Remarkable reversal of C-NMR assignment in d, d compared to d, d acetylacetonate complexes: analysis and explanation based on solid-state MAS NMR and computations

13 C solid-state MAS NMR spectra of a series of paramagnetic metal acetylacetonate complexes; [VO(acac) 2 ] (d 1 , S = ½), [V(acac) 3 ] (d 2 , S = 1), [Ni(acac) 2 (H 2 O) 2 ] (d 8 , S = 1), and [Cu(acac) 2 ] (d 9 , S = ½), were assigned using modern NMR shielding calculations. This provided a reliable assignment of the chemical shifts and a qualitative insight into the hyperfine couplings. Our results show a reversal of the isotropic 13 C shifts, δ iso ( 13 C), for CH 3 and CO between the d 1 and d 2 versus the d 8 and d 9 acetylacetonate complexes. The CH 3 shifts change from about −150 ppm (d 1,2 ) to roughly 1000 ppm (d 8,9 ), whereas the CO shifts decrease from 800 ppm to about 150 ppm for d 1,2 and d 8,9 , respectively. This was rationalized by comparison of total spin-density plots and computed contact couplings to those corresponding to singly occupied molecular orbitals (SOMOs). This revealed the interplay between spin delocalization of the SOMOs and spin polarization of the lower-energy MOs, influenced by both the molecular symmetry and the d-electron configuration. A large positive chemical shift results from spin delocalization and spin polarization acting in the same direction, whereas their cancellation corresponds to a small shift. The SOMO(s) for the d 8 and d 9 complexes are σ-like, implying spin-delocalization on the CH 3 and CO groups of the acac ligand, cancelled only for CO by spin polarization. In contrast, the SOMOs of the d 1 and d 2 systems are π-like and a large CO-shift results from spin polarization, which accounts for the reversed assignment of δ iso ( 13 C) for CH 3 and CO. The variation in 13 C NMR paramagnetic shifts as a function of d-electron configuration was explained by NMR shielding calculations.