Cyanide linkage isomerism in cerium(III) and uranium(III) complexes. A relativistic DFT study

International audienceThe chemistry of cyanide complexes of the f-elements has witnessed significant recent advances, showing in particular the remarkable ability of the cyanide ligand to stabilize uranium compounds in their +3 to +6 oxidation states and its capacity to adopt two distinct ligation modes, namely cyanide M−CN or isocyanide M−NC, towards trivalent f-ions. Here, we have theoretically investigated the tris(cyanide) complexes [ML$_2$X$_3$]$^{2–}$ (M = Ce$^{3+}$, U$^{3+}$; L = C$_5$H$_4$Me, N(SiMe$_3$)$_2$; X = CN, NC), using relativistic DFT computations, focusing on cyanide (CN$^−$) and isocyanide (NC$^−$) coordination competition and differences between their electronic structures. X-ray crystal data reveal distinct coordination modes of the CN ligand towards uranium and cerium metal centers. Interestingly, in the case of the cerium complexes, and contrarily to the uranium ones, the coordination mode of the cyanide ligand depends on the co-ligand L attached to the metal. The observed coordination in these tris(cyanide) complexes is driven by the binding energies of the CN$^−$/NC$^−$ ligands to the metals. The solvent is shown to play a determining role for the chosen coordination. The developed methodology can help to confirm the cyanide or isocyanide coordination mode when X-ray crystal data are not conclusive or to predict the configuration of a complex to be synthesized. Another application is in the field of lanthanide (III)/actinide (III) differentiation