The crystal structures of dehydrated Mn46Si100Al92O384 (a = 24.721(5) Angstrom) and of its ethylene sorption complex Mn46Si100Al92O384. 30C(2)H(4) (a = 24.690(5) Angstrom) have been determined by single-crystal X-ray diffraction techniques in the cubic space group Fd (3) over bar at 21(1)degrees C. Each crystal was prepared by dehydration at 380 degrees C and 2 x 10(-6) Torr for 2 days; the complex was then prepared by exposure to 300 Torr of ethylene gas at 24(1)degrees C, and its structure was determined in this atmosphere. Their structures were refined to the final error indices, R-1 = 0.050 and R-2 = 0.044 with 316 reflections and R-1 = 0.064 and R-2 = 0.061 with 313 reflections, respectively, for which I > 3 sigma(I). In each structure, 16 Mn2+ ions, each octahedrally coordinated by six oxygens of the zeolite framework, fill site I at the centers of the double six-rings. The remaining 30 Mn2+ ions in each structure are at site II in the supercage. Each site-II Mn2+ ion in the empty structure is three-coordinate and is quite close to the plane of the three oxygens to which it binds, projecting only 0.14 Angstrom into the supercage. To coordinate laterally to an ethylene molecule, each moves 0.24 Angstrom deeper into the supercage. These Mn2+ ions are 2.119(11) Angstrom from three framework oxygens and 2.76(6) Angstrom from each carbon atom of an ethylene molecule (C=C = 1.10(8) Angstrom). The long Mn2+-ethylene distance is indicative of physisorption.