gold catalysts , both homogeneous and heterogeneous , have seen a meteoric rise in importance in the last three decades , and interest continues unabated . this has been amply documented , for example , in special editions of major review journals . for organic transformations , gold in the oxidation state + i is overwhelmingly used as the catalyst , as documented in recent reviews concerning synthetic applications . important aspects in homogeneous gold(i ) catalysis include , inter alia , the role of oxidation states , silver activators , ligand structure and the role of couteranions , the formation of heterocycles , the nature of gold carbene intermediates , the influence of aurophilic interactions on au(i ) catalysis , silver - free gold catalysts , as well as the important role of counterions and their effect on chemo- and regioselectivity and of chiral brnsted acids in gold catalysis . gold(iii ) catalysts have also found widespread applications in synthetic methodology , most commonly in the form of simple gold salts such as aucl3 as catalysts or catalyst precursors . while such catalysts can be fast and efficient , as schmidbaur pointed out in a recent review , the nature and oxidation state of the catalyst is in most cases difficult to determine and remains a matter of conjecture . it is the most electronegative metallic element , with a pauling electronegativity value close to that of carbon and hydrogen ( au : 2.54 ; c : 2.55 ; h : 2.20 ) . as gold(i ) , with d configuration , it is akin to a main group element , while as gold(iii ) it behaves as a transition metal and displays a coordination chemistry which mirrors that of other heavy - metal d ions . the 3 row elements platinum , gold and mercury are strongly affected by relativistic effects , which cause an expansion of the 5d orbitals and a contraction of the 6s shell and reach their maximum for gold . there are certainly many chemical and structural similarities between the d systems platinum(ii ) and gold(iii ) , which have led to platinum and gold catalysts often being compared side - by - side . numerous reaction mechanisms and intermediates proposed for gold catalysed reactions have been drawn up in analogy to organometallic palladium or platinum catalysts . however , there are also significant differences between those two neighbours in the periodic table . most prominent are the redox potentials , which are very much more positive for gold than for platinum ( values under standard conditions , 1 m aqueous acid , 25 c):au / au  1.52 vau / au  1.36 vau / au  1.83 vpt / pt  1.19 v gold(iii ) is therefore very much more readily reduced than platinum(ii ) , to either au(i ) or au(0 ) , and the formation of purple gold nanoparticles during the course of unsuccessful reactions is only too familiar to practitioners of gold chemistry . the redox chemistry of gold is illustrated , for example , by the ability of  naked  solvated au ions to oxidatively cleave benzylic c  h bonds of arenes with an oxidation potential of up to 1.82 v , and even the c c bonds of diarylmethanes , to give carbocation intermediates , with metallic gold being generated in the process . oxidation catalysts based on mixtures of aucl and the diketiminate anion [ hc(mec 
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	 nc6h3pr 
 2)2 ] have been reported , the assumption being that under such conditions au(i ) diketiminato chelate complexes are generated in situ as catalytically active species . however , it was shown subsequently that even at 0 c au(i ) and [ hc(mecnar)2 ] anions lead to the immediate reduction to metallic gold , which would seem to suggest that the observed oxidation catalysis is in all probability effected by gold nanoparticles . in other organic transformations , reduction of au(iii ) vinyl intermediates to au(i ) could be demonstrated , while the formation of gold nanoparticles could be excluded by x - ray absorption spectroscopy ( xas ) methods . on the other hand , there are also notable cases where au(iii ) vinyl reaction intermediates of the type [ ( vinyl)alcl3 ] and [ ( vinyl)2aucl2 ] , could be isolated and structurally characterised . the particular bonding and redox characteristics of gold are of course reflected in the reactivity of the types of intermediates that , by analogy to other metal - catalysed reactions , are assumed to be involved in catalysis : complexes with alkenes and alkynes , co , metal hydrides and alkyls are all quite reasonable proposals , but , in the case of gold , how much is really known about such compounds ? for which ligands or ligand combinations , and for which gold oxidation states , is it realistic to assume the existence , or indeed non - existence , of such species ? gold , as the embodiment of a noble metal , had long been regarded as unreactive and chemically uninteresting ; it is therefore a late - comer to catalysis . this means that many aspects of fundamental organometallic chemistry , which after all forms the basis of our understanding of catalytic mechanisms , remained unexplored for gold . the most dramatic example of this are olefin complexes : whereas platinum(ii ) provided the first alkene complex of any transition metal , zeise 's salt k[ptcl3(-c2h4 ) ] , reported in 1827 , ethylene complexes of the isoelectronic au(iii ) ion are conspicuous by their absence ( scheme 1 ) . similarly , hydride complexes of platinum(ii ) have been known for over 50 years , whereas those of au(iii ) remained unknown . this brief perspective presents some recent results on the identification and characteristics of some key types of catalysis - relevant gold complexes and tries to highlight the similarities and differences in bonding and reactivity of species with au as will be seen , in some cases these gold complexes display a reactivity pattern that is rather different from that of most other transition metal compounds . the chemistry of gold hydrides and compounds with hydrogen bonds to gold have recently been the subject of a comprehensive review . early attempts to generate gold hydride complexes and to stabilise gold(iii ) with complex anions like bh4 
  and alh4 
  at 120 c were reported by wiberg ; the result in all cases was reduction to metallic gold . the formation of gold hydrides and their stability is of course particularly interesting in connection with catalytic cycles since h - transfer , -h elimination or m  c bond hydrogenolysis are common reaction steps in transition metal catalysis . the first examples of isolable gold hydrido species were binuclear metal complexes ( co)5m(-h)aupph3 ( m = cr , mo , w ) and lau(-h)mln [ mln = irh2(pph3)3 or pt(c6cl5)(pet3)2 ] ; all these bimetallic compounds show strongly shielded hydride nmr resonances between  2 and 6 ppm ( in contrast to terminal au hydrides of gold in oxidation states i , ii and iii could be identified by vibrational spectroscopy in the reactions of laser - ablated gold atoms condensed into frozen gas matrices at 5 k or below , including ( h2)auh , ( h2)auh3 , auh2 , auh4 
  and auh2 
