The reaction of a new hexadentate Schiff base hydrazide ligand (LH3) with rare earth(III) chloride salts in the presence of triethylamine as the base afforded two planar tetranuclear neutral complexes: [{(LH)(2)Dy-4}(mu(2)-O)(4)](H2O)(8)center dot 2CH(3)OH center dot 8H(2)O (1) and [{(LH)(2)Ho-4} (mu(2)-O)(4)](H2O)(8)center dot 6CH(3)OH center dot 4H(2)O (2). These neutral complexes possess a structure in which all of the lanthanide ions and the donor atoms of the ligand remain in a perfect plane. Each doubly deprotonated ligand holds two Ln(III) ions in its two distinct chelating coordination pockets to form [LH(Ln)(2)](4+) units. Two such units are connected by four [mu(2)-O](2-) ligands to form a planar tetranuclear assembly with an Ln(III)(4) core that possesses a rhombus shaped structure. Detailed static and dynamic magnetic analysis of 1 and 2 revealed single molecule magnet (SMM) behavior for complex 1. A peculiar feature of the chi(M)'' versus temperature curve is that two peaks that are frequency dependent are revealed, indicating the occurrence of two relaxation processes that lead to two energy barriers (16.8 and 54.2 K) and time constants (tau(0) = 1.4 x 10(-6) S, tau(0) = 7.2 X 10(-7) S). This was related to the presence of two distinct geometrical sites for Dy(III) in complex 1.