Structure. Single crystal X-ray diffraction studies reveal that Cf(18-crown-6)I2 (Figure ) adopts a structure-type first established with Ln II cations (Ln II = Eu, Sm, Yb, Dy, Tm) that has been the subject of recent investigations for single molecule magnetism with Tm(18-crown-6)I2. The Cf(18-crown-6)I2 molecule consists of a Cf II cation bound within the cavity of the crown ether by the six etheric oxygen atoms and by two trans iodide anions. This yields a slightly distorted hexagonal bipyramidal environment where the Cf II cation is located on an inversion center. Cf-O bond distances range from 2.685(5) to 2.666(5) Å and are slightly shorter than those found with Eu(II), but longer than those of Dy(II) as shown in Figure . Although the Cf-O and Cf-I bonds are mostly ionic, there is a small covalent contribution to these bonds where the 5forbital participation is essentially negligible. The Cf-I bond distance is 3.1563(4) Å which is notably shorter than that of Eu II -I, but statistically longer than that observed for Dy II -I. The shortening of this bond is ascribed to the mixing of iodide 4pz orbitals with Cf II 6𝑑 z 2 orbitals (Figure ). Seeing as this is the first condensed-matter Cf II molecule that has been isolated and characterized, these bond metrics help bracket the upper and lower limits of the ionic radius of Cf II , which is currently not established. As previously mentioned, crystals can be obtained in low yield from reaction mixtures where the mother liquor has been removed from the amalgam. These crystals, while complex in nature, shed some light on the reductive process. X-ray diffraction studies show that these crystals are composed of [Cf(18-crown-6)(H2O)2-x(CH3CN)Ix]I2 as shown in Figure . The structure contains a mixture of an intermediate Cf II complex, [Cf(18-crown-6)(H2O)2(CH3CN)]I2, and the Cf III parent [Cf(18-crown-6)(H2O)(CH3CN)I]I2. The key difference between these two complexes exists primarily in the apical position that can be occupied by either a H2O molecule or an iodide anion. The structure refinement shows that the occupancy of the superimposed atomic positions are 0.6865 (15) for the oxygen atom and 0.3135 (15) for the iodide anion. In short, it contains ~69%