A Gold(I)-Acetylene Complex Synthesised using Single-Crystal Reactivity

Using single-crystal to single-crystal solid/gas reactivity the gold(I) acetylene complex [Au(L1)(η2-HC≡CH)][BArF 4] is cleanly synthesized by addition of acetylene gas to single crystals of [Au(L1)(CO)][BArF 4] [L1=tris-2-(4,4'-di-tert-butylbiphenyl)phosphine, ArF=3,5-(CF3)2C6H3]. This simplest gold-alkyne complex has been characterized by single crystal X-ray diffraction, solution and solid-state NMR spectroscopy and periodic DFT. Bonding of HC≡CH with [Au(L1)]+ comprises both σ-donation and π-backdonation with additional dispersion interactions within the cavity-shaped phosphine.

Classical Gold Carbonyl Complexes in Tetrahedral and Trigonal-Planar Settings

A unique four-coordinate, classical gold(I)-carbonyl complex with substantial backdonation from gold has been isolated by using a B-methylated and fluorinated tris(pyridyl)borate chelator. Its lighter silver(I) and copper(I) analogs enabled a study of trends in the coinage-metal family. The B-arylated ligand version also afforded a gold-carbon monoxide complex that displays a notably low C-O stretch value, but with trigonal planar geometry at the gold. A computational analysis shows that the AuI -CO bonds of these tris(pyridyl)borate ligand-supported molecules consist of electrostatic attraction, OC→Au σ-donation, and very significant Au→CO π-back-bonding components. The latter is responsible for the observed C-O stretching frequencies, which are lower than in free CO.

Gold(I)-N-Heterocyclic Carbene Synthons in Organometallic Synthesis

The prominent role of gold-N-heterocyclic carbene (NHC) complexes in numerous research areas such as homogeneous (photo)catalysis, medicinal chemistry and materials science has prompted organometallic chemists to design gold-based synthons that permit access to target complexes through simple synthetic steps under mild conditions. In this review, the main gold-NHC synthons employed in organometallic synthesis are discussed. Mechanistic aspects involved in their synthesis and reactivity as well as applications of gold-NHC synthons as efficient pre-catalysts, antitumor agents and/or photo-emissive materials are presented.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

Vibrational Properties of CO Adsorbed on Au Single Atom Catalysts on TiO2(101), ZrO2(101), CeO2(111), and LaFeO3(001) Surfaces: A DFT Study

The nature and local environment of Au single atoms supported and stabilized on four different oxides is studied by means of DFT + U calculations using CO as probe molecule and its stretching frequency, ωe, as a fingerprint of the site where the Au atom is bound. Four oxides are considered, anatase TiO2, tetragonal ZrO2, cubic CeO2, and a perovskite LaFeO3. In this latter case a recently reported experimental study has detected a stretching mode for CO adsorbed on Au1/LaFeO3 of 2215 cm−1, with a large blue shift, ∆ω(CO) = 72 cm−1 with respect to free CO. In order to identify the Au adsorption site that can give rise to this large blue-shift we have considered five cases: (a) Au replacing a lattice cation, (Au)subM; (b) Au replacing a lattice O anion, (Au)subO; (c) Au adsorbed on the surface, (Au)ads; (d) Au bound to an extra O atom on the surface, (AuO)ads, or (e) Au bound to two extra O atoms on the surface, (AuO2)ads. It turns out that the correct reproduction of ∆ω for CO adsorbed on positively charged gold, Auδ+, is challenging for DFT. Therefore, we have performed a comparative study of Auδ+-CO molecular compounds for which ωe(CO) is known experimentally using various kinds of DFT functionals and accurate CCSD and CCSD(T) quantum chemistry methods. Also based on this comparison we propose a tentative assignment for the observed frequency of CO adsorbed on Au1/LaFeO3 single atom catalyst.

Graphic Abstract

A dicoordinate gold(I)-ethylene complex

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold. Besides, the bonding situation of this species has been investigated by means of state-of-the-art Density Functional Theory (DFT) calculations indicating that π-backdonation plays a minor role compared with tricoordinate analogues.

