Halide hypersensitive transition intensities

Table 14 summarizes the trivalent lanthanide halide vapor complexes which have been studied by means of spectrophotometry. Among the binary systems involving Group-IIIA trihalides, chlorides are by far the most extensively studied systems. The overall spectra of vapor complexes are similar to those observed in aqueous solutions (Carnall 1979) or in other (e.g. glass) media (Reisfeld 1975) in terms of transition energies but the hypersensitive transition intensities were foimd to vary. However, the hypersensitive intensity ( G5/2 <— 19/2) of Nd-Al-Cl vapor complexes compares well with the result... 

Plausible though this mechanism is, it came under criticism (7) because, inter alia, it could not account for the intense hypersensitive transitions of the gaseous rare-earth trihalides (8). However, there is recent evidence that the halides are not planar (9,JL0), as had been previously supposed. If this is in fact the case, the importance of the mechanism based on Y terms in V remains undecided. 

Finally, experimental results point to the following outline behavior of the hypersensitive band intensities as affected from the chemical environment of the lanthanide ions (Henrie et al. 1976, Peacock 1975) (i) the intensity of a given hypersensitive transition in a halide compound of constant structure appears to increase in the order r >Br > Cr > F (ii) hypersensitivity is proportional to the nephelauxetic ratio and possibly to the R-X covalency (Henrie and Choppin 1968, Mehta et al. 1971) and (iii) a correlation exists between the hypersensitive intensities and the electron donating ability (basicity) of the ligand according to which the more basic the ligand the more intense the hypersensitive band. 

Regardless of the structure (i.e. R, K3 or S ) of the RX3 XAX3 complexes the A-X interaction will decrease the basicity of the ligand coordinated to R in the sequence In > Ga > Al, and thus the intensity of hypersensitive transitions are expected to decrease in the same sequence. The data presented in table 14 and the above discussion support this view, but no definite conclusions about the type of species and their structures can be made. In relation, however, to the proposed structures in fig. 11 the following statements can be made and might be helpful in choosing certain molecular structures for the lanthanide halide vapor complexes ... 

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