Lead oxide, crystal structure

In other cases, if the fluorination process leads to cardinal changes in the crystal structure of the initial oxide compounds, new compounds with polar structures can be obtained. A demonstrative example of such materials are compounds that belongs to the system Na5(W3 xNbx)09..xF5+x and that have chiolite-type structures, when neither pure fluoride nor oxide display any ferroelectric properties [393 - 395]. 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Oxidative addition of H2 and Ph3SiH to IrBr(CO)(chiraphos), where chiraphos 2,S, 3,S -bis (diphenylphosphino)butane, leads to stable kinetic (115a,115b), followed by thermodynamic (116a,116b) diastereomers as shown in reaction Scheme 15.2 3 The crystal structure of cis-[IrH2(chiraphos)2]BF4 has been reported.204... 

One learns from these molecular complexes that equivalent synthons can lead to virtually identical crystal structures. Synthons in, V and VI are chemically and geometrically equivalent though they originate from different molecules, a nitrile, an N-oxide and a nitro compound. These three synthons are used in crystal design in almost the same way. So, different molecules may yield similar crystal structures if they are capable of forming equivalent synthons. This is a powerful concept because it establishes a many-to-one correspondence between molecular and crystal structures. 

---