 . evidence for more thermally stable hydride complexes was provided by mass spectrometry , to give ions of type 1 , and by nmr studies of the reactions of ( p^p)aucl with phme2sih , which gave binuclear h - bridged compounds 2 and 3 ( scheme 2 ) ; such hydrides are thought to be involved in gold - catalysed alcohol dehydrosilylation . the h nmr hydride signal of 2 was found at  8.29 ( j 
 ph 49.5 hz ) , and while the h signal of 3 could not be detected , the h resonance occurs at  7.0 ppm ( j 
 pd 8.4 hz ) . the first example of a mononuclear gold(i ) hydride was isolated by sadighi in 2008 , ( ipr)auh ( 4 ) , either by the reaction of ( ipr)aucl with lihbet3 , or by treating ( ipr)auobu with hsi(ome)3 ( ipr = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene ) . the hydride ligand gives rise to a signal with positive chemical shift , at  5.11 in c6d6 (  3.38 in cd2cl2 ) , with an au  h ir stretching frequency of 1976 cm (  
 au - d 1407 cm ) . the same compound was also made from ( ipr)auoh . this family of gold hydrides stabilised by n - heterocyclic carbene ( nhc ) ligands was recently increased by ring - expanded carbenes to give 5 and 6 (  
 h 3.57 and 3.15 , respectively , in c6d6 ) ( scheme 2 ) . protonation of 5 or 6 with [ h(oet2][b{c6h3(cf3)2}4 ] at 80 c gave -h cations of type 1 , indicated by a high - field shift of the hydride h nmr signal by 45 ppm ; the suspected intermediate gold dihydrogen complexes [ ( nhc)au(h2 ) ] could not be detected . hydridic  character . and indeed , solid 4 is stable to air and moisture at ambient conditions , does not react with protic solvents like t - butanol , and reacts only very slowly with benzoic acid . on the other hand , dimethyl acetylenedicarboxylate gives a trans - insertion product , and ethyldiazoacetate reacts under c  h activation ( scheme 3 ) . the hydride ligand in 4 can however be abstracted by cph3 
  to give a binuclear hydride - bridged cation of type 1 , indicated by the high - field shift of the h nmr signal to  0.42 . the mesityl - substituted analogue of 4 , ( imes)auh , shows low reactivity in the hydrodefluorination of perfluoroarenes , but the reaction is accelerated by p - dimethylaminopyridine ( dmap ) and involves a spectroscopically detectable -stacked intermediate ( scheme 4 ) . under 19 bar o2 , complex 4 undergoes slow dioxygen insertion into the au  h bond to give the hydroperoxide 7 . product 7 readily decomposes to ( ipr)auoh ( 8) , followed by condensation of 7 with 8 to give the structurally characterised peroxide 9 ( scheme 4 ) . the reaction of 4 with stable radicals was used to estimate the au  h bond energy . whereas 4 does not react with tempo (= 2,2,6,6-tetramethyl-1-piperidyl - n - oxide ) , the hydride ligand is abstracted by galvinoxyl , with formation of the crystallographically characterised galvinoxide salt 10 ( scheme 5 ) . since the o h bond dissociation energies ( bdes ) of tempoh and galvinoxyl - h are 291 kj mol and 329 kj mol , respectively , this provides an estimate of the au gold(i ) hydrides could , in principle , also be formed by -h elimination of gold(i ) alkyls , and conversely , the hydrogenation of unsaturated substrates is usually thought of as involving the -insertion of a coordinated alkene or alkyne into a metal gold(i ) is of course a d system , without suitable energetically accessible vacant orbitals for interacting with a -c  h bond , nor for the coordination of an olefinic substrate . it is therefore consistent that for ( ipr)auet a high barrier for -h elimination was found , such that this pathway becomes kinetically competitive only above the decomposition temperature of the complex ( > 200 c ) . as bertrand has shown , both -h and , much more slowly , -h abstractions from gold alkyls are possible , but only by using an external electrophile such as cph3 
 , not however by intramolecular h - transfer reactions . in line with this , calculations have suggested that for alkene hydrogenations with homogeneous gold(i ) catalysts in polar solvents like ethanol , a heterolytic , solvent - assisted h2 activation pathway prevails over a -bond metathesis - type au  c bond cleavage by an h2 molecule ( scheme 6 ) . alkene hydrogenation is thought to follow an ionic mechanism , in which a proton is delivered from the solvent to the alkene . the rate of alkene hydrogenation increases therefore as the stability of the resulting carbocation increases , e.g. cyclohexene > ethene . whatever the detailed energetics of these solvent - assisted mechanisms , it is clear that for gold catalysts the ionic reaction profiles are always energetically favoured over the homolytic pathway . in addition to gold(i ) catalysts , gold(iii ) catalyst precursors are also frequently employed in organic transformations . however , in the large majority of cases the fate of the gold species , the oxidation state and the coordination environment of the resulting gold species under such conditions are a matter of conjecture . the facile reduction of au(iii ) to au(i ) suggests that the active species formed in situ are most probably gold(i ) , particularly if catalysts with labile ligands are used , such as aucl3 . the inadvertent formation of catalytically active gold nanoparticles is also not easy to rule out . hydride complexes such as haucl2 are occasionally invoked in catalytic cycles , but as yet no such species has actually been identified . schiff - base gold(iii ) complexes , supported and unsupported , proved to be potent catalysts for the hydrogenation of diethyl itaconate , comparable in activity to palladium catalysts . the process of hydrogen activation ( heterolytic , ethanol - assisted ) and the likely intermediacy of au(iii ) hydride species was explored by computational methods . square - planar au(iii ) hydride and hydrido - ethylene complexes are energetically favoured ; the alternative formation of a 5-coordinate [ ( o^n^n)auh(c2h4 ) ] species was disfavoured by an increased energy barrier ( scheme 7 ) . this confirms the general picture that , although au(iii ) complexes with coordination numbers 5 and 6 are not unknown , the square planar geometry is strongly preferred . there was , however , no experimental evidence that the postulated species could actually be detected . polydentate ligands offer the possibility of stabilising complexes kinetically and may also shift the redox potentials sufficiently to allow isolation and characterisation of such elusive hydride species . cyclometallated mono- and diarylpyridine ligands provide particular stability ; e.g. 2,6-diphenylpyridine derivatives form c^n^c bonded pincer complexes with au(iii ) . an example is the gold hydroxide 11 which has proved to be a highly versatile starting material ; for example , it undergoes aryl transfer reactions with boronic acids in toluene under neutral conditions and c  h activates fluoroarenes , to give photoluminescent gold aryl complexes in essentially quantitative yields . surprisingly , the reaction of 11 with  superhydride  , lihbet3 , at 78 c followed by stirring at room temperature proceeds without reduction of the metal centre and gives the gold hydride 12 as a yellow crystalline complex , the first example of an isolable au(iii ) hydrido complex ( scheme 8) . 
