Unusual structure, bonding and properties in a californium borate

The participation of the valence orbitals of actinides in bonding has been debated for decades. Recent experimental and computational investigations demonstrated the involvement of 6 p , 6 d and/or 5 f orbitals in bonding. However, structural and spectroscopic data, as well as theory, indicate a decrease in covalency across the actinide series, and the evidence points to highly ionic, lanthanide-like bonding for late actinides. Here we show that chemical differentiation between californium and lanthanides can be achieved by using ligands that are both highly polarizable and substantially rearrange on complexation. A ligand that suits both of these desired properties is polyborate. We demonstrate that the 5 f , 6 d and 7 p orbitals are all involved in bonding in a Cf( III ) borate, and that large crystal-field effects are present. Synthetic, structural and spectroscopic data are complemented by quantum mechanical calculations to support these observations. Actinides generally form ionic compounds, however, when electron-rich ligands with large hyperpolarizabilities are used, partially covalent bonds can also form. Now a rare californium borate is shown to exhibit significant differences from other f -elements in its structure and bonding. Quantum mechanical calculations support Cf and ligand orbital interactions, also indicating partial covalent bonding.