¹³C_carbene Nuclear Magnetic Resonance Chemical Shift Analysis Confirms Ce^IV=C Double Bonding in Cerium(IV)-Diphosphonioalkylidene Complexes

Diphosphonioalkylidene dianions have emerged as highly effective ligands for lanthanide and actinide ions, and the resulting formal metal-carbon double bonds have challenged and developed conventional thinking about f-element bond
multiplicity and covalency. However, f-element-diphosphonioalkylidene complexes can be represented by several resonance forms that render their metal-carbon double bond status unclear. Here, we report an experimentally-validated ¹³C Nuclear Magnetic Resonance computational assessment of two cerium(IV)-diphosphonioalkylidene complexes, [Ce(BIPM^TMS)(ODipp)₂] (1, BIPM^TMS = {C(PPh₂NSiMe₃)₂}^2-; Dipp = 2,6-diisopropylphenyl) and [Ce(BIPM^TMS)₂] (2). Decomposing the experimental alkylidene chemical shifts into their corresponding calculated shielding (σ) tensor components verifies that these complexes exhibit Ce=C double bonds. Strong magnetic coupling of Ce=C σ/π* and π/σ* orbitals produces strongly deshielded σ₁₁ values, a characteristic hallmark of alkylidenes, and the largest ¹³C chemical shift tensor spans of any alkylidene complex to date (1, 801 ppm; 2, 810 ppm). In contrast, the phosphonium-substituent shielding contributions are much smaller than the Ce=C σ- and π-bond components. This study confirms significant Ce 4f-orbital contributions to the Ce=C bonding, provides further support for a previously proposed inverse-trans-influence in 2, and reveals variance in the 4f spin-orbit contributions that relate to the alkylidene hybridisation. This work thus confirms the metal-carbon double bond credentials of f-element-diphosphonioalkylidenes, providing quantified benchmarks for understanding diphosphonioalkylidene bonding generally.