  the complex was crystallographically characterized and the gold - bound h atom was located in the electron density map . the infrared spectrum of the compound shows a sharp  
 au this is higher than the value for ( ipr)auh ( 1976 cm ) but comparable to the frequencies assigned to auh and ( h2)auh in an argon matrix ( 2226.6 and 2173.6 cm , respectively ) . in the h nmr spectrum the hydride ligand gives rise to a broad singlet at  6.51 in cd2cl2 and at  5.73 in c6d6 . the deuteride shows a h nmr resonance at  6.58 ( cd2cl2 ) . 
  the observed au h h nmr chemical shift for 12 is in line with theoretical predictions ( taking relativistic effects into account ) , which show that systems with fully filled d - shell are dominated by deshielding spin  orbit coupling and signals are therefore found at low field , ( e.g. the au(i ) complexes 46 ) , whereas for d systems the spin orbit coupling is strongly shielding , resulting in hydride resonances high - field of tms , as is indeed observed . the chemical shift therefore does not allow any conclusions to be drawn concerning the polarity or the au(iii ) hydride 12 is stable to air and moisture at ambient conditions , an indication of the highly covalent character of the au the compound was found to be moderately light sensitive , and exposing ch2cl2 solutions to sunlight for a few hours leads to chloride abstraction from the solvent , with formation of ( c^n^c)aucl . complex 12 does not reduce co2 or benzaldehyde in the dark or under photolytic conditions , and attempted insertion reactions with ethylene , 3-hexyne , phenylacetylene , and even dimethyl acetylenedicarboxylate were unsuccessful . however , it does react with allenes under 1,2-insertion to give isolable au(iii ) vinyl complexes . however , treating dichloromethane or toluene solutions with the strong brnsted acid [ h(oet2)2][h2n{b(c6f5)3}2 ] at 70 c led to the disappearance of the hydride resonance , together with signals that resembled the product of cleavage of one of the au  phenyl bonds , [ as for example in ( hc  n^c)au(c6h4f)(o2ccf3 ) ] . similarly , upon reacting the deuteride 12-d with [ h(oet2)2][h2n{b(c6f5)3}2 ] or trifluoroacetic acid at 70 c gave no evidence of the characteristic triplet of hd . c(phenyl ) bond in preference to the hydride , in agreement with the covalent nature of the au a more unusual reaction of 12 occurs if excess ( c^n^c)auoh is present : water is eliminated in a slow reductive condensation reaction , to give the gold gold bonded dimeric au(ii ) complex 14 , a rare example of a thermally stable au(ii ) complex with an unsupported au  au bond . for comparison , the recently reported dimer [ au(cf3)2(py)]2 follows the thermodynamic trend and spontaneously disproportionates in solution to give au(i ) and au(iii ) products . by contrast , 14 can be heated to 120 c without decomposition , is stable to air and moisture in the solid state , and in some reactions turns out to be the unreactive end product . on uv irradiation no reaction was observed between 14 and elemental sulfur in toluene ( 60 c , 12 h ) or with o2 under 9 bar pressure , even though calculations suggest that o2 insertion into the gold the lack of reactivity is most probably a reflection of the steric shielding provided by the four t - butyl substituents that envelope the au  au bond . on the other hand , the reduction of the hydride 12 to the au(ii ) dimer 14 was studied by cyclic voltammetry . 12 shows three reduction waves with peak potentials at 2.00 v ( corresponding to a metal - centred au(iii  ii ) reduction ) , 2.25 v and 2.60 v vs. fc , respectively . the 2.25 v wave was tentatively assigned to the reduction of a hydride - bridged species , such as [ ( c^n^c)au  h au(c^n^c ) ] , formed from the reaction of 12 with a ( c^n^c)au radical ( scheme 9 ) . the reduction wave at 2.60 v corresponds to ligand based multi - electron processes . the reduction process of 12 differs from that of the hydroxide 11 , where no such binuclear intermediate could be identified , which suggested that under these conditions the au  o bond is more readily cleaved than the au h bonds in 11 and 12 ( neglecting differences in solvation between hydride and hydroxide ) , which showed that the au this energy difference has a number of consequences and makes gold chemistry distinctive from other metals ( vide infra ) . the oxidation of the dimer 14 ( + 0.59 v vs. fc ) was found to be irreversible at all voltage scan rates . this seems to suggest that the 1-electron oxidation product of the au(ii ) dimer is unstable and likely undergoes rapid follow  up chemistry . there was no evidence for detectable amounts of a mixed - valence radical cation intermediate [ ( c^n^c)au  au(c^n^c ) ] in the oxidation process . electrochemistry also allowed the determination of the au  au bond energy of 14 . neglecting entropic contributions , the au au bond enthalpy was estimated as 198  1 kj mol , in reasonably good agreement with dft calculations ( 225 kj mol ) . for comparison , pyykk described the au  au bond as a 6s6pz5dxy hybrid and calculated a bond energy of 255 kj mol for [ au(cf3)2(py)]2 . this suggests that the stability differences between this complex and 14 is due to kinetic rather than thermodynamic factors . heterogeneous gold catalysts are involved in a number of oxidation reactions , and the reaction pathways and the surface species that are likely to be involved have been the subject of numerous experimental and computational studies . selective oxidations and the mode of o2 activation on oxide - supported gold catalysts for co oxidation have been reviewed . a number of studies are concerned with oxygen activation and the role of au(-o)au species in co oxidation and at the au / oxide support interface . au moieties have been proposed as key species in o2 activation of supported gold catalyst supported on a variety of metal oxides . gold hydroperoxides are likely involved in the catalytic synthesis of h2o2 , as gold surface species and in catalytic methanol oxidation . evidence has also been provided for peroxide and superoxide adsorbed on anionic and cationic gold clusters , and a variety of species including hydrides , hydroxides , peroxides and hydroperoxides are thought to be involved in propene oxidation . with few exceptions these proposed mechanisms and surface intermediates key to rationalising all these processes is a detailed understanding of the gold  oxygen bond , including possible -bonding contributions and preferred coordination geometries . oxide bond has been the subject of considerable debate , notably the possibility ( or impossibility ) of forming terminal oxo species lnmo with metals in groups 911 ( the  oxo wall  ) . indeed , the only structurally confirmed example of a noble metal oxo complex is wilkinson 's oir(mes)3 ( 15 , d ) ( scheme 10 ) , and there is indirect evidence for pt 
 o ( d ) . earlier claims of an octahedral au(iii ) ( d ) oxo species with an auo double bond proved to be based on error . 
