13C Chemical Shift of N-Heterocyclic Carbenes in Coinage Metal (I) Complexes and Clusters: trans-Influence and Spin-Orbit Coupling

N-heterocyclic carbenes (NHCs) have emerged as versatile ligands in organometallic chemistry, prized for their strong σ-donating and tunable electronic properties. They stabilize diverse organometallic motifs, as well as clusters, and nanoparticles, particularly those based on the coin-age metals—Cu, Ag, and Au. Importantly, the carbene 13C NMR isotropic chemical shift (δiso) of NHC-coinage metal complexes varies significantly across these elements, reflecting the nuanced interplay of electronic and structural factors. Here, we investigate the nature of the car-bene carbon chemical shift in NHC-Au(I)-X complexes (X = H, OH, Cl, Br, I, CN, N3 and neutral ligands such as pyridine and NHC) compared to the Cu and Ag congeners. Using DFT calculations, the chemical shielding tensor (CST) components are analyzed to uncover the underlying factors influencing the carbene carbon δiso, revealing the roles of σ-donation and spin-orbit coupling in modulating chemical shifts. A clear trend is observed, where stronger σ-donor X-ligands lead to greater deshielding in both paramagnetic and spin-orbit contributions that can reach similar magnitude for Au. A correlation between spin-orbit contribution to the CST (σso) and Au-carbene bond distance highlights how trans-influence drives spin-orbit coupling and the overall chemical shift in this series of compounds. In-depth analysis of σso on each principal component shows that stronger σ-donor ligands, associated with a greater trans-influence and elongated Au-carbene bond, lead to a higher-lying NHC-Au σ-bond and lower-lying π*-bond, ultimately yielding greater deshielding and higher 13C chemical shift. This work provides insight into how structural and electronic factors govern carbene chemical shifts in NHC-based Au complexes and clusters, establishing a direct link between NMR spectroscopic descriptor and electronic structure, thus opening avenues for developing structure-activity relationships in catalysis and material science applications.