According to the 450-180cm^1 region of the IR spectra, tri-or di-aquo acetyl-acetonto complexes of the lanthanide can be distinguished from the mono-aquo complexes. Some IR bands in this region are slightly shifted to higher frequency with the increasing of the atomic number of the lanthanide ions. This indicates that these bands are the M -- O bond stretching mode absorptions. Using group theory method and taking adequate configurations, the number of IR active &C (M-O) modes have been calculated for MO8 and MO7 site structures. Comparing the calculation with the observed spectra of lanthanide -- tri- or di-aquo and mono-aquo acetylacetonato complexes, we reasoned that the tri-and di-aquo complexes with coordination number 8 probably possess dodecahedron configuation and most of the mono-aquo complexes with coordination number 7 possess mono-capped trigonal prism configuration.

Key words: INFRARED SPECTROPHOTOMETRY, PENTANEDIONE P, HYDRATE, RARE EARTH METAL COMPLEX, CONFIGURATION, LANTHANIDE SERIES COMPOUNDS