1.1- Dihalides from 2 molecules

Despite the large body of literature discussing the preparation and reaction chemis try of dichloromethylene in solution, very few reports of the isolation of the molecule have appeared. The technique of forming Group IV dihalides from the reduction of the tetrahalide with the metal has proved to be of great utility for production of SiX2 and GeX2, but has not been successful in the case of carbon. 

The product of the second addition reaction is a geminal dihalide, a molecule with two halogens on the same carbon. Geminal comes from geminus, which is Latin for twin. If the alkyne is a terminal alkyne, the first electrophilic addition reaction is also regioselective the adds to the less substituted sp carbon (that is, the one bonded to the hydrogen). 

Although the small molecule most commonly split out is water this is not necessarily the case. In the formation of polysulphides from dihalides and sodium polysulphide, sodium chloride is produced. 

The radiation nozzle system has been used for studying a series of transition metal dihalide molecules. Typical molecular intensity distributions are shown in Fig. 4 for manganese(II) chloride. The quickly damping character of the intensity distribution relates to the large-amplitude motion in the molecule due to the high temperature ( 750 °C) conditions of the experiment. Fig. 5 shows the radial distribution from the same experiment which also well demonstrates the straightforward manner of structure determination of such simple molecules. 

Raman spectral studies of the species MX(n" 2) (n = 2—4 M = Zn, Cd, or Hg X = Cl, Br, or I) in anhydrous tributyl phosphate have been reported.24 For the MX2 molecules, sufficient metal dihalide-solvent interaction exists to suggest bent X—M—X species with C2v rather than >ooh symmetry. The effect appears most marked for zinc(n) and least marked for mercury(n), which is in accord with the Lewis acidity sequence ZnX2 > CdX2 > HgX2. A similar analysis of the anionic MX3 complexes formed from a 1 1 mixture of LiX and MX2 again demonstrates solvent interaction, and a tetrahedral C3v species is indicated, rather than the planar structure found in the solid state. Studies involving the halogeno-complexes of zinc, cadmium, and mercury in DMSO and DMF have also been reported.25,26... 

Diorgano tellurium dihalides and ammonium, phosphonium, arsonium, or stibonium halides react in refluxing chloroform or ethanol to form onium diorganotetrahalo-tellurates(I V)5,1. When an onium halide with a halogen different from that in the diorgano tellurium dihalide was used, tetrahalotellurates with two different halogens in the molecule were isolated. 

In some cases, we can generate a carbon-carbon triple bond by eliminating two molecules of HX from a dihalide. Dehydrohalogenation of a geminal or vicinal dihalide gives a vinyl halide. Under strongly basic conditions, a second dehydrohalogenation may occur to form an alkyne. 

For floppy systems, such as many metal halide molecules, the rg/re differences may be even much greater than those listed in Table 3. Alkaline earth metal, zinc, and transition metal dihalides, for example, have been extensively investigated by gas-phase electron diffraction (see e.g., [33-36]). The structure determinations have involved a joint electron diffraction/vibrational spectroscopic analysis (cf. [37]). Depending on the model potential used, and among them on the manner in which anharmonic effects are taken into account, even the r distances are rather different. This is illustrated by the data of Table 4. The re distances obtained from experimental data applying various model potentials [32] have the following... 

The moisture-sensitive complexes 215 and 217c have been prepared from dilithium perfluoropinacolate and the appropriate dihalide in tetrahydro-furan. One molecule of THF is also coordinated to the metal 64). 

ABj molecules For bent species (Ca,.,), three vibrations should be detected in the IR spectrum as well as in the Raman spectrum. Sulfur dihalides provide a good example to demonstrate this fact SF2 (Ne matrix) shows the following absorptions in the IR spectrum i(SF2) = 834 cm" (FSF) = 358 cm" Vas (SF2) = 808 cm (Haas and Willner, 1978). Since SF2 is un.stable with respect to disproportionation, no Raman spectrum has yet been obtained. However, the IR and the Raman spectrum of SCI2 can easily be measured r j(S- Cl2) = 527 cm r u.dS Ch) = 525 cm" (IR, Ne matrix), (Bielefeld and Willner, 1980) <5(C1SC1) = 208 cm" (Frankiss and Harrison, 1975). In both cases, the expected bent structure is derived from the IR spectrum as well as from the Raman spectrum by simply counting the number of bands. 

CUCI2 has been studied spectroscopically at 1000 °C in the presence of CI2 (to prevent decomposition to CuCl and Cb). Binary copper compounds with very soft iodide ligands are not known by simply adding 1 to a solution of Cu +, Cul and I2 are quickly formed. This reaction can be used for the volumetric analysis of Cu + in solution by titrating the I2 produced with sodium thiosulfate (equation 8). Hydrates of the dihalides (F, Cl, Br) are common, easily prepared from aqueous haloacids, and contain coordinated water molecules. 

Dehaiogenation of vicinal (Latin vicinalis, neighboring) dihalides is severely limited by the fact that these dihalides are themselves generally prepared from the alkenes. However, it is sometimes useful to convert an alkene to a dihalide while we perform some operation on another part of the molecule, and then to regenerate the alkene by treatment with zinc this procedure is referred to as protecting the double bond. 

---