The reaction of Nd(SePh)(3) with SeO2 and Hg in pyridine gives the dodecanuclear cluster [(py)(18)Nd12O6Se4(Se-2)(4)(SePh)(4)(Se2Ph)(2)Hg-2(SePh)(4)][(Hg(SePh)(3 )](2). In this compound the 12 Nd(III) ions are stacked in four sets of Nd-3, with pairs of tetrahedral oxo ligands separating the Nd-3 planes and Se, SeSe, SePh, pyridine, and HgSePh groups encapsulating the oxo core. Both the Nd-O bond lengths and the geometries about the oxo ions are remarkably similar to those found in solid-state Nd2O3. Near-IR emission experiments indicate that the cluster emission properties are less intense than those of highly emissive (DME)(2)Nd(SC6F5)(3) or (THF)(8)Nd8O2Se2(SePh)(16) but brighter than the nonemissive solid-state compound Nd2O3. Intensity variations are interpreted in terms of concentration quenching and phonon relaxation.