Isostructural Condensed Anions

FIGURE 5.32 Crystal structure of Pb2Cs3(P40[2)(P03)3. Showing independent P40j2 rings and (P03) chains. Filled circles = Pb, open circles = Cs. (Adapted from M.T. Averbuch-Pouchot, A. Durif, Ann. Rev. Mat. Sci., 21, 65, 1991.) 

More complex isostructural arrangements such as trimeta rings (5.194a) and the whole series of hexameta rings (5.194b) are now known to exist, in the solid state at least. Sheets and three-dimensional analogue structures also exist. 

In the case of condensed anions where X = Al, Si or Ge, each XO4 tetrahedron can share up to a maximum of all four of its comers with neighbouring tetrahedra. 

When X = P or As, a maximum of only three comers can be shared with similar tetrahedra and this places some restriction on the number of possible arrangements compared to those possible in the first group. When X = S or Cr, only two comers can be shared with similar tetrahedra, and only chains or simple rings are feasible. 

Silicate minerals are long known for the enormous variety of structural arrangements they exhibit. This is helped by the hydrolytic stability of the Si-O-Si linkage compared to most other X-O-X-type linkages and is presumably why condensed species of other than silicates are of comparatively rare occurrence in nature. 

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The reaction of Mo(CO)6 and acetic acid is still the most convenient preparative route known for condensing two monomeric units into a compound containing a quadruple bond. In fact, facile conversion of Mo2(02CCH3)4 to chloro species occurs in concentrated hydrochloric acid to form the Mo2 Clj ion (36), which is isostructural with the Re2Clg ion. The eclipsed configuration of Mo2Clg" and the molybdenum atom separation of 2.14 A are entirely consistent with a quadruple bond in the anion as expected (33). 

Larger binary chalcogeno phosphate anions are almost exclusively based on similar tetrahedral [PE4] phosphorus (V) building blocks, which can condense to form rings or clusters. This is well illustrated by the isostructural [1,4-P2Eg] anions (E = Se ) (3) which contain chair-like six membered P2E4... 

Aza phosphorus compounds were first studied in the nineteenth century, notably by Liebig and later by Stokes, who characterised the imidopolyphosphates and the multiply-bonded polyphosphazenes. Since that time azaphosphorus chanistry has grown considerably, culminating in the recent discovery that phosphorus and nitrogen can form anions which are isostructural with the condensed phosphates. 

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