A FAR-INFRARED STUDY AND INVESTIGATION OF THE DIFFERENCE BETWEEN THE ASYMMETRIC AND SYMMETRICAL POO- STRETCHING FREQUENCIES IN METAL DIPHOSPHONATE COMPLEXES

Lanthanide complexes of P,P'-di(2-ethylhexyl) methanediphosphonic acid , H2DEH[MDP], were prepared and investigated by infrared spectroscopy. Qualitative information about the nature of the phosphonate binding i n the complexes was obtained by measuring the asymmetric and symmetric POO- stretching frequencies of the ligand. These vibrational modes ap pear at lower energy in the complexes than in the sodium salt. This in dicates symmetrical coordination of the phosphonate groups through che late and/or bridging interactions. The frequency difference between th e POO- stretching bands Delta v becomes smaller as the ionic potential ell of the metal ion increases. The correlation is valid when both ox ygen atoms are involved in the bonding and no other absorption bands i nterfere with the POO- vibrations. The origin of this empirical correl ation is the change in the P-O bond order which arises from a change i n the strength of the metal-diphosphonate interaction. The far-infrare d spectra of the lanthanide complexes, an Fe-54-labelled compound, and the sodium salt of H2DEH[MDP] were measured to provide insight into t he nature of the metal-ligand interaction. The appearance of metal-oxy gen stretching bands in the iron compound indicates that the iron-diph osphonate interaction has a substantial covalent component whereas the absence of metal-oxygen stretching bands in the lanthanide complexes indicates the binding is predominantly ionic. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.