Timothy J. Bergendahi North Adarns State College North Adarns, Massachusetts 01247
The Oxidation States of Gold
The chemistry of gold involves primarily the 1+ and 3+ oxidation states. The free gold(1) ion is unstable in solution and prefers to form two or four coordinate complexes. Two coordinate examples include K+AuC12-, K+Au(CN)z-, and [(&H~)~P]~AU+CI(I), whereas K+[A~(dipy)(CN)~l-, dipy = 2.2'-dipyridine (2), and Au(diars)2+CI-, diars = ophenylenebis(dimethylarsine) (3), are four coordinate svstems. Most gold(II1) complexes are four coordinate square planar species, examples being K+AuC14-, [Au(py)2C12]+CI-, phen = 1,lO-phenpy = pyridine, and [A~(~hen)Clz]+Cl-, anthroline. There are, however, five and six coordinate gold(1II) complexes, [A~(diars)2I]~+(C104)2representing the former and [A~(diars)~I2]+C104the latter (3). I t appears that uncoordinated gold(II1) ions do not exist in solution. Several gold complexes have been prepared in which the oxidation state of gold appears to he 2+. X-Ray investigations, however, have revealed these species to he mixed valence systems which consist of equimolar portions CNn) of gold(1) and gold(II1). The CsAuC13 entity is actuin a tetragonal in a square ally CsfAu(I)C12-.Cs+Au(III)C14- (4). (C6H5CH2)2SAuCl2 field. ~lanarfield. (C6H5CH2)2SA~(I)C1.(C~H~CH2)2SA~(III)Cls (5), is Au(dmg)Cl (dmg = dimethylglyoximate) is Au(III)(drng)~+. Au(1)CIz- ( 6 ) , and Au(dbtc)CI, dhtc = N,N-di-n-hutyldiA comparison of the d orbital arrangements for Cu(l1) in a tebagonal lieu and (7). thiocarbamate, is Au(III)(dtc)z+~Au(I)Cl~A@) in a square p l a ~ field. r In addition to the mixed valence gold systems which have been characterized by X-ray methods i t is plausible that AuSO4 (8), AuO (9), AuS (lo), (CGHSCHZ)ZSAUB~~ (11). ( C ~ H ~ C H Z ) ~ S(A l lU ) , ICsAuIa ~ (12), CsAuBrs (13), (nh)AuaCla (14), (nb)nAuzCl4 (14), (nh)3AuzCla (14), nb = s s norhornadiene, (CsH5)3PAu(mnt) (151, and (C6Hs)aABergendahl and Bergendahl (18) investigated (I) with L = sAu(mnt) U5), mnt = maleonitriledithiolate = cis-1,2(n-C4Hy)zNCS2- and obtained results analogous to those (CN)2C2Sz2-, which all possess the correct stoichiometry of Vanngard and Akerstrom. I t has been impossible to isofor authentic gold(Il), are complexes which consist of equilate the Au(1I)Lz complexes due to their instability. molar amounts of gold(1) and gold(II1). In 1965 MacCragh and Koski (19) reported the isolation The 2+ oxidation state of gold was unknown until 1954. of gold(I1) phthalocyanine, A u C ~ ~ H The I~N complex ~ was At that time Rich and Tauhe (16) presented kinetic eviprepared by heating a mixture of gold monobromide and dence which suggested the formation of Au(II)Clr2- as an 1,3-diiminoisoindolin. Due to solubility problems, however, intermediate in the iron(I1) catalyzed exchange of *C1the complex was not isolated pure (19). In addition the with AuC14-. Shortly thereafter Vanngard and Akerstrom magnetic properties of the complex are not consistent with (17) observed four line electron spin resonance spectra its formulation as an authentic goldlII) species at least a t (esr) when they dissolved the diamagnetic compound room temperature. Au(III)(detc)s; detc = N,N-diethyldithiocarbamate,in henLater in 1965 Waters and Gray (20) reported the prepazene. Such esr spectra would be consistent with an authenration of ditetra-n-butylammonium bis(maleonitri1edithiotic gold(I1) species since a divalent gold complex would lato)aurate(II), [(n-C4Hy)4N]2Au(mnt)2, when they allowed possess a 5dy electronic configuration with one unpaired the corresponding gold(II1) complex to interact with tetraelectron and naturally occurring gold consists of 100% n-hutylammonium horohydride in carefully degassed tetradY6.Y7Auwhich has a nuclear spin of 312. Gold(1) and hydrofuran. Although obtained pure, the yield of the goldgold(II1) species would he diamagnetic with 5d1" and 5d8 (11) complex was low. The analytical data and the magnetic electronic configurations, respectively. properties, a four line esr spectrum, and n magnetic moA reaction pathway which involved the following equilihment of 1.85 BM, support the formulation of the system as rium was proposed by Viinngard and Akerstrom to account an authentic gold(I1) species. The gold(I1) mnt complex for their observations has also been prepared in good yield by allowing the mixed valence compounds (C~H~CH~)ZSAU(I)X.