Evaluating f-Element Bonding from Structure and Thermodynamics

The practical goal to measure and understand the thermodynamic properties of molecules and materials containing f‐elements is often achieved through indirect methods. Of the characterization tools available to inorganic chemists, few are more powerful than X‐ray crystallography. Yet for lanthanides and actinides, interpretation of a bond length is a challenging undertaking that involves a complex interplay of steric and electronic forces. In this Concept article, we perform an analysis of selected examples in which structural criteria alone have been used to draw qualitative conclusions about chemical bonding. In other instances for which such an analysis is not valid, thermodynamic information is evaluated side by side with structural data to provide reasonable interpretations of a covalent/ionic mode of bonding. A geometric variation larger than 3σ is not necessarily correlated to a change in bonding, nor is an increase in bond energy related to a bond with more covalent character. However, careful consideration of thermodynamic information can lead to reasonable interpretations of electronic structure, and may provide a more reliable benchmark for the theoretical methods which can describe f‐elements. Assessing bonding criteria: Correlating physical data, such as bond length and vibrational frequency, to electronic structure has proven difficult in systems containing f elements. A new approach to the interpretation of a bond length is proposed in consideration of the bond's energetic properties (see figure). The discussion also suggests that assignment of merit to a theoretical method based on agreement with a solid‐state bond length would be, at best, a dubious process.