Unlocking Structural Diversity in Gold(III) Hydrides: Unexpected Interplay of cis/trans-Influence on Stability, Insertion Chemistry, and NMR Chemical Shifts

The synthesis of new families of stable or at least spectroscopically observable gold­(III) hydride complexes is reported, including anionic cis-hydrido chloride, hydrido aryl, and cis-dihydride complexes. Reactions between (C^C)­AuCl­(PR3) and LiHBEt3 afford the first examples of gold­(III) phosphino hydrides (C^C)­AuH­(PR3) (R = Me, Ph, p-tolyl; C^C = 4,4′-di-tert-butylbiphenyl-2,2′-diyl). The X-ray structure of (C^C)­AuH­(PMe3) was determined. LiHBEt3 reacts with (C^C)­AuCl­(py) to give [(C^C)­Au­(H)­Cl]−, whereas (C^C)­AuH­(PR3) undergoes phosphine displacement, generating the dihydride [(C^C)­AuH2]−. Monohydrido complexes hydroaurate dimethylacetylene dicarboxylate to give Z-vinyls. (C^N^C)Au pincer complexes give the first examples of gold­(III) bridging hydrides. Stability, reactivity and bonding characteristics of Au­(III)–H complexes crucially depend on the interplay between cis and trans-influence. Remarkably, these new gold­(III) hydrides extend the range of observed NMR hydride shifts from δ −8.5 to +7 ppm. Relativistic DFT calculations show that the origin of this wide chemical shift variability as a function of the ligands depends on the different ordering and energy gap between “shielding” Au­(dπ)-based orbitals and “deshielding” σ­(Au–H)-type MOs, which are mixed to some extent upon inclusion of spin–orbit (SO) coupling. The resulting 1H hydride shifts correlate linearly with the DFT optimized Au–H distances and Au–H bond covalency. The effect of cis ligands follows a nearly inverse ordering to that of trans ligands. This study appears to be the first systematic delineation of cis ligand influence on M–H NMR shifts and provides the experimental evidence for the dramatic change of the 1H hydride shifts, including the sign change, upon mutual cis and trans ligand alternation.