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

N-Heterocyclic carbenes (NHCs) are versatile ligands in organometallic chemistry, prized for their strong σ-donating and tunable electronic properties. They are used to stabilize a wide range of motifs, including clusters and nanoparticles, based in particular on coinage metals─Cu, Ag, and Au. Notably, the carbene ¹³C NMR isotropic chemical shift (δ_iso) of NHC-coinage metal complexes varies significantly across these elements, reflecting the nuanced interplay of electronic and structural factors. Here, we study the carbene carbon chemical shift in NHC-Au(I)-X complexes (X = H, OH, halides, CN, N₃, and neutral ligands such as pyridine and NHC) compared to the Cu and Ag congeners. Density functional theory calculations are used to analyze the chemical shielding tensor components, revealing that stronger σ-donor X-ligands lead to greater deshielding of δ_iso through enhanced paramagnetic contributions and, for Au, spin–orbit contributions of comparable magnitude. Moreover, a correlation between the spin–orbit contribution to the chemical shift (σ_so) and the Au-carbene bond distance highlights the critical role of trans-influence in modulating spin–orbit coupling and the overall chemical shift. Analysis of σ_so 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*-orbital, 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 descriptors and electronic structure, thus opening avenues for developing structure–activity relationships in catalysis and materials science.