Nature of Cr–NO Bonding from ¹⁵ N Solid-State NMR and X-ray Absorption Spectroscopic Signatures

Transition metal-nitrosyl species holds prime importance in chemistry, yet describing their bonding remains a great challenge. Envisioning to develop a spectroscopic descriptor that addresses nitrosyl electronic structure, 15 N solid-state NMR (ssNMR) spectroscopy, augmented by computational analysis, is herein applied to probe and disentangle the bonding in a family of low-valent Cr-nitrosyl complexes. Combined with complementary techniques (scXRD, IR, and X-ray absorption spectroscopy) pointing at a significant π-back-donation, 15 N ssNMR-assisted orbital analysis highlights the formation of filled π(Cr–N) orbitals. Bonding analysis, anchored on spectroscopic data, reveals an increased Cr–N bond order reminiscent of a potential Cr–N multiple-bond character, which revisits the known analogy between nitrosyl and imido/nitrido ligands. Further analysis hints to a non-negligible influence of ancillary ligand, where strong σ-donor enhances Cr–N π-bonding. The sensitive response to the nitrosyl bonding and the ease to record 15 N ssNMR signature and its link to the underlying electronic structures makes it privilege descriptor to understand metal-nitrosyl bonding at molecular (orbital) level.