Heterometallic clusters containing lanthanides and the group 12 metals can be isolated as crystalline compounds in high yields. These products [(py)(8)Ln(4)M(2)Se(6)(SePh)(4) (Ln = Er, Yb, Lu; M = Cd, Hg)] adopt a double cubane structure with the covalent M occupying an opposing pair of external metal sites. Both Er/M compounds are strongly emissive materials, with emission lifetimes of 1.41 ms (Er/Cd) and 0.71 ms (Er/Hg) and with the Er/Cd radiative quantum efficiency twice that of the Er/Hg compound. Thermal decomposition of the Er/Cd and Yb/Cd compounds at 650 degrees C give the ternary solid-state materials CdLn(2)Se(4).