A series of heterometallic complexes of formula [Mn(2)(III)Ln(3)(III)(bdeaH)(3)(bdea)(2)(PiV)(8)]center dot MeCN, where bdeaH(2) = N-butyldiethanolamine, piv = pivalate, and Ln = Y (1), Tb (2), Dy (3), Ho (4), and Er (5), has been prepared. 1-5 are isostructural, with the metal centers forming a novel semicircular Ln(III)-Mn(III)Ln(III)-Mn-III-Ln(III) strand. Only 2 and 3 exhibit slow relaxation of the magnetization, suggesting that the highly anisotropic Ln(III) ions (Tb-III and Dy-III) are essential for an energy barrier to spin reversal.