13 C carbene nuclear magnetic resonance chemical shift analysis confirms Ce IV [double bond, length as m-dash]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 13 C Nuclear Magnetic Resonance computational assessment of two cerium(iv)-diphosphonioalkylidene complexes, [Ce(BIPM TMS )(ODipp) 2 ] (1, BIPM TMS = {C(PPh 2 NSiMe 3 ) 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[double bond, length as m-dash]C double bonds. Strong magnetic coupling of Ce[double bond, length as m-dash]C σ/π* and π/σ* orbitals produces strongly deshielded σ 11 values, a characteristic hallmark of alkylidenes, and the largest 13 C 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[double bond, length as m-dash]C σ- and π-bond components. This study confirms significant Ce 4f-orbital contributions to the Ce[double bond, length as m-dash]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.