C nuclear magnetic resonance chemical shift analysis confirms Ce[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 C Nuclear Magnetic Resonance computational assessment of two cerium(iv)-diphosphonioalkylidene complexes, [Ce(BIPM)(ODipp)] (1, BIPM = {C(PPhNSiMe)}; Dipp = 2,6-diisopropylphenyl) and [Ce(BIPM)] (2).