Abstract: In this work, the Laser Raman and infrared spectra of 15 rare earth acetylacetonate complexes in the region 300-3300cm^-1 are studied.The bands near 401 and 415 cm^-1 are sensitive to a change in rare earth ion.The frequencies of these two bands change periodically with atomic number, showing the "tetrad effect" and ihe "inclined Weffect" characteristic of lanthanides, and the former was first observed in spectra of rare earth complexes.These effects provide strong evidence for assigning the band near 401cm^-1 to the MO₂ symmetric stretch and that near 415cm^-1 to the MO₂ antisymmetric stretch.Adiatomic molecule approximation may be used to estimate the M-Ostretching force constants from the M-Ovibrational frequencies.If the force constant is plotted against the atomic number, the curve shows also the "tetrad effect" .If the force constant is plotted against the logarithm of the stability constants of these complexes, two straight lines having different slope were obtained for light and heavy rare earth ions which cross at Eu.Since the force constants vary appreciably compared to the variation in stability constants for heavy rare earth acetylacetonate complexes, the force constant is a more precise structure parameter for describing these complexes.The C-Ostretching frequency in the infrared spectrum of acetylacetone decreases upon coordination with rare earth metal ions, which indicates that the charge transfer occurs.The negative charge in the acetylacetonate ring tends to be delocalized.Rare earth acetylacetonate complexes are usually considered to be ionic, but the present result indicates that a certain amount of covalency is present.