Trihalides actinide

Table 10.2 Coordination number and structure types of the actinide trihalides...

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. 

The absorption spectra of these complexes can be obtained in the visible and near infrared region using solutions of the triphenylphos-phonium salts, but if the ultraviolet spectrum is desired, the complex must be formed in solution using an aliphatic quaternary ammonium halide and the actinide trihalide or oxyhalide. 

The potassium salts of bis([8]annulene)uranate(III),2 -neptunatedll). s -plutonatedll) and -americiatedll) anions have all been reported. These complexes have been synthesized by the reaction of two moles of [8]annulene dianion with one mole of the actinide trihalide. 

Some relevant data are compiled in table 4 and the enthalpies of solution are plotted in fig. 6. We note that, as is the case for sesquioxides, all enthalpies of solution of actinide trihalides are less exothermic than those of structurally similar lanthanide trihalides. See section 4.1.1 for further discussion. 

It has been noted (Morss 1986, fig. 17.10 this chapter, section 4.1.1 and fig. 3) that the actinide trihalides have less exothermic heats of solution than structurally similar lanthanide trihalides. 

It has already been noted (section 4.1.1) that the enthalpies of solution of actinide trihalides and elpasolites Cs2NaAnClg are systematically less exothermic than enthalpies of solution of structurally similar lanthanide compounds. This lanthanide-actinide bonding difference may be due to lattice energies that are larger for actinide compounds than for structurally similar lanthanide compounds (fig. 8). 

Fig. 1. A structure-field map of the lanthanide and actinide trihalides. Packing efficiency, determined from the sum of the (assumed spherical) volume calculated from the anion and cation radii divided by the actual volume derived from the lattice parameters, is plotted against radius ratio. Only those trihalides that mark the limits of the structure field are indicated.

---