Oxides of Ge, Sn, and Pb

Dark-brown crystalline GeO is obtained when germanium powder and Ge02 are heated or Ge(OH)2 is dehydrated, but this compound is not well characterized. 

Crystal structure of SnO (and PbO) (a)viewed from the side of a part of one layer, (b) the arrangement of bonds and lone pair at a metal atom. 

The sesquioxide Pb2C 3 is a black monoclinic crystal, which consists of PbIVC 6 octahedra (PbIV-0 218 pm, mean) and very irregular six-coordinate Pbn06 units (Pbn-0 231, 243, 244, 264, 291 and 300 pm). The Pb11 atoms are situated between layers of distorted PbIVC 6 octahedra. 

Crystal structure of Pb304 (large circles represent Pb atom, white PbIV and shaded Pb11 small circles represent O atom). 

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Another study on the electrosynthesis of (alkyl) M compounds (M = Ge, Pb, Sn n = 2, 4) provides illustrative examples37. Sacrificial cathodes of Cd, Zn and Mg were used to produce the corresponding metal alkyls which are subsequently oxidized on sacrificial anodes of Ge, Sn and Pb. The cells are of very simple construction, with the proper metal electrodes. Diethylcadmium is utilized in this way for the manufacture of tetraethyllead from lead acetate and triethylaluminum in the following reaction sequence ... 

A second important characteristic of the mass spectra of Ge-, Sn- and Pb-containing compounds is the tendency to display abundant peaks corresponding to the M(IV) and M(II) oxidation states of these elements. 

Trends in ionization energies have been illustrated previously but other examples are of particular interest. The halides of Ge, Sn and Pb can be singled out because they have been extensively characterized for both the (IV) and (II) oxidation states. As in the case of C and Si, the tetrafluoro derivatives display very high ionization energies, and the observed trends in Ge demonstrate the decrease in IE in going from F to I, and the decrease in going from the (IV) to the (II) oxidation state, i.e. 

Herrmann and coworkers183 reported a series of Cp-manganese carbonyl complexes which bind Ge, Sn and Pb as central atoms linearly coordinated in clusters, to two Mn atoms in one series and trigonal-planar coordinated to three Mn atoms in another series 8 and 9. The group 14 atoms are double-bonded to two Mn atoms in these compounds, or carry one double bond and two single bonds to three Mn atoms. Potentiometric measurements of these compounds show irreversible reductions and oxidation by CV. No products could be isolated from either reduction or oxidation. The exceptionally high oxidation potential of (/i-Pb) r/ -CsHs )Mn(CO)2]2 as compared to the apparently similar Sn compound is noteworthy (Table 15). 

Most decaphenylmetailocenes also exhibit an unsurpassed thermal stability for sandwich complexes. The decaphenyl Ge, Sn, and Pb derivatives do not decompose until above 350°C (under nitrogen) (39), in contrast with around 100°C for the decabenzyl analogs (106). For sym-penta- and decaphenylferrocene and -ruthenocene an extraordinary degree of thermal and oxidative stability is noted (40) they are unchanged in air ( ) at 315°C and volatilize only at 250-300° C in the mass spectrometer. 

The elements Si, Ge, Sn, and Pb all exhibit the oxidation states of +2 and +4. However, the +2 state for Si is rare. One reason is that SiO is not stable and the halides SiF2 and SiCl2 are polymeric solids. A few Ge(II) compounds are known (e.g., GeO, GeS, and Gel2). The +2 and +4 oxidation states are about equally common for Sn and Pb. For example, Sn02 is the most common ore of Sn, and numerous compounds contain Sn(II) (stannous compounds). As we will see later, there are also numerous common compounds of both Pb(II) and Pb(IV). 

Many ionic compounds of AX2 stoichiometry possess the CaF2 (fluorite), or Na20 (antifluorite) structures shown in Figure 3.15. Fluorite is similar to CsCl, but with every other eight coordinate cation removed. Each fluoride anion is tetrahedrally coordinated by calcium ions. This structure is adopted by several fluorides and oxides. In the antifluorite structure, the coordination numbers are the inverse. Most oxides and other chalcogenides of the alkali metals (e.g. Na2Se, K2Se) possess the antifluorite structure, but so do some more covalent compounds, such as the silicides of Mg, Ge, Sn, and Pb. 

The He(l) photoelectron spectrum shows the first ionization potential at 9.4 eV assigned to the Ge-C (t2) orbital a broad envelope of ionizations follows between 11.5 and 13.5 eV as represented by a figure comparing Ge(C2H5)4 with Ge(C2H5)nH4 n compounds [26]. A linear relationship has been found between the total ionization cross-section and the metal polarizability for M(C2H5)4 compounds where M = Si, Ge, Sn, and Pb [20]. For various MR4 compounds (M = Ge, Sn, and Pb) a relationship also exists between the first ionization potential and the rate of homogeneous electron transfer to an oxidant [27]. 

The thermal and chemical stability of C—M bonds (M = Si, Ge, Sn, Pb), and therefore of all organic compounds of the silicon subgroup elements, decreases, both in homolytic and in heterolytic processes, when the atomic number of the M element increases. For example, the thermal stability of tetraalkyl derivatives R4M diminishes essentially when M is changed consecutively from Si to Pb151 l52. The ease of oxidation of R4M compounds and the ease of cleavage of C—M bonds by halogens, protic and aprotic acids, etc.,... 

The dihalides of Si and Ge are polymeric solids that are relatively unimportant compared to those of Sn and Pb. The latter elements are metallic in character and have well-defined +2 oxidation states. Physical data for the divalent halides are shown in Table 11.1. The compounds of Si(II) are relatively unstable because the reaction... 

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