Over the last decade the organometallic chemistry of gold(III) has seen remarkable advances. This includes the synthesis of the first examples of several compound classes that have long been hypothesized as being part of catalytic cycles, such as gold(III) alkene, alkyne, CO and hydride complexes, and important catalysis-relevant reaction steps have at last been demonstrated for gold, such as migratory insertion and β-H elimination reactions. Also, reaction pathways that were already known, such as the generation of gold(III) intermediates by oxidative addition and their reductive elimination, are much better understood. A deeper understanding of fundamental organometallic reactivity of gold(III) has also revealed unexpected mechanistic avenues, which can open when the barriers for reactions that for other metals would be regarded as "standard" are too high. This review summarizes and evaluates these developments, together with applications of gold(III) in synthesis and catalysis, with emphasis on the mechanistic insight gained in these investigations.

Organometallic compounds [Cp*Ir(κ 2 -N,O)X] (κ 2 -N,O = 2-pyridinecarboxylic acid, ion(−1) (1), 2,4pyridinedicarboxylic acid, ion(−1) (2), 2,6-pyridinedicarboxylic acid, ion(−1) (3); X − = Cl − (a), NO 3 − (b)) and [Ir(κ 3 -N,O,O)(1-κ-4,5-η 2 -C 8 H 13 )(MeOH)] (κ 3 -N,O,O = 2,6-pyridinedicarboxylic acid, ion(−2) (4)) are effective catalysts for the oxidative splitting of water to O 2 driven by Ce 4+ . They show similar TOF LT values (long-term TOF, 2.6−7.4 min −1 ) while TOF IN values (initial TOF) strongly depend on the catalyst (1 ≫ 2 > 3 > 4), reaching a maximum value of 287 min −1 (4.8 s −1 ) for 1a, which is the highest TOF value ever reported for an iridium catalyst. Voltammetric measurements indicate that the oxidative processes of compounds 1−4 are located at values substantially less positive than that of [Cp*Ir(bzpy)NO 3 ] (bzpy = 2benzoylpyridine; ΔE ≈ 0.2−0.3 V), taken as reference catalyst for water oxidation. In particular, compound 3, having a pendant −COOH moiety in close proximity to an iridium coordination site, as shown by the structure determined by single-crystal X-ray diffraction, exhibits several low-potential oxidation processes.

(19)F,(1)H HOESY, diffusion, and temperature-dependent (19)F and (1)H NMR studies allowed us to unequivocally probe the association between the frustrated PR3/B(C6F5)3 (1, R = CMe3; 2, R = 2,4,6-Me3C6H2) Lewis pairs in aromatic solvents. No preferential orientation is favored, as deduced by combining (19)F,(1)H HOESY and DFT results, suggesting association via weak dispersion rather than residual acid/base interactions. The association process is slightly endoergonic [K = 0.5 M(-1), ΔG(0)(298 K) = +0.4 kcal/mol for 2], as derived from diffusion NMR measurements.

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.