Tetrahalide

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 

Numerous crystal structures have been published of compounds containing the pyramidal cations Se Cl3+, Se Br3+, Te Cl3, etc. and the anions Se CLj , Sej Se3Cli3,  

Se3Bri3- SeCls , TeClj-, TeCle , etc. The anion structures are much as expected with the Se species featuring square planar (pseudo-octahedral) units, and the trinuclear Se anions as in the tellurium analogue above. See also p. 776. There are, in addition, a fascinating series of bromoselenate(II) dianions based on fused planar SeBr4 units, e.g. Se3Brg , Se4Bri4 , 

Llthrs, R. Willmer and F.-P. Ahlers, Z. anorg. allg. Chem. 592, 17-34 (1991). See also H. Folkerts, 

Dehnicke, J. Magull, H.Goesmann and D. Fenske, Z anorg. allg. Chem. 620, 1301-6 (1994). 

More than thirty metals form a tetrahalide with at least one of the halogens, but in relatively few cases are all four tetrahalides of a given metal known. All four tetrahalides are known of Ge, Sn, Ti, Zr, Hf, Nb, Ta, Mo, W, Th, and U, and at the ether extreme MF4 is the only tetrahalide known of Cr, Mn, Pd, Ru, Rh, Ce, Pr, and Tb no tetrahalides of Ir have been prepared. The crystal structures of more than one-half of the seventy or so known tetrahalides are known, and they include 

The (layer) structure of PuBt3. The planes of the layers are normal to that of the paper. 

Octahedral structures (see Chapter 5) include two closely related chain structures, a-Nbl4 and TcCl4 and the layer structure of Snp4 (and the isostructural NbF4( and PbF4). In the a-Nbl4 chain pairs of Nb atoms are alternately closer together and further apart (Fig. 9.10(a)), and the interaction 

In Snp4 octahedral SnF groups form a layer by sharing four equatorial F atoms as in the KjNiF4 structure (p. 171). 

Other octahedral structures include that suggested for high-MoCl4 that is, random occupation of three-quarters of the metal positions in the Bils structure. There would be isolated MoClg octahedra and portions of edge-sharing chains. 

Only the fluorides of Ce, Pr, and Tb exist, the three lanthanides with the most stable (-1-4) oxidation state. Fluorine is most likely to support a high oxidation state, and even though salts of ions like [CeCle] are known, the binary chloride has not been made. CeF4 can be crystallized from aqueous solution as a monohydrate. Anhydrous LnF4 (Ln = Ce, Pr, Tb) 

---

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

When carbon forms four covalent bonds with halogen atoms the second quantum level on the carbon is completely filled with electrons. Most of the reactions of the Group IV tetrahalides require initial donation by a Lewis base (p. 91) (e.g. water, ammonia) which attaches initially to the tetrahalide by donation of its electron pair. Hence, although the calculated free energy of a reaction may indicate that the reaction is energetically favourable, the reaction may still not proceed. Thus we find that the tetrahalides of carbon... 

Silicon tetrafluoride is a colourless gas, b.p. 203 K, the molecule having, like the tetrahalides of carbon, a tetrahedral covalent structure. It reacts with water to form hydrated silica (silica gel, see p. 186) and hexafluorosilicic acid, the latter product being obtained by a reaction between the hydrogen fluoride produced and excess silicon tetrafluoride ... 

Hafnium is resistant to concentrated alkalis, but at elevated temperatures reacts with oxygen, nitrogen, carbon, boron, sulfur, and silicon. Halogens react directly to form tetrahalides. 

Like all the lead tetrahalides, lead tetrafluoride [7783-59-7] is very reactive. It is relatively the most stable halide, however. PbF is a white crystalline... 

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... 

Similar results are obtained for dre deposition of the carbides of these metals using methane as a source of carbon, atrd silicon tetrahalides for the preparation of silicides. These reactions are more complex than dre preparation of the diborides because of the number of carbides atrd silicides that the tratrsition metals form, some of which have wide ranges of non-stoichiometry. The control of the ratio of the partial pressures of dre ingoing gases is therefore important as a process variable. 

Diphosphorus tetrahalides and other tower halides of phosphorus... 

Several ternary diphosphorus tetrahalides, P2X Y4- , (X, Y = C1, Br, I) have recently... 

Figure 16.13 Structures of some tetrahalides of Se and Te (a) Sep4 (gas), (b) crystalline Sep4, and schematic representation of the association of the pseudo-tbp molecules (see text), (c) coordination environment of Te in crystalline Tep4 and schematic representation of the polymerized square pyramidal units, (d) the tetrameric unit in crystalline (TeCl4)4, and (e) two representations of the tetrameric molecules in Te4li6 showing the shared edges of the Telg octahedral subunits.

Table 21.3 Some physical tetrahalides properties of titanium...

All the tetrahalides, but especially the chlorides and bromides, behave as Lewis acids dissolving in polar solvents to give rise to series of addition compounds they also form complex anions with halides. They are all hygroscopic and hydrolysis follows the same pattern as complex formation, with the chlorides and bromides being more vulnerable than the fluorides and iodides. TiCU fumes in and is completely hydrolysed by... 

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 ... 

---