From tetrahalides

The allenes, RC=C=CR, are readily prepared by this method from tetrahalides and dihalo olefins. Typical procedures are given for methyl-allene (72%) and 1-phenyl-l,2-butadiene (77%). A convenient route to the allenes from allyl halides is as follows ... 

The separation of niobium from tantalum tluough the gaseous chlorides is carried out at higher temperature, about 900 K, and it is therefore to be expected, as is the case, that the thermodynamic data will provide a useful guide. These metals form a number of chlorides, mainly the ui- tetra- and pentachlorides. These latter are much more volatile than the tetrahalides, and the exchange reaction at 900 K... 

The other tetrahalides can all readily be made by direct reactions of the elements. Crystalline SeCU, TeCU and -SeBr4 are isotypic and the structural unit is a cubane-like tetramer of the same general type as [Me3Pt(/Z3-Cl)]4 (p. 1168). This is illustrated schematically for TeCU in Fig. 16.13d each Te is displaced outwards along a threefold axis and thus has a distorted octahedral environment. This can be visualized as resulting from repulsions due to the Te lone-pairs directed towards the cube centre and, in the limit, would result in the separation into... 

The known halides of this triad are listed in Table 26.3. It can be seen that, apart from C0F3, C0F4 and the doubtful iridium tetrahalides, they fall into three categories ... 

I. Halides from Alcohols by Triphenylphosphine-Carbon Tetrahalide... 

A halogenating system related to the preceding case is formed by the reaction of triphenylphosphine with molecular bromine or chlorine. The system is not as sensitive to moisture as the phosphine-carbon tetrahalide system (see preceding section), but it suffers from the disadvantage that hydrohalic acids are produced as the reaction proceeds. Nevertheless, sensitive compounds can be successfully halogenated by the system, as exemplified by the preparation of cinnamyl bromide from the alcohol. 

Reductive methods form B—B bonds from B—X bonds. For B2X4 (X = Cl, Br, I) from BXj, an electric discharge is supplemented by the presence of a metal, or metal atoms, as halide scavenger. The passage of BX3 at low pressure through a rf discharge in the presence of Hg produces the diboron tetrahalides B2X4 at 300 mg h ... 

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 first isolable product in the reaction of triphenylphosphine and carbon tetrachloride is the salt (73), which reacts rapidly with further phosphine to give the stable phosphorane (74).62 In contrast, tris-t-butylphosphine reacts with germanium and tin tetrahalides to form the salts (75) 3 compounds of the latter type have long been postulated as arising from the reactions of phosphines with carbon tetrahalides but so far have defied detection. 

The B-B bonded l,2-dimethoxydiborane(4) precursor67 was selected on the basis that it could be obtained readily from (MeO)2B-B(OMe)268 by reaction with the lithium aryl. In contrast to diborane(4) tetrahalides,... 

Recently, molecular orbital calculations on some iron complexes have been made by Gray and his co-workers (2, 30). These values for ferrous complexes are plotted in Figure 2 as the dashed line [3d 4s (MO)]. Hence, Mossbauer spectroscopy provides estimates for the 4s electron contribution for molecular orbital calculations. This correlation does not hold for high spin complexes such as FeCb" and FeFe ". Bersuker (3, 4) has attempted to relate both S and AEq directly to molecular orbital parameters. Using the equations developed from this approach, Bersuker, Gordanskii, and Makarov (5) have concluded that in tin tetrahalides the role of dx-Px bonding is significant. 

Such serious deviations from the predictions of electronegativity are not often found in Mossbauer experiments. In particular, Alexsandrov et al. ( ) and others have found a linear relation between 8 and electronegativity for the tin tetrahalides. Since the differences in electronegativity are much larger for the halogens than the alkalies, the difference in ionicity is much more important for the tin tetrahalides than for the alkali iodides. 

Fig. 5. Sigma orbital populations of the halogen atoms in Group IV tetrahalides obtained from the halogen nuclear quadrupole coupling constants by use of Equation (1) with an = 2.0...

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