The use of the exceptionally bulky tris-2-(4,4′-di-tert-butylbiphenylyl)phosphine ligand allows the isolation and complete characterization of the first dicoordinate gold(i)–ethylene adduct, filling a missing fundamental piece on the organometallic chemistry of gold.

Experimental and theoretical studies on gold(III) carbonyl complexes: reductive C,H- and C,C bond formation

The reactivity of cationic (C^C)gold(iii) carbonyl complexes was investigated. While the in situ-formed IPrAu(bph)CO+ complex (bph = biphenyl-2,2'-diyl) does not undergo a migratory insertion of CO into the neighboring gold-carbon bond, nucleophiles can attack the coordinated CO moiety intermolecularly. Water as a nucleophile initiates a CO2 extrusion combined with a reductive C,H bond formation. The rapid formation of a gold(i) species from an intermediary gold(iii) carbonyl has not been observed before and shows a significant difference in reactivity between (C^C) and (C^N^C)gold(iii) carbonyls. The latter have been reported to form stable gold(iii) hydrides via the WGS reaction. In the case of methanol acting as a nucleophile attacking the gold(iii) carbonyl, no extrusion of CO2 is observed. Instead an intermediary gold(iii) carboxyl complex forms an aryl carboxylate via reductive C-C bond elimination. Experimental and theoretical studies on the mechanism explain the observed selectivities and give new insights into the reactivity of elusive gold(iii) carbonyls.

Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12 X11 ]- (X=F, Cl, Br, I, CN)

Electrophilic anions of type [B₁₂ X₁₁ ]^- posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B₁₂ X₁₁ ]^- with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N₂ as σ-donor-π-acceptors. Temperature-dependent formation of [B₁₂ X₁₁ NG]^- indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B₁₂ X₁₁ ]^- and steric effects. The binding of CO and N₂ to [B₁₂ X₁₁ ]^- is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B₁₂ X₁₁ CO]^- and [B₁₂ X₁₁ N₂ ]^- were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N₂ stretching frequencies ν C O and ν N 2 , respectively. Observed shifts of ν C O and ν N 2 are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B₁₂ X₁₁ ]^- reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions.

Synthesis of Gold(I)-Trifluoromethyl Complexes and their Role in Generating Spectroscopic Evidence for a Gold(I)-Difluorocarbene Species

Readily prepared and bench-stable [Au(CF₃ )(NHC)] compounds were synthesized by using new methods, starting from [Au(OH)(NHC)], [Au(Cl)(NHC)] or [Au(L)(NHC)]HF₂ precursors (NHC=N-heterocyclic carbene). The mechanism of formation of these species was investigated. Consequently, a new and straightforward strategy for the mild and selective cleavage of a single carbon/fluorine bond from [Au(CF₃ )(NHC)] complexes was attempted and found to be reversible in the presence of an additional nucleophilic fluoride source. This straightforward technique has led to the unprecedented spectroscopic observation of a gold(I)-NHC difluorocarbene species.

Synthesis and characterization of N-heterocyclic carbene-MOEt2 complexes (M = Cu, Ag, Au). Analysis of solvated auxiliary-ligand free [(NHC)M]+ species

We report the synthesis, characterization and computational analysis of coinage metal-ether complexes supported by N-heterocyclic carbenes (NHC), SIPr and Et2CAAC. The related water adducts are also included. The [(NHC)M]+(M = Cu, Ag, Au) species show the noteworthy ability to bind Et2O and H2O. This interaction towards Et2O and H2O is partly ascribed to a σ-hole bonding with an almost linear disposition, taking advantage of the enhanced σ-hole potential evaluated for such [(NHC)M]+ species. This enhanced ability is larger than those found for non-covalent interactions involving main group species.

