How 5 f Electron Polarisability Drives Covalency and Selectivity in Actinide N-Donor Complexes

We report a series of isostructural tetravalent actinide (Th, U-Pu) complexes with the N-donor ligand N,N'-ethylene-bis((pyrrole-2-yl)methanimine) (H2L, H(2)pyren). Structural data from SC-XRD analysis reveal [An(pyren)(2)] complexes with different An-N-imine versus An-N-pyrrolide bond lengths. Quantum chemical calculations elucidated the bonding situation, including differences in the covalent character of the coordinative bonds. A comparison to the intensely studied analogous N,N '-ethylene-bis(salicylideneimine) (H(2)salen)-based complexes [An(salen)(2)] displays, on average, almost equal electron sharing of pyren or salen with the An(IV), pointing to a potential ligand-cage-driven complex stabilisation. This is shown in the fixed ligand arrangement of pyren and salen in the respective An(IV) complexes. The overall bond strength of the pure N-donor ligand pyren to An(IV) (An=Th, U, Np, Pu) is slightly weaker than to salen, with the exception of the Pa-IV complex, which exhibits extraordinarily high electron sharing of pyren with Pa-IV. Such an altered ligand preference within the early An(IV) series points to a specificity of the 5f(1) configuration, which can be explained by polarisation effects of the 5 f electrons, allowing the strongest f electron backbonding from Pa-IV (5f(1)) to the N donors of pyren.