  the gold  oxygen bond in au(i ) complexes has been the subject of some detailed theoretical investigations , e.g. ( nhc)au  ocf3 ( 16 ) which , with an au  o  c angle of 118.3(4) , shows little evidence of -overlap . for ( nhc)au  o  aryl complexes of type 17 it has been argued that there is interaction between the phenolate lone electron pairs and the au  c antibonding orbital . c(aryl ) angles are indeed much wider than expected for sp hybridisation [ in the range of 121.2126(3) ] , but significant steric repulsion between the bulky nhc and aryloxide ligands must be taken into account . in contrast to gold(i ) , which prefers coordination to  soft  donors like phosphorus and sulfur , gold(iii ) has long been known as a  hard  , lewis acidic metal ion . this is probably best exemplified by the square - planar tetrahydroxoaurate anion , [ au(oh)4 ] ( 18 ) which forms numerous salts , as well as by the classical work of tobias concerning the identification of dimethylgold aquo ions [ me2au(oh2)2 ] , na[me2au(oh)2 ] and dimethylgold hydroxide 19 ( scheme 11 ) . the latter is a dimer in solution but forms a tetrameric structure in the crystal , based on square - planar me2au(-oh)2 building blocks . vicente showed that gold complexes with aryl ligands form similar compounds with au  o bonds , including phenolate ( 20 ) and hydroxide ( 21 ) derivatives . the structure of the hydroxide 22 has also been determined . 
  like other lewis acids , gold(iii ) readily forms adducts with pyridine and related n - donors , such as 2,2-bipyridyl and terpyridyl . the formation of cyclometallated ortho - arylpyridine derivatives has proved a particularly successful strategy for the preparation of gold(iii ) complexes with oxo and hydroxo ligands . scheme 11 shows a number of representative examples . while terminal auo complexes are unknown , gold(iii ) compounds with bridging oxo ligands are well established . in 23 , for example , the square - planar metal centres form part of a planar au2o2 core , with au o single - bond distances of 1.976(3 ) and 1.961(3 )  ( for r = ch2bu ) . tridentate ligands form more stable pincer complexes and have been useful in providing dicationic terpyridyl - ligated gold hydroxides such as 26 , as well as the cyclometallated n^n^c - bonded complexes 27 . 
  however , as of 2012 there were no reports of isolable gold complexes containing the types of o - ligands that were being postulated in heterogeneous gold - catalysed oxidations , such as superoxides au - oo , peroxides au we were therefore interested in the synthesis and characterisation of such compounds . given that the hydroxide ( c^n^c)auoh ( 11 ) had proved an ideal starting material for hydride synthesis ( scheme 8) , we chose this compound as the starting point for reactions with peroxides . the reaction of 11 with t - buooh readily affords the alkylperoxide 28 . similarly , treatment of 11 with h2o2 resulted in the rather delicate hydroperoxide 29 , which has a tendency to condense with excess 11 to give the peroxide 30 . such a dehydration reaction also became apparent when 11 was stirred in dry solvents ( e.g. dry acetone ) , which led to the formation of the -oxide 31 ( scheme 12 ) . subsequently a gold(i ) t - butylhydroperoxide has also been prepared by an analogous route . the crystal structure of the -oxide 31 also answers the question concerning possible -bonding contributions to the au o -bonding leads to a widening of the m  o  m angle up to 180 and m the au  o  au angle in 31 , on the other hand , is 113.5 , only slightly less acute than the tetrahedral angle of 109.5 , a rather small steric effect given the bulkiness of the two ( c^n^c)au fragments . there is therefore no evidence for any interaction between the au(iii ) centres and the lone pairs on the bridging o ligand . this is further borne out by dft calculations on 11 , which show that the lobes of the gold(iii ) d - orbitals and of the oxygen lone pairs in the relevant mo ( here homo1 ) are out of phase ( fig . attempts to generate these au peroxide species directly from o2 failed : neither the hydride 12 nor the au(ii ) dimer 14 insert o2 , even under forcing conditions , in spite of the calculated exothermic character of the reaction . the reactivity of these ( c^n^c)au(iii ) pincer complexes differs therefore from the behaviour of palladium and platinum , where o2 insertions into m the reactivity of these gold(iii ) hydride and oxide species is in keeping with the au  x bond energies . treatment of the hydroperoxides with a phosphine par3 led to selective and stepwise o - atom abstraction , with formation of the phosphine oxide ( ar = 4-c6h4r ; r = h , f , me , ome ) ( scheme 13 ) . remarkably , even the hydroxide 11 could be deoxygenated , to give the hydride 12 . h > au  o . the transformation of a metal hydroxide into a metal hydride ( in the absence of a hydride transfer agent ) is unique to au(iii ) and is not known for any other metal . for platinum(ii ) , the order of bond energies is reversed , pt  h < pt  o , and the hydroxide - to - hydride transformation is thermodynamically unfavourable . this is therefore an example of a reaction which gold can do but platinum can not . oh deoxygenation works for au(iii ) , gold(i ) hydroxides like ( ipr)auoh do not react . the hydroxide to hydride conversion is , in principle , a key step in a water splitting cycle which does not require oxidation state changes of the metal ( scheme 14 ) . however , so far it has not been possible to close the cycle because the gold hydride is too stable and does not react with acids under h2 elimination . the kinetics ( over the temperature interval 10 to 55 c , toluene ) show a secondary kinetic isotope effect k 
 h / k 
 d = 1.45 , consistent with an au  o dft studies confirmed that the oxygen is abstracted via an external attack by phosphine ( pme3 as model ) ; intermediates involving a 5-coordinate phosphine adduct of gold could not be identified . o - labelling confirmed that the phosphine oxide formed was derived from the gold hydroxide . the reaction is 1 order in both [ au ] and [ pr3 ] ; the kinetic parameters ( e 
 a = 31.4  1.7 kj mol ; h 
  = 35.02(67 ) kj mol ; s 
  = 105.9(3 ) j mol k ) show a strongly negative entropy of activation , in agreement with an associative mechanism . the rates for different phosphines with electron donating and withdrawing para - substituents follow a hammett relationship (  = 3.15 ) , with electron - donating groups such as me or ome accelerating the rates substantially . the negative  value ( 1.05 per aryl ring ) suggests that positive charge is built up on the reaction centre and is in good agreement with a zwitterionic transition state ( scheme 15 ) . 