Mercury(II) Complexes of Anionic N-Heterocyclic Carbene Ligands: Steric Effects of the Backbone Substituent

Mercury(II) complexes (Me-maloNHC_Dipp)HgCl (1b), (t-Bu-maloNHC_Dipp)HgCl (2b) and (t-Bu-maloNHC_Dipp)HgMe (2c) supported by anionic N-heterocyclic carbenes have been obtained in good yields from the reaction of the potassium salt of N-heterocyclic carbene ligand precursors and mercury(II) salts, HgCl₂ and MeHgI. These molecules have been characterized by ¹H-NMR, ¹³C-NMR and IR spectroscopy and elemental analysis. X-ray crystal structures of 1b and 2b are also presented. Interestingly, complex 1b is polymeric {(Me-maloNHC_Dipp)HgCl}_n in the solid state, as a result of inter-molecular Hg-O contacts, and features rare three coordinate mercury sites with a T-shaped arrangement, whereas the (t-Bu-maloNHC_Dipp)HgCl (2b) is monomeric and has a linear, two-coordinate mercury center. The formation of T-shaped structure and the aggregation of complex 1b is attributable to the reduced steric demand of the N-heterocyclic carbene ligand backbone substituent.

Syntheses and Reactivity of New Zwitterionic Imidazolium Trihydridoborate and Triphenylborate Species

In this study, four new N-(alkyl/aryl)imidazolium-borates were prepared, and their deprotonation reactions were investigated. Addition of BH₃•THF to N-benzylimidazoles and N-mesitylimidazoles leads to imidazolium-trihydridoborate adducts. Ammonium tetraphenylborate reacts with benzyl- or mesityl-imidazoles with the loss of one of the phenyl groups yielding the corresponding imidazolium-triphenylborates. Their authenticity was confirmed by CHN analysis, ¹H-NMR, ¹³C-NMR, ¹¹B-NMR, FT-IR spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). 3-Benzyl-imidazolium-1-yl)trihydridoborate, (HIm^Bn)BH₃, and (3-mesityl-imidazolium-1-yl)trihydridoborate, (HIm^Mes)BH₃, were also characterized by X-ray crystallography. The reactivity of these new compounds as carbene precursors in an effort to obtain borate-NHC complexes was investigated and a new carbene-borate adduct (which dimerizes) was obtained via a microwave-assisted procedure.

Synthesis and characterization of heterometallic complexes involving coinage metals and isoelectronic Fe(CO)5, [Mn(CO)5]- and [Fe(CO)4CN]- ligands

The chemistry of coinage metal ions with Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- has been explored using Mes₃P and N-heterocyclic carbene supporting ligands. A comparison of [(SIPr)Au-Fe(CO)₅][SbF₆], [(^Et2CAAC)Au-Fe(CO)₅][SbF₆] and [(Mes₃P)Au-Fe(CO)₅][SbF₆] shows that the ligand donor strength towards Au(i) follows the order Mes₃P > ^Et2CAAC > SIPr. These Fe(CO)₅ complexes show significant blue shifts in [small nu, Greek, macron]_CO bands relative to those observed for free Fe(CO)₅ as a result of it serving as a net electron donor to Au(i). Au(i) is a much stronger acceptor in (SIPr)Au-Mn(CO)₅ compared to Ag(i) in (SIPr)Ag-Mn(CO)₅. The structural details of Mes₃PAu-Mn(CO)₅ are also presented. [Fe(CO)₄CN]^- afforded CN bridged coinage metal complexes with (IPr*)Au⁺, (SIPr)Ag⁺ and (SIPr)Cu⁺ moieties, rather than molecules with direct Fe/coinage metal bonds. The computed total interaction energies indicate that both [Mn(CO)₅]^- and [Fe(CO)₄CN]^- are stronger donors toward Au(i) than Fe(CO)₅. A detailed analysis of the bonding interactions between the coinage metal ions and Fe(CO)₅, [Mn(CO)₅]^- and [Fe(CO)₄CN]^- suggests that the largest contribution comes from electrostatic attraction, while the covalent component follows the Dewar-Chatt-Duncanson model. The σ-donor interactions of these organometallic ligands with coinage metal ions are considerably stronger than the π-backbonding from the coinage metal ions.

