A new californium complex provides more evidence that later actinides can form covalent bonds to ligands rather than just engage in ionic interactions, reports a team led by Samantha K. Cary and Thomas E. Albrecht-Schmitt of Florida State University (Nat. Commun. 2015, DOI: 10.1038/ncomms7827). Historically, chemists thought that early actinides, such as uranium, would form covalent bonds but that orbital contraction in later actinides would cause a shift to ionic bonding. Prior work by Albrecht-Schmitt’s group on a californium borate complex upended that trend by showing that Cf–O bonds have significant covalent character. But the electron-rich borate ligands could have made that complex an exception. Now, the group sees similar covalency in a compound with 2,6-pyridinedicarboxylate ligands—Cf(HDPA)3 · H2O. Several of its properties suggest that the metal-ligand bonds have covalent character, with the ligands donating electron density to the 5f, 6d, 7s, and 7p orbitals of Cf. In particular, the team pins the complex’s green photoluminescence on a typically covalent ligand-to-metal charge-transfer transition. The source of the covalency is likely the relative stability of the Cf(II) oxidation state plus effects from the smaller size of Cf(III)—both characteristics that should also lead to covalent bonding in later actinides, the researchers say.