  in summary , studies in gold(iii ) oxide systems have confirmed the absence of double - bond character and of -bonding contributions in the au  o bond . both au(i ) and au(iii ) hydrides are highly covalent species , stable to air and moisture and surprisingly resistant to weak acids . one interesting consequence of these bond energy differences was the facile o - abstraction from a gold(iii ) hydroxide to give the corresponding gold hydride , a reaction that is so far unique to au(iii ) and found neither for pt(ii ) nor for au(i ) . the chemistry of -complexes of gold in various oxidation states with alkenes and alkynes has recently been the subject of a very detailed and comprehensive review . the first alkene adducts of aucl were reported in the 1960s by chalk and httel ; the latter developed this chemistry throughout the 1970s and 80s . the advent of very non - coordinating anions made it possible to isolate complexes where au(i ) is exclusively ligated to alkenes , as in the trigonal - planar [ au(c2h4)3 ] ( 32 ) , [ au(norbornene)3 ] and tetrahedral [ au(cod)2 ] cations ( cod = 1,5-cyclooctadiene ) ( 33 ) ( scheme 16 ) . the reaction of the gold(iii ) oxide [ au2(-o)2(bipy)2 ] with alkenes proceeds with reduction to give the au(i ) bipyridyl - stabilised alkene complexes of type 34 ( bipy = variously 6,6-substituted bipyridyl ) , with alkene = styrene , p - methoxystyrene , -methylstyrene , cis - stilbene ; the complexes with norbornadiene , cod and dicyclopentadiene are alkene - bridged binuclear species . tris(pyrazolato)borates act as bidentate n - donor ligands in the zwitterionic complex 35 . 
  strongly donating phosphine and nhc ligands complexes with alkenes carrying 14 substituents are now known , including the crystallographically characterised isobutene ( l = ipr ) and tetramethylethylene derivatives ( l = bu 
 2pc6h4 - 2-ph ) . structurally similar complexes have been reported for 1,3-dienes , which are -coordinated , as well as for cyclic dienes , allenes , and vinyl ethers . an -cyclopentadiene complex of type 36 has also been characterised ( l = bu 
 2pc6h4 - 2-ph ) , and mixtures of ph3paucl and agsbf6 were found to polymerise cyclopentadiene and 1,3-cyclohexadiene . a mechanism was not suggested , although au or h initiated carbocationic diene polymerisation seems most likely . the structural chemistry of gold(i ) alkyne complexes follows the pattern set out for alkene adducts and was recently discussed in detail by schmidbaur . the involvement of au(i ) alkyne compounds in homogeneous gold catalysed alkyne reactions has been reviewed . two - coordinate adducts prevail , and complexes of cyclic alkynes tend to be the most stable due to the stronger -interactions of strained c 
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	 c bonds with metal centres it has proved possible to crystallographically characterise the 3-hexyne complex aucl(etccet ) , which shows a cc bond length of 1.224(6 )  , only marginally longer than that of a typical non - coordinated alkyne ( around 1.202  ) . in line with this very small degree of bond elongation , structural and dft studies on au(i ) complexes with cyclic alkynes l bond ( l = nhc ) is 34 times larger than d(au )  *(cc ) back - donation . these observations explain the electrophilic activation of alkynes by lau fragments in gold(i ) catalysed reactions . with silyl alkynes , desilylation has been observed , which leads to a number of structurally characterized oligonuclear complexes ( 37 ) , not dissimilar to the interactions postulated for di - gold activation of alkynes . nmr spectroscopic studies ( h  f hoesy experiments ) showed that the relative cation  anion positions in gold(i ) alkyne complexes [ ( l)au(alkyne)]bf4 
  have been found to vary , depending on the ligand structure ; the anion is located close to gold in the phosphine complex 39 but has close interactions with the alkyne ligand in the nhc derivative 40 ( scheme 18 ) . anions also play an important role in the intermolecular alkoxylation of alkynes catalysed by carbene - based gold(i ) salts , ( nhc)aux ( x = [ b{c6h3(cf3)2}4 ] , bf4 
