13Ccarbene nuclear magnetic resonance chemical shift analysis confirms CeIV=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-13 Nuclear Magnetic Resonance computational assessment of two cerium(iv)-diphosphonioalkylidene complexes, [Ce(BIPMTMS)(ODipp)(2)] (1, BIPMTMS = {C(PPh2NSiMe3)(2)}(2-); Dipp = 2,6-diisopropylphenyl) and [Ce(BIPMTMS)(2)] (2). Decomposing the experimental alkylidene chemical shifts into their corresponding calculated shielding (sigma) tensor components verifies that these complexes exhibit CeC double bonds. Strong magnetic coupling of CeC sigma/pi* and pi/sigma* orbitals produces strongly deshielded sigma(11) values, a characteristic hallmark of alkylidenes, and the largest C-13 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 CeC sigma- and pi-bond components. This study confirms significant Ce 4f-orbital contributions to the CeC 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.