MO6 octahedra

Mo-Mo distances in the triangles of Mo3 perpendicular to the threefold axis are almost constant, and Mo-Mo distances between the triangles become longer for the more oxidized clusters. Therefore, the distortion of the Mo6 octahedra is largest for the binary phases Mo6E8 (20 e). 

The change of chalcogen atoms from sulfur to selenium and tellurium diminishes the intertriangle distances and the distortion of Mo6 octahedra. 

In order to make these points clearer, interatomic distances in representative compounds are listed in Table VII. The distortion of Mo6 should be caused by electronic effects from the individual cluster units and/or steric effects due to the linkage of the clusters. At one time the electronic effects were considered more important based on the band calculations (58, 84). The strong eg Mo rf-bonding character of the conduction band causes contraction of the Mo octahedron, and the filling of the tlu(Oh) - eu (S6) and t2u (Oh) —> au (S6) band is related to the smaller intratriangle distance. Namely, the larger shift of electrons from Mx and chalcogen atoms to the molybdenum atoms is related to smaller distortion of the Mo6 octahedra. 

The hexanuclear Mo11 methoxides Na2Mo6(OMe)14 and Na2Mo6Cl8(OMe)6 contain central 24-electron octahedral Mo6 clusters.223 There are eight /t3-X ligands (X = Cl or OMe) and six terminal OMe ligands which radiate from the center of the Mo6 octahedra. 

The binary phase (Mo6S8) contains quite distorted Mo6 octahedra, elongated on a three-fold axis to become trigonal antiprisms. This can be attributed to the fact that there are only 20 cluster electrons. This number is insufficient to form a full set of Mo—Mo bonds, that is, to populate all the 12 bonding orbitals with the result that there is a distorted octahedron with somewhat longer Mo—Mo bonds compared to the 24-electron clusters. In the Chevrel phases the additional metal atoms add electrons to the Mo6 core and it becomes more regular in structure with shorter Mo—Mo bonds. 

Mo6 octahedron) the cluster is electron-precise, the valence band is fully occupied and the compounds are semiconductors, as, for example, (Mo4Ru2)Se8 (it has two Mo atoms substituted by Ru atoms in the cluster). In PbMo6Sg there are only 22 electrons per cluster the electron holes facilitate a better electrical conductivity below 14 K it becomes a superconductor. By incorporating other elements in the cluster and by the choice of the electron-donating element A, the number of electrons in the cluster can be varied within certain limits (19 to 24 electrons for the octahedral skeleton). With the lower electron numbers the weakened cluster bonds show up in trigonally elongated octahedra. 

The cores of the W6 clusters are very similar to those of the Mo6 analogues, and they are composed of the regular octahedra of six tungsten atoms capped by eight sulfur atoms (41, 43, 44). The W-W distances are almost the same, and difference in terminal ligands has little effect on the geometry (Table I). Only in the case of tert-butylpyridine complexes has a very small compression of the octahedron been observed in the c-axis direction (44). The bond order for the W—N bonds in the pyridine complexes is much weaker than that for the triethyl-phosphine analogue (43). 

The relationship between the trigonal distortion of the octahedra of Mo6 in the Chevrel phases and the oxidation states (number of cluster valence electrons) of Mo6Eg, the difference of chalcogens, the oxidation states, and the sizes of M have been repeatedly discussed (84). The main issues can be summarized as follows. 

The cubic AMO3 perovskite structure consists of an MO3 array of comer-shared MO6/2 octahedra with a large A cation at the body-center position. As is illustrated in tig. 1, this structure allows formation of the Ruddlesden-Popper (1957,1958) rock-salt/perovskite intergrowth structures MO (AMO3 ) . In all these structures, the mismatch between the equilibrium (A-O) and (M-O) bond lengths is given by the deviation from unity of the geometric tolerance factor... 

Heteropolyacids (HPA s) [27] and their salts are polyoxo compounds incorporating anions (heteropolyanions) having metal-oxygen octahedra (MO6) as the basic structural unit. They contain one or more heteroatoms (Si, Ge, P, As, etc.) that... 

Cluster complexes that obey Cotton s strict definition such as [ Nb6 Cli8]K4 and [ Mo6 C1h]Cs2 have 16 and 24 electrons, respectively, for bonding interactions within the octahedral niobium and molybdenum clusters. In a VB picture, these electrons are consumed for eight two-electron-three-center (2e-3c) or 12 2e-2c bonds, situated in the eight triangular faces and the 12 edges of the respective octahedra. 

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