Trigonal antiprism structures

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. 

There are two allotropic forms of polonium, both primitive, with one atom per unit cell. a-Po is simple cubic (3PO) and each Po atom has six nearest neighbors (3.366 A) at the vertices of a regular octahedron. The simple cubic structure is the same as that of NaCl (Figure 3.6) with all atoms identical. The structure of (3-Po is a primitive rhombohedral lattice. Each Po atom has six nearest neighbors forming a slightly flattened trigonal antiprism (or a flattened octahedron). 

Cages with nine vertices are rare. Representative ones are Bi + (in Bi24Cl26), B9Hi , and B7QH9, all of which have the tricapped trigonal prism structure (Fig. 1-5), and Sn9, which is a square antiprism capped on one square face. 

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. 

The crystal and molecular structure of tetrachlorobis(triphenylphosphine oxide) uranium(vi) has been determined from three-dimensional AT-ray diffraction data. The complex was found to adopt a distorted octahedral configuration with the neutral ligands co-ordinated in the cis positions. The single-crystal study of chlorohexakis-(trimethylphosphine oxide)uranium(iv) trichloride showed the uranium atom to be co-ordinated to one chlorine and six oxygen atoms. The co-ordination polyhedron was described in terms of a distorted monocapped trigonal antiprism or a distorted monocapped octahedron. 

Reaction of Ybl2 with NaTp gives the highly insoluble homoleptic [Yb(Tp )2], structurally characterized as a trigonal antiprism.615 Its reaction with nitric oxide gave the seven-coordinate [Yb(N02)(Tp )2].626 [Yb(Ti2-02)(Tp)2] and [Yb(ri2-02)(Tp )2] have been prepared by the reactions of [Yb(Tp)2] and [Yb(Tp )2], respectively, with dioxygen in toluene at —75°C.617... 

The XYg-type molecule may take the form of (I) a cube (O/,), (II) an archimedean antiprism (D4J), (III) a dodecahedron (D2j), or (IV) a face-centered trigonal prism (C21 ). Although XYg molecules are rare. X-ray analysis indicates that [TaFg] and [CrOg] ions take structures II and III, respectively [1339,1340]. The infrared and Raman spectra of crystalline Na3[TaFg] are in accord with structure II, proposed by X-ray analysis [1341]. However, the vibrational spectra of the [IFg] and [TeFg] ions support the archimedean antiprism structures [1332] shown below ... 

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