The syntheses, structures, and magnetic properties of two new Mn-7 complexes containing phenylseleninate ligands are reported. [Mn7O8(O2SePh)(8)(O2CMe)(H2O)] (1) and [Mn7O8(O2SePh)(9)(H-2)] (2) were both prepared by the reaction of 18 equiv of benzeneseleninic acid (PhSeO2H) with [Mn12O12(O2CMe)(16)(H2O)(4)] in MeCN. Complex 1.6MeCN crystallizes in the triclinic space group P (1) over bar, and complex 2.2CH(2)Cl(2) crystallizes in the monoclinic space group P2(1)/m. Both compounds possess an unprecedented [Mn7O8](9+) core comprising a central [Mn-3(III)(mu(3)-O)(4)](+) unit attached to [Mn-2(IV)(mu-O)(2)](4+) and [Mn-2(IV)(mu-O)(mu(3)-O)](4+) units on either side. In each cluster, the PhSeO2- groups function as bridging ligands between adjacent Mn centers. The structure reveals strong Se...O intermolecular contacts between Mn-7 units to give a one-dimensional chain structure, with weak interchain interactions. Solid-state DC magnetic susceptibility measurements of complexes 1 and 2 reveal that they have very similar properties, and detailed studies on 1 by AC susceptibility measurements confirm an S = 2 ground-state spin value. In addition, out-of phase AC signals are observed, suggesting slow magnetization relaxation. Magnetization versus DC field sweeps down to 0.04 K reveals hysteresis loops, but the temperature dependence of the coercivity is not what is expected of a single-molecule magnet. Instead, the behavior is due to single-chain magnetism, albeit with weak antiferromagnetic interactions between the chains, with the barrier to relaxation arising from a combination of molecular anisotropy and ferromagnetic intermolecular exchange interactions mediated by the Se...O contacts. An Arrhenius plot was constructed from the magnetization versus time decay data. The thermally activated region at >0.5 K gave an effective relaxation barrier (U-eff) of 14.2 K. Below similar to0.1 K, the relaxation is independent of temperature, which is characteristic of magnetization quantum tunneling through the anisotropy barrier. These Mn-7 compounds are thus the first single-chain magnets to comprise polynuclear metal clusters and also the first for which the temperature-independent relaxation characteristic of tunneling has been identified. The work also emphasizes that out-of-phase AC signals for ostensibly molecular compounds are not sufficient proof by themselves of a single-molecule magnet.