 , otf , ots , tfa , or oac ) . the anion is involved in alcohol deprotonation , and the intermediate basicity of the ots anion gave the most efficient catalysis . in addition to the mono - alkyne adducts of aucl and [ ( l)au ] , the bis- and tris - alkyne compounds 4143 have also been characterised ( scheme 19 ) . the structures are analogues of alkene adducts , but the tris - cyclooctyne complex 42 shows a slight propeller - type twist of the ring ligands , with torsion angles of 3.0 , 6.3 and 43.9. the au c bond lengths to the cyclooctyne with the largest twist are significantly larger than the au this indicates weaker bonding of one of the alkyne ligands , and indeed on recrystallization some alkyne is lost and crystals of the bis - alkyne adduct could also be obtained . in the [ au(cyclooctyne)2 ] cation 43 the cc bonds in these compounds are slightly elongated compared to the corresponding free alkynes , with the largest elongation being displayed by the neutral aucl adduct , which can be expected to display the strongest back - donation . interestingly , in nmr studies on the reaction of the thermally unstable gold(iii ) diaryl complex [ ( c6f5)2aucl]2 at 80 to 0 c laguna et al . were able to detect some tantalising indications for the formation of an alkyne complex with the tentative structure ( c6f5)2aucl(-phcch ) (  
 h 3.76 , toluene - d 
 8) . however , on removal of the solvent above 50 c the compound decomposed to metallic gold . by contrast to alkynes , the first examples of alkene complexes of gold(iii ) have recently been reported , by two very different routes . initial attempts seemed to confirm the susceptibility of gold(iii ) alkene species to reduction ; for example , efforts to displace the ether ligands in uson 's bis(pentafluorophenyl)gold(iii ) cation [ ( c6f5)2au(oet2)2 ] with norbornene or 1,5-cyclooctadiene were unsuccessful , while chloride abstraction from [ ( c6f5)2aucl2 ] with agsbf6 in the presence of norbornene led to reductive elimination of perfluorobiphenyl and formation of [ au(norbornene)3 ] . the strategy of taking advantage of the stability of the c^n^c pincer ligand framework proved more successful . the trifluoroacetate ligand in ( c^n^c)au(oac ) ( 44 ) is easily abstracted by b(c6f5)3 in dichloromethane , and adding alkenes to this solution at 40 c resulted in the clean formation of the corresponding -alkene complexes 4547 ( scheme 20 ) . dichloromethane solutions of the cyclopentene and ethylene complexes are stable up to about 0 and 20 c , respectively , whereas solutions of the norbornene complex 47 can be handled at 20 c . removal of volatiles afforded the complexes as yellow powders which can be stored indefinitely under nitrogen . once isolated , these complexes proved stable in the solid state for hours at room temperature and in air with minimal degradation . in the absence of excess ethylene , complex 45 shows a sharp h nmr singlet at  6.29 , downfield shifted from the  5.38 signal of free c2h4 ( cd2cl2 , 40 c ) , in contrast to au(i ) and pt(ii ) ethylene complexes , which show an upfield shift of the ethylene resonance ( table 2 ) , suggestive of -electron donation which is poorly compensated by back - bonding . in the presence of excess ethylene the h nmr signals broaden on warming from 70 to 10 c , as expected for ligand exchange . the c2h4 complex shows a c{h } nmr resonance at  108.9 , up from  122.8 for the free alkene ; j 
 ch increased on coordination from 156 to 166 hz . similar j 
 ch values have been found for gold(i ) ethylene complexes , although in those cases the changes in the c nmr chemical shifts  ( c ) are much larger , of the order of 60 + ppm , for both au(i ) and pt(ii ) . the au(iii)alkene bond is evidently weaker , particularly the back - bonding contribution , than the pt(ii)alkene bond . 
 in cd2cl2 unless indicated otherwise . this remarkably low au(iii)ethylene back - bonding contribution , compared to pt(ii ) , is further underlined by the calculated rotational barriers . the energy difference for the in - plane and perpendicular ethylene orientations for 45 was estimated to be only 2.4 kj mol , less than 10% of the value for isostructural cationic platinum pincer compounds ( scheme 21 ) . this agrees with the donation / back - donation ( d / b ) ratio obtained by charge decomposition analysis , which gives a value of d / b = 3.41 for au(iii ) but d / b = 1.70 for pt(ii ) , i.e. back - donation is much more important for platinum than for gold . this general picture , i.e. the remarkably small contribution of back - bonding for au(iii ) , has since been confirmed for other systems , including chelating olefin and co complexes ( vide infra ) . cyclopentene coordination in 46 was indicated by a downfield shift of the olefinic proton signal from  5.74 to 6.3 ppm , as well as by the inequivalence of the methylene protons . there was no indication of ligand exchange , which would have resulted in the interchange of the methylene - protons ha and hb ( scheme 20 ) ; the ch2 grous of -cyclopentene are diastereotopic , and coordination is via one -face only . unlike the cyclopentene and ethylene compounds , the chch signal of the norbornene complex 47 shows only a small upfield shift from  6.02 to 5.8 ; in this case a clearer indication of cc coordination is provided by the bridgehead ch signals , which are shifted from  2.84 for free norbornene to  3.49 . 
