Comer sharing

Tin(II) chlorides are similarly complex (Fig. 10.5). In the gas phase, SnCh forms bent molecules, but the crystalline material (mp 246°, bp 623°) has a layer structure with chains of comer-shared trigonal pyramidal SnClsl groups. The dihydrate also has a 3-coordinated structure with only I of the H2O molecules directly bonded to the Sn (Fig. I0.5c) the neutral aquo complexes are arranged in double layers with the second H2O molecules interleaved between them to form a two-dimensional H-bonded network... 

A rather different structure-motif is observed in the chain polyphosphates these feature comer-shared PO4 tetrahedra as in the polyphosphoric acids (p. 522). The general formula for such anions is [Pn03 4.i] + , of which the diphosphates, P2O7 ", and tripolyphosphates, PsOjo , constitute the first two members. Chain polyphosphates have been isolated with n up to 10 and with n infinite , but those of intermediate chain length (10 < n < 50) can only be obtained as glassy or amorphous mixtures. As the chain length increases, the ratio (3n + l)/n approaches 3.00 and the formula approaches that of the polymetaphosphates [P03 ]oo-... 

Pairs of comer-shared SO4 tetrahedra are found in the disulfate.s, (S-O -S 124°,... 

The transition-metal monopnictides MPn with the MnP-type structure discussed above contain strong M-M and weak Pn-Pn bonds. Compounds richer in Pn can also be examined by XPS, such as the binary skutterudites MPn , (M = Co, Rh, Ir Pn = P, As, Sb), which contain strong Pn-Pn bonds but no M-M bonds [79,80], The cubic crystal structure consists of a network of comer-sharing M-centred octa-hedra, which are tilted to form nearly square Pnn rings creating large dodecahedral voids [81]. These voids can be filled with rare-earth atoms to form ternary variants REM Pnn (RE = rare earth M = Fe, Ru, Os Pn = P, As, Sb) (Fig. 26) [81,82], the antimonides being of interest as thermoelectric materials [83]. 

RuF3 can be made by iodine reduction of RuF5. It is obtained as a dark brown powder that contains comer-shared RuF6 octahedra [15]. RuC13 exists in a- and /3-phases ... 

Bismuth Tungstates. A family of structures recently identified (23) to occur in the Bi/W/0 system is illustrated schematically in Figure 5. The individual members are made of interleaved 2 3 an< 3 ayer> t ie latter consisting of comer-sharing WO, octahedra. The resulting homologous series has a formula n 3n+3 ... 

Figure 4.20 (a) Idealized cubic structure of WO3 shown as comer-shared octahedra (shaded... 

The CS planes are associated with oxygen loss and serve to reduce the composition of the parent comer-shared octahedral structure from NbC>3 to Nb2Os, while retaining octahedral coordination for the Nb5+ cations. The centers of the blocks have a composition Nb03 and the block boundaries have a composition Nb02. 

Zeolites are crystalline alumino-silicate materials with Mj [Alj Si, 62] X n H2O, where M is a cation of valence m. Their frameworks are built from comer-shared TO -tetrahedra (T stands for Si, Al, P, Ga, Ge, etc.) to produce channels or cavities (spaces) of molecular dimensions. Based on the characteristic stmctures, zeolites have attracted much attention not only for catalytic applications but also as containers in which materials are confined. It is becoming more and more important to understand the fine stmctures of zeolites in order to synthesize high quality or novel zeolites not only for producing better catalysts but also for better containers of confined materials. 

KsMsSisOis (M =Ta and Nb) [119] possesses a unique pillared structure, in which three comer sharing MOe chains are bridged by ditetrahedral SisOy units. No photocatalytic activity for water splitting was observed from KsNbsSisOis, 4.1 eV bandgap, with or without cocatalyst addition. The M-O-M bond angles in... 

The block iron silicate Si2 xFex Fe3ijFe 05(0H)4, cronstedtite, is representative of several mixed-valent minerals. Its structure consists of alternate layers of comer-shared tetrahedra and edge-shared octahedra. The tetrahedra Si " and Fe ions are coordinated by four oxide ions, the octahedral Fe " and Fe " ions share three common oxide ions with tetrahedral-site layers on one side and on the other are coordinated by three hydroxyl ions that hydrogen bond to the next tetrahedral-site layer. Mossbauer data at 35 K are able to resolve distinguishable Fe " and Fe " ions on the octahedral sites but in the range 300 < T < 500 K a rapid (t), < 10 s) charge transfer was found ... 

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