Subvalent halides

Subvalent halides of heavier group 14 elements MX2 (M = Ge, Sn, and Pb X = F, Cl, Br, and I) and their complexes exhibit the following structural features and properties. 

The structural chemistry of MX2 is complex, partly because of the stereochemical activity (or nonactivity) of the lone pair of electrons and partly because of 

In the solid state, GeF2 has a unique structure in which trigonal pyramidal GeF3 units share two F atoms to form an infinite spiral chain. Reaction between GeFi and F gives GeF3, which is a trigonal pyramidal ion. 

The structure of SnCl2 is shown in Fig. 14.5.1(a). In the crystalline state it has a layer structure with chains of corner-shared, trigonal-pyramidal SnCb units. The commercially available solid hydrate SnCl2-2H20 also has a puckered-layer structure. 

The preference for the +2 over +4 oxidation state increases down the group, the change being due to relativistic effects that make an important contribution to the inert pair. The inert pair concept holds only for the lead ion Pb2+(aq), which could have a 6s2 configuration. In more covalent Pb11 compounds and most Sn11 compounds there are stereochemically active lone pairs. In some MX2 (M = Ge or Sn) compounds Ge and Sn can act as donor ligands. 

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It is known in inorganic chemistry that all refractory and rare earth elements tend to form subvalent halides from their iodides, bromides and chlorides. 

In addition to the above normal valence materials, recent research has uncovered a number of mixed valence and apparent subvalent halides. For example, discreet compounds with halogen to rare earth ratios of2 200, 2 167, and 2 140 have been observed for chlorides and bromides of Dy, Yb, Ho, and Sm. These are really mixed valence 3-E, 2-E compounds with formulas such as DysCln, Yb6Cli3, and SmnBr24. The complex structures of these materials have been the subject of considerable research. 

We were quite optimistic in the beginning as the second reduction process corresponds to the formation of a black deposit which was potentially the first electrochemical route to make thick tantalum layers. After having washed off all ionic liquid from the sample we were already a bit sceptical as the deposit was quite brittle and did not look metallic. The SEM pictures and the EDX analysis supported our scepticism and the elemental analysis showed an elemental Ta/Cl ratio of about 1/2. Thus, overall we have deposited a low oxidation state tantalum choride. Despite the initial disappointment we were still eager to obtain the metal and found some old literature from Cotton [122], in which he described subvalent clusters of molybdenum, tungsten and tantalum halides. In the case of tantalum the well-defined Ta6Cli22+ complex was described with an average oxidation number of 2.33 and thus with a Ta/Cl molar ratio very close to 1/2. Such clusters are depicted in Figure 4.15. 

Metastable solutions of the monohalides AIX and GaX (X = Cl, Br, I) have been prepared using this technique, and oligomeric species (MX) -Em containing a variety of donors (E) have been crystallised. The solutions disproportionate to the trihalide and the metal (equation 2) when warmed to temperatures in the range -40 to 4-50 °C, depending on the halide, the donor, and the concentration. Species with oxidation states both higher and lower than +1 (i.e. on the path to both disproportionation products) have been isolated. The reduced species (0 < Nox < 1) are discussed in the section on metalloid clusters, and the monohahdes and more oxidized species (1 < Nox < 3) are discussed here. A sonochemical synthesis of a subvalent galhum species, possibly Gal, has also been developed. ... 

This rearrangement from isopropyl to n-propyl Grignard implies that any Grignard may be used to provide the intermediate subvalent titanium compound, and that addition of externally generated olefin would lead to the synthesis of organomagnesium compounds directly from olefins, without the intermediate formation of alkyl halide. 

Finally, it is possible that such halide, or subvalent mercury forms are the source of the conflicting data 109-112). 

In a similar way as mentioned above for subvalent Ge(I) halides Schnepf and coworkers have also reported several reactions of Sn(I) halides with sUyl anions which gave silylated Zintl-type clusters 809 and Snio [248-251], In addition to these clusters smaller units such as the stannyl anion [ (Me3Si)3Si 3Sn] , the cyclotristannene [ (Me3Si)3Si 4Sn3], and the novel cluster [ (Me3Si)3Si 4 (Me3Si)2SiSii4] formed in the course of fliese reactions [252, 253]. 

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