  a different route to an au(iii ) alkene complex was developed by tilset and co - workers , by reacting the cyclometallated dimethylgold complex 48 with trifluoroacetic acid at 78 to 0 c in the presence of cod . under these conditions the phenylpyridine ligand is displaced , to give cis-[me2au(cod ) ] ( 49 ) ( scheme 22 ) . the olefinic h nmr signal is found at  6.39 , 0.8 ppm higher than the signal of free cod . the au cod bonds in the analogous platinum complex ( 1,5-cod)ptme2 ( average au : 2.389 ; pt : 2.228  ) , while the cc bond lengths in both complexes are identical within experimental error ( average au : 1.356(5 ) ; pt : 1.355(17 )  ) ; both show slight elongation compared to free 1,5-cod [ cc 1.340(3 )  ] . the au  c bond length to the two c - atoms of the same cc bond differ by over 0.04  , a consequence of conformational strains in the cod backbone . dft modelling showed that back - bonding from the metal dxz and dyz orbitals is substantially weaker for au(iii ) than for pt(ii ) ; this reinforces the bonding picture suggested by the spectroscopic results for compounds 4547 . the norbornene complex 47 was found to react with traces of moisture or with alcohols to give the corresponding hydroxy- or alkoxyalkyl complexes 50 , respectively ( scheme 23 ) . the reaction involves the protolytic cleavage of one of the au  phenyl bonds of the pincer ligand . an unusual reaction that was discovered in the course of this work is the direct insertion of ethylene into the au - trifluoroacetate bond , to give the crystallographically characterised 2-acetatoethyl complex 51 ( scheme 24 ) . the insertion proceeds slowly ( 1472 h ) at room temperature and is quantitative ; it is accelerated by agoac impurities which catalyse the acetate abstraction . the reaction is yet another expression of the thermodynamic trend in au  o and au  c bond strengths ( cf . the mechanism of ethylene insertion into gold(iii)trifluoroacetate bonds was subsequently investigated in detail by tilset and co - workers . in trifluoroacetic acid as the solvent the ethylene insertion is fast ( 530 min ) . the reaction is reversible , and there is ethylene exchange with c2d4 . with z - c2h2d2 the threo isomer 52-d2 is formed , arising from external anti attack by acetate on ethylene in the likely intermediate [ ( c^n)au(oac)(-c2h4 ) ] ; this ethylene complex itself could however not be detected . in trifluoroethanol as solvent the corresponding 2-alkoxyethyl complex 53 is formed ( scheme 25 ) . whereas alkene insertions into au  x bonds lead to functionalised alkyls , the formation of new c  c bonds requires alkene insertion into a gold  alkyl bond . these are generally not at all facile . as amgoune and bourissou were recently able to show , such a pathway becomes feasible if a coordinatively unsaturated , formally three - coordinate gold(iii ) species can be generated . this was successful in the case of the cyclometallated phosphinonaphthalene complex 54 ( scheme 26 ) . while cold ( 80 c ) mixtures of 54 with b(c6f5)3 as methide abstractor failed to react with ethylene or styrene and gave au  c6f5 species on warming , the reaction with norbornene led to the first observation of an alkene insertion into a gold(iii ) alkyl bond . the addition of neutral n - donors allowed the isolation of 55 , while chloride addition afforded the crystallographically characterised complex 56 . the syn-1,2 character of the insertion process suggests an intramolecular migratory pathway via an ( undetected ) alkene complex [ ( c^p)aume(-norbornene ) ] as the intermediate . the picture that has emerged is that au(iii ) alkene complexes can indeed be isolated and characterized , over 180 years after the first preparation of the famous pt(ii ) analogue , zeise 's salt . they differ however substantially from the alkene complexes of both au(i ) and pt(ii ) in terms of bonding and reactivity and rely almost entirely on the ( cc)au(iii ) donor interaction , with very little back - bonding contribution and very low rotational barriers . this makes gold(iii ) alkene compounds highly susceptible to nucleophilic attack , even by traces of moisture or weak nucleophiles such as trifluoroacetate in acidic media . as yet there are no examples of isolable gold(iii ) alkyne adducts , most likely because alkyne complexes rely for stability even more on -back - donation . the first carbonyl complex of gold was manchot 's aucl(co ) , first reported in 1925 . since then a number of neutral and cationic co complexes of gold(i ) have been described . the bonding of co to gold in compounds of the type [ ( l)au(co ) ] ( l = phosphine or nhc ) has recently been analysed in detail . these complexes are linear . in gold  co complexes the donation of the non - bonding electron pair on carbon dominates over back - donation , which leads to an increase of the co stretching frequency to a value higher than that of free co ( 2143 cm ) and more akin to the formyl cation hco ( 2184 cm ) , a situation fund for quite a number of highly electrophilic metal ions ( often referred to as  non - classical co complexes  , a term that is not without objections ) . for linear gold complexes [ ( l)auco ] it was shown that the  
 co stretching frequencies do not correlate readily with the familiar tolman electronic parameters of the l ligands ; instead the  
 co frequencies are mainly determined by the flux of -electron density from the l  au bond . a three - coordinate co complex with a chelating carboranyl bis(phosphine ) ligand , [ ( p^p)au  co ] ( 57 ) , has also been reported ; here the increased electron density provided by the chelating phosphine increases the -back - bonding sufficiently to give the first classical  gold co complex (  
 co = 2113 cm ) ( scheme 27 ) . the situation resembles that in the three - coordinate styrene complex 34 , which also shows stronger back - bonding than two - coordinate alkene complexes . since the metal has a full complement of d - electrons and there are no energetically accessible orbitals cis to co , au(i ) carbonyls do not participate in carbonylation catalysis . on the other hand , au co adducts are likely involved in heterogeneous gold catalysis , including the much - studied catalytic oxidation of co and of methanol . this includes surface species of tio2-supported co oxidation catalysts where gold is thought to be present as au ions ; and co adducts with  
 co = 2158 cm have been identified . however , isolable co complexes of gold in the oxidation state + iii were unknown . indeed , as early as 1905 it was reported that treating haucl4 with co leads to reduction and is in fact an excellent method for the preparation of colloidal gold nanoparticles ; this method continues to be used for this purpose . given that au(iii ) and pt(ii ) are isoelectronic and pt(ii ) provided the first transition metal carbonyl complex ever made , ptcl2(co)2 ( 1868 ) , the lack of isolable au(iii ) co complexes is rather surprising . redox potentials can of course be shifted by suitable ligands , and in spite of the reducing power of co , we found recently that co complexes of au(iii ) are in fact readily accessible . thus , treating mixtures of ( c^n^c)au(oac ) ( 44 ) and b(c6f5)3 in dichloromethane at 30 c with co generates [ ( c^n^c)au  co][(c6f5)3boac ] ( 58 ) ( scheme 28 ) . the complex can be isolated by precipitation with light petroleum as a yellow microcrystalline solid . [ ( c^n^c)au  co ] ( 58 
