The synthesis and characterization of a family of Mn2(III)Mn(2)(II)Ln(III) L complexes (Ln = Gd (1), Tb (2), DY (3), and Ho (4)) of formula [Mn(4)Ln(2)O(2)(O2CBut)(6)(edteH(2))(2)(NO3)(2)] are reported, where edteH(4) is N,N,N',N'-tetrakis(2-hydroxyethy)ethylenediamine. The analogous Mn4Y2 (5) complex has also been prepared. They were obtained from reaction of Ln(NO3)(3) or Y(NO3)(3) with Mn(O2CBut)(2), edteH(4), and NEt3 in a 2:3:1:2 molar ratio. The crystal structures of representative 1 and 2 were obtained, and their core consists of a face-fused double-c-ubane [Mn(4)Ln(2)(mu(4)-O2-)(2)(mu(3)-OR)(4)] unit Such double-cubane units are extremely rare in 3d metal chemistry and unprecedented in 3d-4f chemistry. Variable-temperature, solid-state dc and ac magnetic susceptibility studies on 1-5 were carried out. Fitting of dc chi T-M vs T data for 5 gave J(bb) (Mn-III center dot center dot center dot Mn-III) = -32.6(9) cm(-1), J(wb) (Mn-III center dot center dot center dot Mn-III) = +0.5(2) cm(-1), and g = 1.96(1), indicating a vertical bar n, 0, > n = 0-5) 6-fold-degenerate ground state. The data for 1 indicate an S = 12 ground state, confirmed by fitting of magnetization data, which gave S = 12; D = 0.00(1) cm-(1), and g = 1.93(1) (D is the axial zero-field splitting parameter). This ground state identifies the Mn-III center dot center dot center dot Gd-III interactions to be ferromagnetic The ac susceptibility data independently confirmed the conclusions about 1 and 5 and revealed that 2 displays slow relaxation of the magnetization vector for the Mn4Tb2 analogue 2. The latter was confirmed as a single. molecule magnet by observation of hysteresis below 0.9 K in magnetization vs dc field scans on a single crystal of 2.MeCN on a micro-SQUID apparatus. The hysteresis loops also displayed well-resolved quantum tunneling of magnetization steps, only the second 3d-4f SWIM to do so.