Orbital Decomposition of the Carbon Chemical Shielding Tensor in Gold(I) N-Heterocyclic Carbene Complexes

The good performance of N-heterocyclic carbenes (NHCs), in terms of versatility and selectivity, has called the attention of experimentalists and theoreticians attempting to understand their electronic properties. Analyses of the Au(I)-C bond in [(NHC)AuL]+/0 (L stands for a neutral or negatively charged ligand), through the Dewar-Chatt-Duncanson model and the charge displacement function, have revealed that NHC is not purely a σ-donor but may have a significant π-acceptor character. It turns out, however, that only the σ-donation bonding component strongly correlates with one specific component of the chemical shielding tensor. Here, in extension to earlier works, a current density analysis, based on the continuous transformation of the current density diamagnetic zero approach, along a series of [(NHC)AuL]+/0 complexes is presented. The shielding tensor is decomposed into orbital contributions using symmetry considerations together with a spectral analysis in terms of occupied to virtual orbital transitions. Analysis of the orbital transitions shows that the induced current density is largely influenced by rotational transitions. The orbital decomposition of the shielding tensor leads to a deeper understanding of the ligand effect on the magnetic response properties and the electronic structure of (NHC)-Au fragments. Such an orbital decomposition scheme may be extended to other magnetic properties and/or substrate-metal complexes.

Organic Azide and Auxiliary-Ligand-Free Complexes of Coinage Metals Supported by N-Heterocyclic Carbenes

Organic azide complexes of copper(I) and silver(I), [(SIPr)CuN(1-Ad)NN][SbF₆], [(SIPr)CuN(2-Ad)NN][SbF₆], [(SIPr)CuN(Cy)NN][SbF₆], and [(SIPr)AgN(1-Ad)NN][SbF₆] have been synthesized by using Ag[SbF₆] and the corresponding organic azides with (SIPr)CuBr and (SIPr)AgCl (SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene). The copper and silver organic azide complexes were characterized by various spectroscopic techniques and X-ray crystallography. Group trends of isoleptic Cu(I), Ag(I), and Au(I) organic azide complexes are presented on the basis of experimental data and a detailed computational study. The ν_asym(N₃) values of the metal-bound 1-AdNNN in [(SIPr)MN(1-Ad)NN]⁺ follow the order Ag < Cu < Au. DFT calculations show that gold(I) forms the strongest bond with 1-AdNNN in this series, while silver has the weakest interaction. Furthermore, auxiliary ligand free coinage metal N-heterocyclic carbene complexes, [(SIPr)M][SbF₆], have been synthesized via metathesis reactions of (SIPr)MCl (M = Cu, Ag, Au) with Ag[SbF₆]. X-ray crystal structures of dinuclear [(SIPr)Ag]₂[SbF₆]₂ and [(SIPr)Au]₂[SbF₆]₂ are also reported. They show close metallophilic contacts. [(SIPr)Au]₂[SbF₆]₂ reacts with OEt₂, SMe₂, and CN^tBu to afford [(SIPr)Au(OEt₂)][SbF₆], [(SIPr)Au(SMe₂)][SbF₆], and [(SIPr)Au(CN^tBu)][SbF₆] adducts, respectively.

Bond dissociation energies of carbonyl gold complexes: a new descriptor of ligand effects in gold(i) complexes?

Ligand electronic effects in gold(i) chemistry have been evaluated by means of the experimental determination of M-CO bond dissociation energies for 16 [L-Au-CO]⁺ complexes, bearing L ligands widely used in gold catalysis. Energy-resolved analyses have been made using tandem mass spectrometry with collision-induced dissociation. Coupled with DFT calculations, this approach enables the quantification of ligand effects based on the LAu-CO bond strength. A further energy decomposition analysis gives access to detailed insights into this bond's characteristics. Whereas small differences are observed between phosphine- and phosphite-containing gold complexes, carbene ligands are shown to stabilize the gold-carbonyl bond much more efficiently.

