Hexanuclear clusters cluster geometries

It seems natural to suppose that the tetragonal distortion of the tri-anion results from the Jahn-Teller effect. In order to study the problem more thoroughly we undertook recently the DFT calculations of this cluster as well as of several other hexanuclear rhenium chalcohalide clusters. The technical details of these calculations can be found in the original publication [8]. Here we only want to note that the introduction of relativistic corrections for Re atoms is crucial for the correct reproduction of the geometry of clusters. In our calculations, this was done by the zero order regular approximation (ZORA) Hamiltonian [9] within ADF 2000.02 package [10]. 

The electronic structures for many hexanuclear clusters adopting the basic geometries displayed in Figs. 1 and 2 have been investigated using a variety of computational methods (30, 35-47). We will therefore only briefly summarize the results obtained from DFT calculations performed—by methods described in detail elsewhere (30, 44, 45)—on a few representative examples. 

Scheme 12 shows the molecular geometries of the hexanuclear clusters discussed in this review. 

Hexanuclear Organometallic Clusters. Although the octahedral arrangement of metal atoms is the most representative one for hexanuclear cluster compounds, a number of other geometries have also been observed. The structural characteristics of a selected group of hexanuclear compounds are briefly described in Table 2.6. The different types of structures found for these clusters are illustrated schematically in Fig. 2.25. 

An interesting feature being observed in Table 2.6 and in Fig. 2.25 is that from the types of geometries adopted by hexanuclear clusters all, but the octahedron and the trigonal prism, derive from other more simple arrangements by sharing additional atoms to edges or faces of polyhedra of lower nuclearity. Thus, the bicapped tetrahedron also with twelve metal-metal bonds is an alternative to the octahedron. 

The thermolysis of [Os3H3(CO)9(BCO)] in toluene has been reinvestigated and a new hexanuclear cluster [Os6(p-H)3( i6-B)(CO)i6] isolated. The 86 c.v.e. cluster adopts a geometry based on a pentagonal bipyramid with a vertex removed and containing an interstitial boride, a geometry unlike that previously seen for 86 c.v.e. hexaosmium clusters, but duplicating instead that observed for the 84 c.v.e. Os6(CO)i8 with the additional interstitial boron. ... 

Three effects are observed in the reactions of Os6(CO)18 with diphenylacetylene and ethylene. There is modification of the metal framework, rupture of a C=C triple bond, and dimerization of ethylene (228-230). When the activated clusters Os6(CO)17(MeCN) (230) and Os6(CO)20(MeCN) (231) react with mono- and disubstituted alkynes, different penta- and hexanuclear framework geometries are obtained, and the alkyne-derived ligands adopt a range of coordination modes (Fig. 8). 

However this approach is not so easy to handle when the complexity of clusters increases for instance, Rh6(CO)i6, Co6(CO)i6, Ir6(CO)i6 share 86 electrons in the valence shell, which according to the noble gas rule formalism, would result in 11 metal-metal bonds. Since there is no simple geometry to accommodate 11 metal-metal bonds using 6 metal atoms, it is not surprising to observe an octahedral geometry with 12 metal-metal bonds. In this case the molecular clusters have an excess of two electrons which are located in bonding orbitals [2, 7]. If one increases the number of electrons in the hexanuclear frame, for example by introducing a carbon... 

The zigzag chain Ca cluster, the cyclic octanuclear Ca(II) complex and the hexanuclear Na(l) phosphate complex have unusual coordination geometry... 

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