The reaction of Ln(acac)(3)(.)3H(2)O (Ln = Sm, Eu, Gd, Dy, Yb) with K[Cr-2(CO)(10)(mu-H)] at different molar ratios and solvents leads to the formation of nonanuclear lanthanide hydroxo acetylacetonate complexes of general formula [Ln(9)(OH)(10)(acac)(16)][HCr2(CO)(10)]. The compounds are isomorphous, and the common cationic cluster core consists of a novel square antiprismatic arrangement of nine Ln atoms connected by mu(3), mu(4) hydroxo bridges and/or by acetylacetonate ligands as it results from the single-crystal X-ray analysis of the Sm derivative for which the most suitable crystals were obtained.