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) (H(2) L, 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.