Octanuclear heterobimetallic complexes, [Ln(4)Ni(4)(H3L)(4)(mu(3)-OH)(4)(mu(2)-OH)(4)](4)Cl center dot xH(2)O center dot yCHCl(3) (Dy3+, x = 30.6, y = 2 (1); Tb3+, x = 28, y = 0 (2); Gd3+, x = 25.3, y = 0 (3); Ho3+, x = 30.6, y = 3 (4)) (H5L = N-1,N-3-bis(6-formyl-2-(hydroxymethyl)-4-methylphenol)diethylenetriamine) are reported. These are assembled by the cumulative coordination action of four doubly deprotonated compartmental ligands, [H3L](2-), along with eight exogenous -OH ligands. Within the core of these complexes, four Ln(3+)'s are distributed to the four corners of a perfect square grid while four Ni(2+)s are projected away from the plane of the Ln(4) unit. Each of the four Ni(2+)s possesses distorted octahedral geometry while all of the Ln(3+)'s are crystallographically equivalent and are present in an elongated square antiprism geometry. The magnetic properties of compound 3 are dominated by an easy-plane single-ion anisotropy of the Ni2+ ions [D-Ni = 6.7(7) K] and dipolar interactions between Gd3+ centers. Detailed ac magnetometry reveals the presence of distinct temperature-dependent out-of-phase signals for compounds 1 and 2, indicative of slow magnetic relaxation. Magnetochemical analysis of complex 1 implies the 3d and the 4f metal ions are engaged in ferromagnetic interactions with SMM behavior, while dc magnetometry of compound 2 is suggestive of an antiferromagnetic Ni-Tb spin-exchange with slow magnetic relaxation due to a field-induced level crossing. Compound 4 exhibits an easy-plane single-ion anisotropy for the Ho3+ ions and weak interactions between spin centers.