 - 
 c ) was obtained similarly and gives a c nmr resonance at  167.6 ( cf . the same product was obtained when [ ( c^n^c)au(c2h4)][(c6f5)3boac ] ( 45 ) was exposed to co. the co complex is thermally labile , and solutions must be kept < 10 c . the  
 co stretch of 58 was observed at 2167 cm ( 58 
 - 
 c : 2143 cm ) . this value is rather close to that of co bound to au centres on titania - supported gold mentioned above ( 2158 cm ) , which may suggest that the co bonding in 58 resembles that found in these supported catalysts . these are prominent in the lumo which , however , is separated from the homo by a large energy gap ( fig . c interaction is a single bond , according to natural bond orbital analysis , with the major contributions from gold derived from the 6s , 6py and 5dx  y molecular orbitals . the low - temperature solution ir spectra of 58 and 58 
 - 
 c were always accompanied by bands at 2338 cm for co2 and 2273 cm for co2 , respectively ( ch2cl2 ) . since the co2 must have originated from the co complex , this observation pointed to a gold - mediated water gas shift reaction due to the presence of traces of moisture condensation under the recording conditions ( 20 c ) . the water - gas shift ( wgs ) reaction ( co + h2o  co2 + h2 ; exothermic , h 
 r = 41.2 kj mol ) is catalysed by heterogeneous gold and platinum catalysts , with gold exhibiting a significantly lower activation energy than platinum . at least three mechanisms are being discussed : ( 1 ) the redox mechanism assumes reduction of the oxide support , which is then re - oxidised by h to generate h2 ; ( 2 ) the formate mechanism assumes co insertion into surface - bound au oc(o)h , which then decomposes to give auh + co2 , and ( 3 ) in the carboxylate mechanism co and auoh give au the nature of the catalytically active species is equally uncertain , and the debate concerns whether the active species are dispersed and mononuclear , whether they are gold nanoparticles or in fact metal ions , and if so , in which oxidation state . there are arguments in favour of nanoparticles and gold clusters , as well for dispersed gold ions . very recent proposals for active sites have included models such as au(o)x(oh)y(na)9 ( e.g. x = 6 , 7 ; y = 2 ) with octahedrally coordinated gold in remarkably high oxidation states , with little apparent account of the coordination chemistry and ionisation potentials of gold . the isolation of a well - defined au(iii)co complex 58 has now allowed the role of au(iii ) in the wgs reaction to be explored . by contrast , for none of the well - known au(i ) and pt(ii ) co complexes has wgs activity been reported . in order to monitor the reaction by nmr spectroscopy , ( c^n^c)auoh ( 11 ) was used as a surrogate for water ; in which case the wgs product would be ( c^n^c)au  h ( 12 ) instead of h2 , which of course has an easily recognisable nmr signature . exposing 11 to co did indeed generate 12 , via a hypothetical carboxylate intermediate ( c^n^c)au  cooh ( 59 ) ( scheme 29 ) . reactions a , b and c are part of a wgs cycle according to the carboxylate mechanism . calculations showed that overall steps a and b are 181 kj mol more favourable for gold than for platinum . once again , the high stability of the hydride 12 prevented the closure of the cycle , since it does not react with h2o or acids to liberate h2 . although intermediate 59 could not be spectroscopically identified , support for its formation comes from the analogous reaction of co with the methoxide ( c^n^c)auome . the product , ( c^n^c)au  coome ( 60 ) , is thermally stable and was isolated as white crystals ( scheme 30 ) . in an effort to explore possible formate formation as observed in heterogeneous wgs catalysis , attempts were made to hydrogenolyse 60 under 4 bar h2 . however , methylformate was not observed ; the reaction was in any case calculated to be almost thermoneutral . in addition , the reaction of 11 with co also produced the co2-bridged complex 61 , which is most probably formed in an off - cycle reaction by condensation of intermediate 59 with excess hydroxide . compound 61 was structurally characterised ; it represents the first co2 complex of gold in any oxidation state . on heating to 80120 c , 61 was found to release co2 in a reductive elimination process , to give the thermally stable au(ii ) complex 14 ( scheme 31 ) . the results show that there is a very significant difference in terms of susceptibility to nucleophilic attack between co complexes of au(iii ) and pt(ii ) . this is in spite of the fact that 58 and , for example , cis - ptcl2(co)2 have very similar  
 co frequencies , which could be taken to mean that the c atoms in the co ligands should possess similar degrees of electrophilicity . drastic differences were however revealed in the donation / back - donation ratio ( d / b ) , as estimated by charge decomposition analysis . [ ( c^n^c)au(co ) ] shows a d / b ratio of 2.26 , compared to a value of only 0.65 for ptcl2(co)2 ( scheme 32 ) . evidently back - donation , even for co complexes with high frequencies , is much more prominent in pt(ii ) than in au(iii ) . the study also demonstrates that it is the d / b ratio , rather than the ir frequency , which provides the more informative guide to chemical reactivity . in summary , this perspective has highlighted some recent advances in gold chemistry , alongside some of its peculiarities . the characteristics that set gold apart from other metals , and even from its neighbour in the periodic table , platinum , are the high electronegativity , with the resulting highly covalent character of the au  h and au  c bonds , and the order of bond strengths : au h > au  c  au  o , which runs counter to experience with other transition metals . these special characteristics need to be kept in mind in mechanistic proposals for gold catalysed reactions . the last few years have also seen the isolation of some long sought - after representatives of important compound classes ; in particular , the first examples of gold(iii ) hydride , olefin and co complexes . bonding studies have highlighted the absence of -bonding contributions to and from the metal centre , which appears to be the overriding bonding characteristic of gold(iii ) . this applies to au  o bonds ( absence of ligand - to - metal donation ) as well as to alkene and co complexes ( absence of metal - to - ligand back - bonding ) . in the case of co complexes this has remarkable consequences for reactivity , and illustrates an unexpectedly drastic difference between gold and platinum . these advances notwithstanding , many types of organometallic complexes that are commonplace for other d metals are still missing , such as complexes with labile sites cis to h , co or alkenes , also dihydrides , dicarbonyls and alkyne complexes , as well as cationic and anionic hydrides and carbonyls , hydrido and carbonyl phosphine complexes , and so forth . the synthesis of these much more reactive species will be both challenging and instructive , not least since these are the types of compounds that are likely to be realistic intermediates in catalytic cycles .