Abstract: The solid complexes of La, Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Ho, Er,Tm, Yb, Lu, and Y with 2-ethylhexyl phosphonic acid mono(2-ethylhexyl)ester were prepared.FTIR spectra ofthese complexes have been studied and the principal infrared absorption bands have beenassigned for region 4000-100 cm^-1.The 150 cm^-1 is believed to be Ln-O stretching vibrationband, and the band near 250 cm^-1 is the coupling band of COPO, CCPO skeleton torsionvibration with Ln-O bond. The experimental result showed that Ln-O bonds in these rareearth complexes are principally ionic.The FO₂ coordination with rare carth is in bridging form. A Volymeric model of the complexes is proposed(Fig. la)

Key words: Rare earth complex, Phosphonic acid ster, FT Infrared spectrum