Bonding of isovalent homologous actinide and lanthanide pairs with chalcogenide donors: effect of metal f-orbital participation and donor softness

Chemistry of f-element gains importance in several fields due to the extensive scope of their applications. The aim of this study is to understand the subtle differences in bonding in the structurally similar actinides (An) and their homologous (isovalent) lanthanides (Ln) with several donors, which may lead to their covalency mediated separation. Several experimental and theoretical studies have been reported to address this aspect. However, to the best of our knowledge, a systematic study on the variation in the bonding patterns of the isovalent ‘Ln’ and ‘An’ pairs encompassing the effect of valence f-orbital participation was not attempted. In this study, the minute differences in covalent interactions of these An/Ln pairs having same number of f-electrons with chalcogenide ions of varying softness was probed using relativistic density functional theory (DFT). The f-electronic configurations of the metal ions were observed to play an important role in the f-orbital participation. [AnX] + /[LnX] + pairs with f 0 , f 7 (half-filled) configurations show resistance to f-orbital directed bonding, unlike the systems having f 5 , f 6 electronic configurations where the f-orbital directed covalency is more pronounced in the [AnX] + systems. The nature of covalency was identified to be near degeneracy-driven. This enhanced covalency was found to increase with the subsequent increase in the softness of the donor centres.