Given that the solution spectrum did not show the presence of Cf III and that O2 has been excluded, two possible oxidants remain: water and radiolysis. In this study, the amount of water is low with no more than six equivalents of water present in the initial reaction, and any remaining water after the CfI3•nH2O is placed under vacuum is removed via reduction by the amalgam. Thus, we conclude from six replications of this experiment and numerous others, that radiolytic oxidation occurs and is the primary cause of oxidation. This latter result is expected given the high specific activity of 249 Cf. The presence of an oxygen atom in the apical position of [Cf(18-crown-6)(H2O)2(CH3CN)]I2 does not necessarily mean that it is a water molecule. It could be a hydroxide anion instead, which would yield a Cf III complex. However, computational analyses (Extended Data Table ) show that a Cf III -OH bond distance would be 2.098 Å resulting in a weighted bond distance of 2.407 Å for Cf III -L (L = 69% OH -, 31% I -), which is much shorter than the observed distances of 2.705(11) and 2.742(2) Å for water and iodide, respectively. The same calculated weighted bond distance for Cf II -L (L = 69% H2O, 31% I -) is 2.791 Å, which is in much better agreement with experimental data. This type of substitutional disorder is well precedented in general (e.g. the origin of the bond shift isomerization misidentification), but is also known from numerous actinide halide compounds such as Pu2[B12O18(OH)4Br2(H2O)3]•0.5H2O and Cm2[B14O20(OH)7(H2O)2Cl] where Br -/H2O or Cl -/H2O disorder occurs. The present system is somewhat different from the latter of these examples and better parallels that of the Mo=O/Mo-Cl averaging where two different molecules are present in the same crystal structure on the same crystallographic sites. The structure of [Cf(18-crown-6)(H2O)2(CH3CN)]I2 consists of a nine-coordinate Cf II ion within a hula-hoop geometry. Similar to the Cf(18-crown-6)I2 structure, the "hoop" is formed from six approximately co-planar etheric oxygen atoms from the 18-crown-6 ligand. The apex is occupied by the aforementioned water molecule, and the two positions below the plane are filled by an acetonitrile molecule and a second water molecule as shown in Figure with two outersphere iodide anions rounding out the asymmetric unit. This structure type is well established from lanthanide crown ether complexes and from this present study with Am III . Spectroscopy. Room temperature solid-state absorption studies of single-crystals of Cf(18-crown-6)I2 yield broadband transitions in the UV-visible region as shown in Figure . By comparison, typical solid-state absorption spectra of Cf 3+ -containing compounds are dominated by distinct 5f→5f transitions. Based on reported absorption data for Cf 2+ binary halides, it is anticipated that compounds containing Cf 2+ would also exhibit characteristic 5f→5f transitions. However, the broadband features observed in the experimental absorption spectra reported here are, in fact, 5f→6d transitions (Figure ), and there is a notable lack of expected 5f→5f transitions. For comparison with the experimental data presented here as well as relevant literature data, Ligand-field DFT calculations were used to predict the absorption spectrum for this system (Figure ). It is worth mentioning that the 5f→6d transitions reported in the literature for CfCl2 and CfBr2 in particular are higher in energy than the 5f→6d transitions exhibited by Cf(18-crown-6)I2. The expected 5f→5f transitions in this region are not observed as a result of either (or both) the presence of a formal inversion center in the molecule or masking of the Laporte forbidden transitions by the electric dipole allowed 5f→6d transitions. and 20,368 cm -1 . These bands are produced by the excitation from different 5f orbitals to the lowest lying 6d orbitals, which in this case correspond to the 6dyz and 6dxz (Extended Data Figure ). In addition, a shoulder resolves on the peak centered at 20,368 cm -1 . This shoulder is indicative of the oxidation of Cf 2+ . Under the N2 atmosphere of the temperature-controlled stage used for solidstate measurements, this oxidation is slowed such that reliable measurements can be obtained before complete oxidation of Cf 2+ occurs. At room temperature, the oxidation occurs more rapidly under ambient conditions indicated by the increased intensity of the observed shoulder at 20,829 cm -1 and the gradual decrease of the 5f→6d transitions in the visible region as shown in Figure .