An extensive series of heterometal-iron-sulfur single cubane-type clusters with core oxidation levels [MFe(3)S(3)Q](3+,2+) (M = Mo, W; Q = S, Se) has been prepared by means of a new method of cluster self-assembly. The procedure utilizes the assembly system [((t)Bu(3)tach)(MS3)-S-VI]/FeCl2/Na(2)Q/NaSR in acetonitrile/THF and affords product clusters in 30-50% yield. The trisulfido precursor acts as a template, binding Fe-II under reducing conditions and supplying the MS3 unit of the product. The system leads to specific incorporation of a mu(3)-chalcogenide from an external source (Na(2)Q) and affords the products [((t)Bu(3)tach)MFe(3)S(3)QL(3)](0/1-) (L = Cl-, RS-), among which are the first MFe3S3Se clusters prepared. Some 16 clusters have been prepared, 13 of which have been characterized by X-ray structure determinations including the incomplete cubane [((t)Bu(3)tach)MoFe2S3Cl2-(mu(2)-SPh)], a possible trapped intermediate in the assembly process. Comparisons of structural and electronic features of clusters differing only in atom Qat one cubane vertex are provided. In comparative pairs of complexes differing only in Q placement of one selenide atom in the core increases core volumes by about 2% over the Q = S case, sets the order Q = Se > Sin Fe-Qbond lengths and Q = S > Se in Fe-Q.:Fe bond angles, causes small positive shifts in redox potentials, and has an essentially nil effect on Fe-57 isomer shifts. Iron mean oxidation states and charge distributions are assigned to most clusters from isomer shifts. ((t)Bu(3)tach = 1,3,5-tertbutyl-1,3,5-triazacydohexane)