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) (H2 L, H2 pyren). Structural data from SC-XRD analysis reveal [An(pyren)2 ] complexes with different An-Nimine versus An-Npyrrolide 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) (H2 salen)-based complexes [An(salen)2 ] displays, on average, almost equal electron sharing of pyren or salen with the AnIV , pointing to a potential ligand-cage-driven complex stabilisation. This is shown in the fixed ligand arrangement of pyren and salen in the respective AnIV complexes. The overall bond strength of the pure N-donor ligand pyren to AnIV (An=Th, U, Np, Pu) is slightly weaker than to salen, with the exception of the PaIV complex, which exhibits extraordinarily high electron sharing of pyren with PaIV . Such an altered ligand preference within the early AnIV series points to a specificity of the 5f1 configuration, which can be explained by polarisation effects of the 5 f electrons, allowing the strongest f electron backbonding from PaIV (5f1 ) to the N donors of pyren.