Despite being synthesized nearly 70 years ago by Seaborg and co-workers,^1,2 the chemistry of californium (Z = 98) was recently revealed to be quite different from what we thought it would be. As noted by Hulet, Cunningham, and Nugent as much as four decades ago, something unpredicted starts to occur toward the end of the actinide series; the elements start favoring the 2+ oxidation state in molecules as well as divalent metals in their pure elemental states.^3-6 In fact, this unexpected behavior stymied the first attempts to isolate nobelium (Z = 102) via ion-exchange chromatography because nobelium is No²⁺ in aqueous media, and thus did not elute where expected based on extrapolation from is earlier neighbor mendelevium, which is trivalent in solution. In fact, nobelium is quite difficult to oxidize to No³⁺,⁷ and while one might be tempted to attribute this solely to No²⁺ attaining a 5f¹⁴ configuration, its lanthanide analog ytterbium is unstable as Yb²⁺ in aqueous media and rapidly oxidizes. Hence, other factors are clearly at play that impact the stabilization of An²⁺ (An = Cf, Es, Fm, Md, and No) cations.

The first actinide element that exhibits a metastable divalent state is californium. Whenever elements and compounds exhibit bistability, phenomena emerge that are not typically observed because many different electronic factors are competing with one another on similar energy scales and this leads to unpredictable chemical and physical behavior, and in some rare cases, quite useful properties like superconductivity. These emergent phenomena have captivated our group’s imagination since we prepared the enigmatic californium(III) borate, Cf[B₆O₈(OH)₅], in 2013 and found that its atomic and electronic structure deviated from predictions.⁸ Foremost among the odd features was broadband photoluminescence centered in the green region of the electromagnetic spectrum where one would have predicted narrow, atomic-like features characteristic of f-elements including californium. The “green glow” has been in fact known since the mid-1950’s, but was spectroscopically obscured by bremsstrahlung radiation from short-lived isotopes contained in the samples like ²⁵²Cf (t_½ = 2.64 y).⁹  Today, nearly pure samples of ²⁴⁹Cf can be prepared, and this isotope possesses a more useful half-life of 351 years; albeit it also releases among the most abundant, high-energy g-rays known for actinides making chemical experiments challenging from both reaction scale and safety perspectives.

More generally, nothing was normal about the properties of Cf[B₆O₈(OH)₅]. The magnetic susceptibility was too low, the Cf‒O bond distances were too short, and quantum mechanical theory also pointed the use of an array of frontier orbitals participating in forming chemical bonds that included 5f, 6d, 7s, and 7p in an area of the periodic table where purely ionic bonds were thought to occur. Thus, we had a significant question on our hands. Were we observing these phenomena because the band structure of Cf[B₆O₈(OH)₅] was unusual, or were we seeing manifestations of properties characteristic of californium itself? Had we just found a fundamental change in chemistry in the actinide series that the early pioneers of the field had the first glimpses of? Answering these questions profoundly changed my group’s research direction and even more satisfyingly inspired other talented radiochemistry centers throughout U.S. and the European Union to get involved.    

Our strategy was to move away from complex solids and instead synthesize small molecules that contain californium. At first these molecules contained simple, well-understood ligands like dipicolinate, but within a few years we were designing ligands to adjust specific electronic features. We found the properties observed in Cf[B₆O₈(OH)₅] also occur in simple tris-chelates like Cf(Hdpa)₃ (dpa = 2,6-pyridinedicarboxylate or dipicolinate) as well as in more complex molecules like Cf(phen)(dtc)₃ (phen = phenanthroline; dtc = N,N´-diethyldithiocarbamate).^10,11 These molecules diverge sharply in terms of their electronic structure from formally isoelectronic analogs containing Dy³⁺, but after six years of challenging experiments we have gained enough control over the factors dictating californium’s properties that we are able to force it to behave more similarly to its lanthanide analog. The frontier of californium chemistry continues to be far into the distance, but today we are pursuing new challenges such as gaining control of its redox properties and isolating compounds containing californium in other oxidation states than 3+.

1. Thompson, S. G., Street, K., Ghiorso, A. & Seaborg, G. T. Element 98. Physical Review 78, 298 (1950).
2. Thompson, S. G., Street, K., Ghiorso, A. & Seaborg, G. T. The new element californium (atomic number 98). Physical Review 80, 790 (1950).
3. Haire, R. G. & Baybarz, R. D. Crystal structure and melting point of californium metal. Journal of Inorganic and Nuclear Chemistry 36, 1295 (1974).
4. Kleinschmidt, P. D., Ward, J. W. & Matlack, G. M. Henry’s Law vaporization studies and thermodynamics of einsteinium-253 metal dissolved in ytterbium. Journal of Chemical Physics 81, 473 (1984).
5. Haire, R. G. & Gibson, J. K. The enthalpy of sublimation and thermodynamic functions of fermium. Journal of Chemical Physics 91, 7085 (1989).
6. Peterson, J. R. & Baybarz, R. D. Stabilization of divalent californium in the solid state: californium dibromide. Inorganic and Nuclear Chemistry Letters 8, 423 (1972).
7. Maly, J., Sikkeland, T., Silva, R. & Ghiorso, A. Nobelium: tracer chemistry of the divalent and trivalent ions. Science 160, 1114 (1968).   
8. Polinski, M. J., Garner, E. B., Maurice, R., Planas, N., Stritzinger, J. T., Parker, T. G., Cross, J. N., Green, T. D., Alekseev, E. V., van Cleve, S. M., Depmeier, W., Gagliardi, L., Shatruk, M., Knappenberger, K. L., Liu, G., Skanthakumar., S., Soderholm, L., Dixon, D. A., Albrecht-Schmitt, T. E. Unusual, structure, bonding and properties in a californium borate. Nature Chemistry 6, 387 (2014).
9. Conway, J. G., Gruber, J. B., Hulet, E. K., Morrow, R. J. & Gutmacher, R. G. Absorption and self-luminescence spectra of Cf³⁺ (5f⁹). Journal of Chemical Physics 36, 189 (1962).
10. Cary, S. K., Vasiliu, M., Baumbach, R. E., Stritzinger, J. T., Green, T. D., Diefenbach, K., Cross, J. N., Knappenberger, K. L., Liu, G., Silver, M. A., DePrince, A. E., Polinski, M. J., van Cleve, S. M., House, J. H., Kikugawa, N., Gallagher, A., Arico, A. A., Dixon, D. A. & Albrecht-Schmitt, T. E. Emergence of californium as the second transitional element at the actinide series. Nature Communications 6, 6827 (2015).
11. Cary, S. K., Su, J., Galley, S. S., Albrecht-Schmitt, T. E., Batista, E. R., Ferrier, M. G., Kozimor, S. A., Mocko, V., Scott, B. L., van Alstine, C. E., White, D. F. & Yang, P. A series of dithiocarbamates for americium, curium, and californium. Dalton Transactions 47, 14452 (2018).