Clusters octahedral

Nitrogenase is another big target of cluster synthesis. The X-ray elucidation of the active center of the Fe-Mo cofactor and P-cluster (5) has accelerated the efforts to find rational preparative methods of trinuclear or cubane-type clusters containing molybdenum (6-9). The raft cores in these cluster complexes are one of the general structural units also in solid-state compounds, and the mutual relationships are very important. A number of review articles are now available on the syntheses, structures, and other properties of metal chalcogenide cluster compounds (6, 7,10-24). 

The structural relationship between the molecular and solid-state compounds has been a hot issue in inorganic chemistry for some time (25-27). The extrusion (or excision) from preformed solid-state cluster compounds is one of the major synthetic methods of the preparation of cluster complexes (26). Use of cluster complexes as precursors to solid-state cluster compounds is the reverse reaction of excision. Both reactions utilize the structural similarity of the metal cluster units. The basic cluster units of polyhedra (deltahedra) or raft structures are triangles, and both molecular and solid-state clusters with octahedral, tetrahedral, and rhomboidal cores have been reported. Similarity of other properties such as electronic structures based on the cluster units is also important. The present review is concerned with the syntheses and structures of the cluster complexes of the group 6 metals and with their relationships to solid-state chemistry. 

Similar reactions have been used for the synthesis of [CoeTealPEtalg] (30), [NigTe0(PEt3)8], and [Ni2oTei8(PEt3)i2] 31). 

The cluster cores MogEa (E = S, Se, Te) are very similar to those of molybdenum dihalides MoXg (X = Cl, Br, I), and their derivatives 

25°C for 6 days. The cluster complex is a little more soluble than the pyridine derivative. 

Nitrogenase is another big target of cluster synthesis. The X-ray elucidation of the active center of the Fe-Mo cofactor and P-cluster 

Similar reactions have been used for the synthesis of [Co6Te8(PEt3)6] 

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Figure 6.9 B12 Cubo-octahedral cluster as found in MBj2. This Bj2 cluster alternates with M atoms on an fee lattice as in NaCI, the Bj2 cluster replacing Cl.

Ooso (structure I) has a regular octahedral cluster of 6 B atoms surrounded by a larger octahedron of 6 radially... 

Examples of recently reported oxychlorides with one- or quasi-one-dimensional frameworks include A2Ti2Nb6Cli405 and Cs2Ti4Nbi5Cli80i5 in which the octahedral cluster unit has 5 or 6 oxygen ligands, respectively. 

The structural features of most niobium oxychlorides known to-date are summarized in Table 6.1. The use of a combination of chloride and oxide hgands leads to compounds with unique structure types [41], characterized by a remarkable variety of cluster frameworks, ranging from discrete cluster units to chains, layers, and three-dimensional nets, some topologies of which are unprecedented in compounds containing octahedral clusters. Most of the niobium oxychlorides known to date have anisotropic structures (the exceptions are Cs2LuNb,5Cli70 and PbLusNbsClisOg). 

However, certain polyhedra allow the inclusion of another electron pair without cleavage of any bond. This applies especially to octahedral clusters which should have 84 valence electrons according to equation (13.12), but they frequently have 86 electrons. The additional electron pair assumes a bonding action as a six-center bond inside the octahedron. An octahedral cluster with 86 valence electrons fulfills the Wade rule discussed below. 

KT1 does not have the NaTl structure because the K+ ions are too large to fit into the interstices of the diamond-like Tl- framework. It is a cluster compound K6T16 with distorted octahedral Tig- ions. A Tig- ion could be formulated as an electron precise octahedral cluster, with 24 skeleton electrons and four 2c2e bonds per octahedron vertex. The thallium atoms then would have no lone electron pairs, the outside of the octahedron would have nearly no valence electron density, and there would be no reason for the distortion of the octahedron. Taken as a closo cluster with one lone electron pair per T1 atom, it should have two more electrons. If we assume bonding as in the B6Hg- ion (Fig. 13.11), but occupy the t2g orbitals with only four instead of six electrons, we can understand the observed compression of the octahedra as a Jahn-Teller distortion. Clusters of this kind, that have less electrons than expected according to the Wade rules, are known with gallium, indium and thallium. They are called hypoelectronic clusters their skeleton electron numbers often are 2n or 2n — 4. 

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. 

Only large clusters usually adopt the face-centered cubic structure of metallic platinum. A novel cuboctahedral cluster [Pt15Hx(CO)8(PBut3)6] has been reported by Spencer et al.512 and the first octahedral cluster [Pt6(CO)6(/i-dppm)3]2+ was only reported recently.573... 

To conclude this section, we consider one more probable way for the stabilization of octahedral clusters, which, as was shown in the previous section, is characteristic of binuclear, trigonal-prismatic and, obviously, all types of technetium acido-clusters [10]. According to [12,77], all technetium clusters with an odd number of metallic electrons have shorter multiple M-M bonds than those in analogous structures, but with an even number of metallic electrons. In our opinion [10, 15], this effect of an odd number of metallic electrons is essentially analogous to the effect of the increase in M-M bond... 

Fig 13. Results of EHT calculations of hypothetical chloride technetium octahedral clusters with disturbed and undisturbed ligands surrounding [15]... 

Triacontanuclear Octahedral Clusters Stabilized by Bidentate Acetate Ligand... 

Fig. 6 a-c Coordination mode of the outer Cu2+ ions and the La3+ ions at the three vertices of the huge octahedral cluster, d-e Polymeric chain with LagCuM clusters as nodes viewed along [111] and [010] directions, respectively... 

Fig. 9 a b Coordination mode of the outer Cu2+ ions and the Nd3+ ions at the two vertices of the huge octahedral cluster Nd6Cu24 j for 9. Symmetry codes for A and B are y, z, x and 0.5 - z, 1 — x, —0.5 + y, respectively, c Each cluster nodes link to 12 other cluster units through 12 trans-Cu(pro)2 groups, d 3D open-framework of 9. e Face-centered cubic network... 

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