Tetrahedron complex Framework

The structures of silicates and related compounds contain anionic tetrahedron complexes of different configuration (single tetrahedra, pyrogroups, rings, chains, layers, frameworks etc.). Their comparative analysis considerably contributes to the sdentiiic ideas on the structures of crystalline materials in general. The interest in... 

Several new types of anionic tetrahedron complexes were discovered during the last years an Isolated complex, built of 48 Sl,0 tetrahedra, pyrogroups [P2O7] In a phosphate mineral, a 18-membered [SiOs] rings, a [PO3] chain with 16 tetrahedra In the period, an interrupted [81205] framework, etc. The new scientific ideas on the structures of the siiicates and their analogues stimulate their appiications in different technologies (ex. shape selective catalysis with zeolites). 

As you become more familiar with transition metal dusters (no nonmetals in the framework) you will come to associate doso structures with numbers of electrons. A trimer will have 48 electrons, a tetrahedron will have 60 electrons, a trigonal bipyramid will have 72 electrons, and an octahedron will have 86. Some care is required, however, as can be illustrated with Os,H2(CO),0. An electron count gives us 46 electrons rather than 48. If, however, we allow for one Os—Os double bond, the electron count is as expected. In accord with this expectation, one osmium-osmium bond is found to be shorter than the other two and the complex shows the reactivity expected for an unsaturated complex. 

In this complex, Zn2(H20)3(0H)4, each zinc ion is surrounded by four oxygen atoms (of OH"" or H O), exactly as in the hydrated zinc cation or the zincate anion the loss of water without decrease in coordination number is achieved by tire dual role played by on hydroxide oxygen atom, which serves as part of the coordination tetrahedron for both zinc ions. By continuing this process ail of the tetrahedra can be linked together into an infinite framework, in c hich each tetrahedron shares its corners with four other tetrahedra. This is the structure of the Zn(OH)o precipitate. 

There are numerous complex fluorides of alkali metals and metals of Groups The structures of RbBe2Fs and CsBe2Fs were mentioned in Chapter 3 as examples of the plane 6-gon net and the cubic (10,3) net. Each BeF4 tetrahedron shares 3 vertices to form respectively a layer or a 3D framework of composition Be2Fj. 

---