Monocapped prismatic structure

Structures of to date nonbiological Fe-S clusters (A) the thioprismane structure (B) basket-handle structure (C) monocapped prismatic structure (D) adamantane structure (E) circular FeigSso core unit. 

Polymetallic anions, prepared by dissolution of alloys of the alkali and post-transition metals in amine solvents (often with a complexand for the alkali metal cation), have been characterized in crystalline and solution phases. Clusters TlSng3, Ge92 (with 20 skeletal bonding electrons), Sn93- (21 skeletal e) and Bi95+ (22 skeletal e) possess a tricapped trigonal prismatic structure, symmetry D3A, with variations of dimensional detail which correlate with the electron population.291 292 This structure is a ctoso-deltahedron, and with 20 (2h + 2) skeletal electrons can be construed to be three-dimensionally aromatic.292 The 22e clusters M94 (M = Ge, Sn, Pb) occur as the C4v monocapped square antiprism, a nido polyhedron. 

Symmetries given in Table 2 are idealized (and refer to the symmetry of the cluster without taking into account heteroatoms) and many of the clusters show distortions from these ideal symmetries. For example, the group 14 clusters Geg, Sng and Pbg have 2n + 4 cluster electrons and exist as the expected nido clusters with C4V monocapped square antiprismatic geometry. On the other hand, the bismuth cluster Big +, which also has a 2n + 4 electron count, instead exists in the closo Z>3h form with a tricapped trigonal prismatic structure. In fact, all these 9-atom clusters show some distortions from the idealised symmetry, and the energy differences between different geometries are small.The arachno clusters Sb4, Bi4 , Se4 + and Te4 + (square planar), and Sbg + and Big + (square antiprismatic) have... 

Treatment of 4 with either PF3 or 13CO results in CO substitution believed to proceed via a dissociative process yielding Ti(CO)2(PF3)-(dmpe)2 (6) and Ti(13CO)3(dmpe)2. Structural characterization of 6 showed it also to be monomeric, but possessing a monocapped trigonal prismatic geometry. Complexes 4, 5, and 6 may be considered phosphine-substituted derivatives of the as yet unisolated Ti(CO)7, thus representing the only isolable titanium carbonyl complexes where the titanium atom is in the zero oxidation state. 

The type-B (monoclinic) structure is related to type-A, but is more complex as it contains three kinds of non-equivalent M atoms, some with octahedral and the remainder with monocapped trigonal prismatic coordination. In the latter type of coordination geometry, the capping O atom is appreciably more distant than those at the vertices of the prism. For example, in Sni203, the seventh atom is at 273 pm (mean) and the others are at 239 pm (mean). This type is favored by the middle lanthanides (Sm, Eu, Gd, Tb, and Dy). 

Structure type ErCuPbSa (Gulay et al., 2005h) (Figure 102, Table 52). SG Cmcm, Z = 4, fl = 0.3916, b = 1.2934, c = 1.0106 nm. The sulfur atoms create octahedral and monocapped trigonal prismatic arrangements... 

The molecular structure has been determined by X-ray diffraction, and the result is illustrated schematically in (25). Seven methyl groups surround the thorium atom in a distorted monocapped trigonal prismatic arrangement. Six of the methyl groups [Th-C = 2.667(8)-2.765(9)A ] are coordinated in a pair-wise manner to Li(TMEDA)+ cations, while the seventh [Th-C = 2.571(9) A] is not. 

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