Heterobimetallic Complexes Featuring Fe(CO)5 as a Ligand on Gold

Iron(0) pentacarbonyl complexes of gold(I), [Mes₃ PAu-Fe(CO)₅ ][SbF₆ ] (1) and [(IPr*)Au-Fe(CO)₅ ][SbF₆ ] (2) (Mes=2,4,6-trimethylphenyl; IPr*=1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene) have been synthesized using Mes₃ PAuCl and (IPr*)AuCl as the gold(I) precursor, AgSbF₆ halide ion abstractor, and the Lewis base Fe(CO)₅ . The Au-Fe bond lengths of these metal-only Lewis pair complexes are significantly shorter than the sum of the experimentally derived covalent radii of Au and Fe. The v̄(CO) bands of the molecules show a notable blueshift relative to those observed for free Fe(CO)₅ , indicating a substantial reduction in Fe→CO backbonding upon its coordination to gold(I) with either Mes₃ P or IPr* supporting ligands (L). The analysis of the electronic structure with quantum chemical method suggests that the Au-Fe bond consists mainly of [LAu]⁺ ←Fe(CO)₅ σ-donation and weaker [LAu]⁺ →Fe(CO)₅ π-backdonation. The donor strength of Fe(CO)₅ is similar to that of CO.

Heterobimetallic Silver-Iron Complexes Involving Fe(CO)5 Ligands

Iron(0) pentacarbonyl is an organometallic compound with a long history. It undergoes carbonyl displacement chemistry with various donors (L), leading to molecules of the type Fe(CO)_x(L)_5-x. The work reported here illustrates that Fe(CO)₅ can also act as a ligand. The reaction between Fe(CO)₅ with the silver salts AgSbF₆ and Ag[B{3,5-(CF₃)₂C₆H₃}₄] under appropriate conditions resulted in the formation of [(μ-H₂O)AgFe(CO)₅]₂[SbF₆]₂ and [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅, respectively, featuring heterobimetallic {Ag-Fe(CO)₅}⁺ fragments. The treatment of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with 4,4'-dimethyl-2,2'-bipyridine (Me₂Bipy) and Fe(CO)₅ afforded a heterobimetallic [(Me₂Bipy)AgFe(CO)₅][B{3,5-(CF₃)₂C₆H₃}₄] species with a Ag-Fe(CO)₅ bond and a heterotrimetallic [{Fe(CO)₅}₂(μ-Ag)][B{3,5-(CF₃)₂C₆H₃}₄] with a (CO)₅Fe-Ag-Fe(CO)₅ core, respectively, illustrating that it is possible to manipulate the coordination sphere at silver while keeping the Ag-Fe bond intact. The chemistry of [B{3,5-(CF₃)₂C₆H₃}₄]AgFe(CO)₅ with Et₂O and PMes₃ (Mes = 2,4,6-trimethylphenyl) has also been investigated, which led to [(Et₂O)₃Ag][B{3,5-(CF₃)₂C₆H₃}₄] and [(Mes₃P)₂Ag][B{3,5-(CF₃)₂C₆H₃}₄] with the displacement of the Fe(CO)₅ ligand. X-ray structural and spectroscopic data of new molecules as well as results of computational analyses are presented. The Fe-Ag bond distances of these metal-only Lewis pairs range from 2.5833(4) to 2.6219(5) Å. These Ag-Fe bonds are of primarily an ionic/electrostatic nature with a modest amount of charge transfer between Ag⁺ and Fe(CO)₅. The ν̅(CO) bands of the molecules with Ag-Fe(CO)₅ bonds show a notable blue shift relative to those observed for free Fe(CO)₅, indicating a significant reduction in Fe→CO back-bonding upon its coordination to silver(I).

The gold(iii)–CO bond: a missing piece in the gold carbonyl complex landscape

Surprisingly, charge-displacement (CD) analysis reveals a significant π back-donation in the gold(iii)–CO bond.

