Electronic Structures of Late versus Early Transition Metal Imido Complexes from 15N NMR Signatures

Imido ligand is a ubiquitous motif in organometallic chemistry, serving roles spanning from ancillary ligands to reactive sites. The nature of M=N bond is highly depended on the metal centres and their d-electron configuration, with late-transition metal (LTM) imido complexes exhibiting contrasting features when compared to their early-transition metal (ETM) analogues. Envisioning to uncover general electronic descriptor for the nature of imido ligands, we computationally investigate the solid-state 15N NMR signatures of LTM imido complexes with various central metals, geometries and d-electron counts, and compare them against these of the corresponding ETM systems. The spectroscopic signatures are mostly driven by the presence of filled, π-symmetry orbitals in LTM imido complexes, suggesting the development of high-lying π(M=N) and low-lying σ/σ*(M=N) orbitals. This contrasts with what is observed for the reported ETM systems, for which high-lying σ-type orbitals determine the NMR signature. Noteworthily, Ni- and Pd-imido complexes with d8 configurations exhibit highly asymmetric nitrogen-15 NMR signature with extremely deshielded principal components, because of the presence of filled, high-lying antibonding π*(M=N) orbitals, consistent with their high reactivity. The sensitive response of 15N NMR signature to the nature of metal sites further highlights that chemical shift is a useful reactivity descriptor.