Correlation of the asymmetric and symmetric POO- frequencies with the ionic potential of the metal ion in compounds of organophosphorus acid extractants:: A short review

The correlation between [(nu(asym)(POO-) - nu(sym)(POO-)], Delta nu, and the ionic potential of the metal ion in metal compounds of alkylsubstituted diphosphonic acid extractants is reviewed. Similar correlations of the ionic potential of the metal ion with Delta v in organosubstituted monophosphate, organosubstituted monophosphonate, carbonate and nitrate compounds have been previously demonstrated. In this paper, we discuss the origin, general validity, and utility of the correlation for organophosphorus compounds. Far-infrared and Fe-54-labeled data were used to identify metal-oxygen stretching vibrations and establish that for some metal ions, e.g., Fe(III) and possibly Th(IV), the metal-oxygen interaction has substantial covalent character. The correlation appears to be valid not only for materials in which the metal-oxygen interaction is predominantly ionic, but also for compounds characterized by a covalent metal-oxygen bonding as well as for polymeric materials, providing the POO- stretching vibrations are assignable. The correlation is useful in identifying changes in M-O bonding across a series of complexes containing similar metals (i.e., lanthanides and actinides), as well as in making comparisons for differences in M-O bonding when the charge on the metal ion changes. The correlation becomes particularly useful for compounds where no X-ray data are available.