How π back-donation quantitatively controls the CO stretching response in classical and non-classical metal carbonyl complexes

The CO stretching response upon coordination to a metal M to form [(L) n M(CO)] m complexes (L is an auxiliary ligand) is investigated in relation to the σ donation and π back-donation components of the M-CO bond and to the electrostatic effect exerted by the ligand-metal fragment. Our analysis encompasses over 30 carbonyls, in which the relative importance of donation, back-donation and electrostatics are varied either through the ligand in a series of [(L)Au(CO)]0/+ gold(i) complexes, or through the metal in a series of anionic, neutral and cationic homoleptic carbonyls. Charge-displacement analysis is used to obtain well-defined, consistent measures of σ donation and π back-donation charges, as well as to quantify the σ and π components of CO polarization. It is found that all complexes feature a comparable charge flow of σ symmetry (both in the M-CO bonding region and in the CO fragment itself), which is therefore largely uncorrelated to CO response. By contrast, π back-donation is exceptionally variable and is found to correlate tightly with the change in CO bond distance, with the shift in CO stretching frequency, and with the extent and direction (C → O or C ← O) of the CO π polarization. As a result, we conclusively show that π back-donation can be an important bond component also in non-classical carbonyls and we provide the framework in which the spectroscopic data on coordinated CO can be used to extract quantitative information on the π donor properties of metal-ligand moieties.

An element through the looking glass: exploring the Au-C, Au-H and Au-O energy landscape

Gold, the archetypal "noble metal", used to be considered of little interest in catalysis. It is now clear that this was a misconception, and a multitude of gold-catalysed transformations has been reported. However, one consequence of the long-held view of gold as inert metal is that its organometallic chemistry contains many "unknowns", and catalytic cycles devised to explain gold's reactivity draw largely on analogies with other transition metals. How realistic are such mechanistic assumptions? In the last few years a number of key compound classes have been discovered that can provide some answers. This Perspective attempts to summarise these developments, with particular emphasis on recently discovered gold(iii) complexes with bonds to hydrogen, oxygen, alkenes and CO ligands.

A hexanuclear gold carbonyl cluster

The hexanuclear gold carbonyl cluster [PPh4]2[Au6(CF3)6Br2(CO)2] (4) has been obtained by spontaneous self-assembly of the following independent units: CF3AuCO (1) and [PPh4][Br(AuCF3)2] (3). The cyclo-Au6 aggregate 4, in which the components are held together by unassisted, fairly strong aurophilic interactions (Au···Au ∼310 pm), exhibits a cyclohexane-like arrangement with chair conformation. These aurophilic interactions also result in significant ν(CO) lowering: from 2194 cm-1 in the separate component 1 to 2171 cm-1 in the mixed aggregate 4. Procedures to prepare the single-bridged dinuclear component 3 as well as the mononuclear derivative [PPh4][CF3AuBr] (2) are also reported.

Gold(III)-CO and gold(III)-CO2 complexes and their role in the water-gas shift reaction

The water-gas shift (WGS) reaction is an important process for the generation of hydrogen. Heterogeneous gold catalysts exhibit good WGS activity, but the nature of the active site, the oxidation state, and competing reaction mechanisms are very much matters of debate. Homogeneous gold WGS systems that could shed light on the mechanism are conspicuous by their absence: gold(I)-CO is inactive and gold(III)-CO complexes were unknown. We report the synthesis of the first example of an isolable CO complex of Au(III). Its reactivity demonstrates fundamental differences between the CO adducts of the neighboring d (8) ions Pt(II) and Au(III): whereas Pt(II)-CO is stable to moisture, Au(III)-CO compounds are extremely susceptible to nucleophilic attack and show WGS reactivity at low temperature. The key to understanding these dramatic differences is the donation/back-donation ratio of the M-CO bond: gold-CO shows substantially less back-bonding than Pt-CO, irrespective of closely similar ν(CO) frequencies. Key WGS intermediates include the gold-CO2 complex [(C^N^C)Au]2(μ-CO2), which reductively eliminates CO2. The species identified here are in accord with Au(III) as active species and a carboxylate WGS mechanism.

What can NMR spectroscopy of selenoureas and phosphinidenes teach us about the π-accepting abilities of N-heterocyclic carbenes?

The relationship between the NMR chemical shifts of phosphinidene and selenourea compounds and the π-accepting ability of the related carbene ligands has been investigated.

