M═CR ₂ (M = U, Y) Diphosphonioalkylidene and U≡O Terminal Oxo Multiple Bond Interactions Encoded in ¹³ C and ¹⁷ O Nuclear Magnetic Resonance Chemical Shift Anisotropies: Correlations of M═C Bond Order to Chemical Shift Tensors

Diphosphonioalkylidene ligands have been central to developing f-element M═CR 2 double bond chemistry, but their wide range of C carbene 13 C NMR isotropic chemical shift (δ iso ) values has rendered their nature open to qualitative debate. Here, we report the use of 13 C and 17 O NMR spectroscopies to quantify the chemical shift anisotropies (CSAs) of two diphosphonioalkylidene complexes─[U(BIPM TMS )(O)(Cl) 2 ] ( 1, J. Am. Chem. Soc. 2012, 134, 10047–10054; BIPM TMS = {C(PPh 2 NSiMe 3 ) 2 } 2– ) and [Y(BIPM TMS )(I)(THF) 2 ] ( 2; Organometallics 2009, 28, 6771–6776)─that exhibit disparate solution/solid-state C carbene 13 C δ iso values (386.2/401.9 and 61.0/54.9 ppm, respectively) and for 1 a highly deshielded solution oxo 17 O NMR δ iso (1333.6 ppm). CSA analysis reveals encoded chemical shift tensor (CST) signatures for the U═C and U≡O bonds of 1 due to strong σ ↔ π* and π ↔ σ* magnetic shielding (σ) couplings; the U═C bond exhibits 13 C δ 11 and span (Ω) values that are both ∼ 50 ppm larger than any alkylidene complex, and the U≡O 17 O δ iso is deshielded >200 ppm compared to uranyl 17 O NMR data. CSA analysis confirms dominant 5f-orbital bonding and is consistent with an inverse- trans -influence in the C═U≡O linkage of 1 . By contrast, the 13 C NMR data for 2 exhibit signatures of Y═C double bonding tensioned with C═P ylidene contributions. For the M═CR 2 bonds, we find that the 13 C δ iso (σ iso ), δ 11 (σ 11 ), Ω, and the paramagnetic contribution to the shielding (σ p ) CSTs correlate strongly to bond order for a range of transition metals, rare earths, and actinides. This work demonstrates that CSA analysis is a powerful method for probing actinide chemical bonding, and it brackets the spectroscopic and bonding variance of diamagnetic diphosphonioalkylidene complexes.