type are, as usual, interstitially stabilized. Two types of trans edge-connected octahedral chains are now known both with the dicarbon imit as the interstitial. The chains are, however, distorted in both cases ... 

The comproportionation route (Corbett, 1983a, 1991) is widely used and is very efficient when pure phases are desired, especially when the phase relationships are known or can be anticipated. It led to a great variety of reduced rare-earth halides, binary, ternary, and higher, simple, and complex salts, and such that incorporate metal clusters interstitially stabilized by a non-metal atom or by a (transition) metal atom, for example,... 

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

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... 

Fig. 2 (a) Snss cluster filled with four Ba cations in Ba]gNa204Sn3io (b) interstitially metal stabilized M Ei2 capped truncated tetrahedra in BaisNa204Sn3io (E = Sn, M = Na), M°NaioSni2 (E = Sn,... 

The calculation shows that the 2 2 2 and the 4 3 2 Willis clusters are essentially interstitial dimers stabilized by a coupled interstitial-lattice relaxation mechanism Oxidized (U ) cations localized at cation sites adjacent the cluster are also needed ... 

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. 

We have shown that A) interstitial hydride formation is observed only with partial occupation of the available holes, B) occupation of the interstitial position in isolated polyhedra is not observed, and C) occupation of all the holes in a close-packed lattice cancels metal-metal interactions. Therefore, it seems that interstitial hydrogen can be tolerated only in a fraction of the total number of holes, and with the weakening of metal-metal interactions. This behavior indicates strong competition between metal-metal and metal-hydrogen bonds, which is unique for hydrogen because interstitial carbon can stabilize some unusual arrangements in carbonyl carbide clusters (29, 30). 

There are as yet no thermochemical data to support the notion that the carbido carbon atom imparts additional stability to the cluster that surrounds it, but on the basis of observations made in the course of synthetic studies with these compounds, it is clear that the presence of the interstitial atom provides additional robustness to the molecule (86). Similar assertions have been made for clusters with other interstitial atoms (87). 

For the beautiful tetracapped octahedral Os cluster, [OsioC(CO)24]2- with an interstitial C atom in the octahedral core, shown in Figure 3.10, the predicted eve count is 14(6) + 2 + 4(12) = 134, which agrees with that of the observed stoichiometry. It s a little bit harder to count the sep but give it a try. Each tetrahedral cap consists of an Os(CO)3 fragment and the other six fragments are Os(CO)2 so we have (4x2 + 6x0 + 4 + 2)/2 = 7 appropriate for an octahedron. If you look ahead in Chapter 6 (Exercise 6.1), you will find that this trigonal bipyramidal ten-atom core can be excised from a cubic close-packed metal lattice (ABC layers). [OsioC(CO)24]2- can be considered a nano-sized metal particle stabilized by the ligands in the same manner as Ni atoms are stabilized when removed from Ni metal by CO as Ni(CO)4 in the Mond process. 

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