The electronic nature of the interaction of NHCs with metal centres is of interest when exploring their properties, how these properties influence those of metal complexes, and how these properties might depend on ligand structure. Selenourea and phosphinidene complexes have been proposed to allow the measurement of the π-accepting ability of NHCs, independent of their σ-donating ability, via the collection of ⁷⁷Se or ³¹P NMR spectra, respectively. Herein, the synthesis and characterisation of selenoureas derived from a range of imidazol-2-ylidenes, 4,5-dihydroimidazol-2-ylidenes and triazol-2-ylidenes are documented. Computational studies are used to explore the link between the shielding of the selenium centre and the electronic properties of the NHCs. Results show that δ_Se is correlated to the energy gap between a filled lone pair orbital on Se and the empty π* orbital corresponding to the Se–NHC bond. Bond energy decomposition analysis indicated no correlation between the orbital σ-contribution to bonding and the chemical shielding, while a good correlation was found between the π-contribution to bonding and the chemical shielding, confirming that this technique is indeed able to quantify the ability of NHCs to accept π-electron density. Calculations conducted on phosphinidene adducts yielded similar results. With the link between δ_Se and δ_P and π-back bonding ability clearly established, these compounds represent useful ways in which to fully understand and quantify this aspect of the electronic properties of NHCs.

Enhanced π-backdonation from gold(I): isolation of original carbonyl and carbene complexes

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π-backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π-bond character.

A systematic investigation of coinage metal carbonyl complexes stabilized by fluorinated alkoxy aluminates

The reaction of Cu(I), Ag(I), and Au(I) salts with carbon monoxide in the presence of weakly coordinating anions led to known and structurally unknown non-classical coinage metal carbonyl complexes [M(CO)n][A] (A = fluorinated alkoxy aluminates). The coinage metal carbonyl complexes [Cu(CO)n(CH2Cl2)m](+)[A](-) (n = 1, 3; m = 4-n), [Au2(CO)2Cl](+)[A](-), [(OC)nM(A)] (M = Cu: n = 2; Ag: n = 1, 2) as well as [(OC)3Cu⋅⋅⋅ClAl(OR(F))3] and [(OC)Au⋅⋅⋅ClAl(OR(F))3] were analyzed with X-ray diffraction and partially IR and Raman spectroscopy. In addition to these structures, crystallographic and spectroscopic evidence for the existence of the tetracarbonyl complex [Cu(CO)4](+)[Al(OR(F))4](-) (R(F) = C(CF3)3) is presented; its formation was analyzed with the help of theoretical investigations and Born-Fajans-Haber cycles. We discuss the limits of structure determinations by routine X-ray diffraction methods with respect to the C-O bond lengths and apply the experimental CO stretching frequencies for the prediction of bond lengths within the carbonyl ligand based on a correlation with calculated data. Moreover, we provide a simple explanation for the reported, partly confusing and scattered CO stretching frequencies of [Cu(I)(CO)n] units.

Isolable, copper(I) dicarbonyl complexes supported by N-heterocyclic carbenes

Cationic copper(I) dicarbonyl complexes supported by N-heterocyclic carbene ligands, SIPr and IPr*, have been synthesized. [(SIPr)Cu(CO)(2)][SbF(6)] and [(IPr*)Cu(CO)(2)][SbF(6)] have a trigonal planar, three-coordinate copper atom with an average Cu-CO distance of 1.915 Å and display C-O stretching frequencies higher than that of the free CO (2143 cm(-1)). The high CO stretching frequencies suggest that the Cu(I)-CO interaction in these cationic adducts is dominated by electrostatic and OC → Cu σ-donor components. [(SIPr)Cu(CO)(2)][SbF(6)] and [(IPr*)Cu(CO)(2)][SbF(6)] readily form the corresponding [(SIPr)Cu(CO)(H(2)O)][SbF(6)] and [(IPr*)Cu(CO)(H(2)O)][SbF(6)] with loss of a CO even with traces of water, but they can be converted back to the dicarbonyl adducts using excess CO. The synthesis and structure of [(IPr*)Cu(H(2)O)][SbF(6)] are also reported. It is a two-coordinate copper adduct with a Cu-O distance of 1.874(2) Å. It reacts with excess CO to form [(IPr*)Cu(CO)(2)][SbF(6)].