Trihalide ions, structure

The transition metal trihalides have mostly structures based upon close packings of halide atoms with the metal ions in octahedral holes. Exceptions include those of W and Re with the metal atoms In a five-coordinate square pyramidal environment, and the series AuXs with square-planar coordinated Au atoms.The stoichiometric ratio in the transition metal trihalides provides structures that can range from ID to 3D arrangements. The most common general structure types are VF3, Bil3, AICI3, and TiR. The differences occur in either the three-dimensional anion packing modes or the distribution of occupied metal sites. 

Gabes W, Gerding H (1972) Vibrational-spectra and structures of trihalide ions. J Mol Struct 14 267-279... 

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

The trihalides MBr3 and MI3 are known for all the lanthanide elements. The early lanthanide tribromide (La to Pr) adopt the LaCh structure, while the later tribromides (from Nd to Lu) and the early triiodides (from La to Nd) form a layer structure with eight-coordinate lanthanide ions. 

Because halide ions form bridges between two metal ions, the trihalides of aluminum exist as dimers in the vapor phase and in solvents that are poor electron donors. In general, the structures can be shown as... 

The general chemistry of Ac3 in both solid compounds and solution, where known, is very similar to that of lanthanum, as would be expected from the similarity in position in the Periodic Table and in radii (Ac3, 1.10 La3, 1.06 A) together with the noble gas structure of the ion. Thus actinium is a true member of Group 3, the only difference from lanthanum being in the expected increased basicity. The increased basic character is shown by the stronger absorption of the hydrated ion on cation-exchange resins, the poorer extraction of the ion from concentrated nitric acid solutions by tributyl phosphate, and the hydrolysis of the trihalides with water vapor at 1000°C to the oxohalides AcOX the lanthanum halides are hydrolyzed to oxide by water vapor at 1000°C. 

The actinide trihalides display a similar pattern of structure to those of the lanthanide trihalides. However, comparing the coordination numbers for Ln + and An + ions with the same number of f electrons ( above one another in the Periodic Table ), it can be seen that the coordination number of the lanthanide halides decreases sooner than in the actinide series, a reflection of the fact that the larger actinide ions allow more halide ions to pack around them. Table 10.3 gives comparative coordination numbers for the trihalides of the lanthanides and actinides. 

As already noted only the sharing of a pair of opposite faces has to be considered an infinite chain of composition AX3 is then formed. A number of crystalline trihalides consist of infinite molecules of this type, and chain ions of the same kind exist in BaNi03 and CsNiCl3. In the Zrl3 structure the metal atoms occupy one-third of the octahedral holes in a close-packed assembly of halogen atoms to form infinite chains perpendicular to the planes of c.p. halogen atoms ... 

The UCI3 structure, in which the coordination group of the metal ion is a tricapped trigonal prism, is adopted by numerous 4f and 5f trihalides (Table 9.19)... 

An electrostatic hydration model, previously developed for ions of the noble gas structure, has been applied to the tervalent lanthanide and actinide ions. For lanthanides the application of a single primary hydration number resulted in a satisfactory fit of the model to the experimentally determined free energy and enthalpy data. The atomization enthalpies of lanthanide trihalide molecules have been calculated in terms of a covalent model of a polarized ion. Comparison with values obtained from a hard sphere modeP showed that a satisfactory description of the bonding in these molecules must ultimately be formulated from the covalent perspective. 

The previous examples (Section II.A) of facile four-centre reactions appear to be enabled by the intrusion of ionic interactions into the electronic structure of the potential energy surfaces. The alkali halide-alkali halide exchange reactions involve purely electrostatic interactions. The alkali dimer-halogen molecule reactions involve an electron jump in the entrance valley of the potential surface and thereafter proceed as ion recombination reactions. The present alkali halide-halogen molecule reactions also appear to be permitted by virtue of ionic interactions. The reaction complex, as illustrated in Fig. 26, has the structure of an alkali trihalide molecule. The observed... 

Since the polyhalide ions are formed by the addition of diatomic halogen molecules or of interhalogen molecules to a halide ion, the total number of halogen atoms in such a complex is always odd. The most common case is that of the trihalides. Some penta-, hepta-, and enneahalides are likewise known. The highest known polyhalogen complex is PCUBr 1 the structure of this has not been determined, but by analogy to other similar complexes it can probably be expressed as [PClsBr]+[Bri7]. Table I lists the known polyhalide ions. 

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