A far-infrared study and investigation of the difference between the asymmetric and symmetric 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 in the complexes was obtained by measuring the asymmetric and symmetric POO- stretching frequencies of the ligand. These vibrational modes appear at lower energy in the complexes than in the sodium salt. This indicates symmetrical coordination of the phosphonate groups through chelate and/or bridging interactions. The frequency difference between the POO- stretching bands Delta v becomes smaller as the ionic potential ell of the metal ion increases. The correlation is valid when both oxygen atoms are involved in the bonding and no other absorption bands interfere with the POO- vibrations. The origin of this empirical correlation is the change in the P-O bond order which arises from a change in the strength of the metal-diphosphonate interaction. The far-infrared spectra of the lanthanide complexes, an Fe-54-labelled compound, and the sodium salt of H2DEH[MDP] were measured to provide insight into the nature of the metal-ligand interaction. The appearance of metal-oxygen stretching bands in the iron compound indicates that the iron-diphosphonate 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 Science Ltd. All rights reserved.