(C~H~CH~)~SAu(III)X3, X = C1 or Br, to react with the lithium salt of the mnt ligand and tetra-n-hutylammonium bromide in deeassed 90% tetrahvdrofuran/lO% methanol (21). The reaction pathway apparently involves the equilibrikn Au(1) Au(II1) = 2Au(II). The latest example of a stable gold(I1) complex was re-
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ported by Warren and Hawthorne in 1968, and involved the T-(3)-1.2-dicarbollvl lieand.. ~-(3)-1.2-BsC~H,~2. . . . . , - - -. (22). When the parent ioldfi11) complex ion, AU(III)(?~(3)-1.2-B&H11)~. , . . . - ....,which was nresent as the tetraethvlammonium salt, was allowed to interact with sodium amalgam, in degassed tetrahydrofuran, the complex [(CZH~)~N]~AU(II)(B~C~HI~)~ was isolated. Although the complex exhihited a magnetic moment of 1.79 BM a t room temperature the reported analytical data was poor and esr information was not provided. The scarcity of g o i d ( ~ lran ) be explained in terms of the e n e r n diagram shown in the figure in which copper(llJ and .. gold@l) a& compared (23). The arrangement of orbitals in the figure is justified since most copper(I1) species are not square planar but rather exhibit tetragonal distortions from octahedral symmetry (24), and evidence obtained thus far indicates that gold(I1) complexes are square planar (18). In addition the energy difference between the 5d,,, 5dy,, and 5d12-y2 orbitals for gold(I1) is nearly twice that of the 3d,,, 3dyz, and 3dX~-y2 energy difference for copper(I1) because of the slze of the 5d orbitals. The lone electron in the 5d12-yl orbital of a gold(I1) complex is of particular importance. The energy of this orbital is much higher than that of the 3d9-9 orbital of copper(I1) and i t would be easy to remove the electron resulting in the oxidation process Au(I1) - le- = Au(1II). If the electron lost in the oxidation was transferred to a second Au(I1) species a reduction of the type AuW) + l e - = Au(1) would occur. The two processes can be combined into the disproportionation reaction ZAu(I1) = Au(II1) Au(1). The 5+ oxidation state of gold was reported by Leary and Bartlett (25) in 1972. These workers allowed AuF3 and XeFz to interact with gaseous fluorine a t elevated tempera-
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732 / Journal of Chemical Education
ture and pressure and isolated a crystalline yellow-green product which was identified as [ X ~ Z F ~ ~ ] + [ A U ( V )The F~]-. crystal structure of this complex has been solved by X-ray methods (26) and has revealed the AuF6- portion t o have octahedral geometry. This would be consistent with a t2,6 configuration for Au(V). The complexes Cs+AuF6- and Oz+AuFs- have also been reported (25). The geometry of the hexafluoroaurate(V) anions in these species is most likely octahedral. Literature Clted (11 Chem. Abair, 62,157388. 12) Dothie, H.J., Llewellyn. F.J., Wardlax W , sod W&.
A. J.
E.,d
Chem. Soe, 426
""" \.."-,. (81 Sehofflsnder. F..Ann., 217.337 (lsgl). (91 Kruas, G., Ann., 237,296 (1887). (101 Hoffmann, L., end K r W G.,Bor 20,2705 (1887). (111 Smith,G.M., J.Amsr Chem. Soe., 44,1769 (1922). 1121 Ferreri,A..andTani,M. E.. Gaaz Chim. Ifal., 89.502(1959). (131 Ferrari, A,, Gmr. Chim. Itol.. 67.94 (1937).
110 Hutfel, R., Reinheimer, H., and N o m k , K., TehhedroLetf,
11,1019(1967).
(151 brgendeh1.T. J.,and Watcn. J. H., unpublished. (16) Rich, R. L,and T a u b , H., J. Phys Chcm.. 58.6 (1954). (17) VBnngard, T., and Akentrom, S.,Nature, 184,16311959). (181 Bergendahl, T. J., and Bergendahl, E. M., Inorg Chem., 11,638 11972). (19) MacCragh. A,. and Kobki, W. S.. J. Amor. Chem. Sac. 87.2496 (19651. (20) Waters. J. H., and Grey, H. B., J. Amer Chem. Sm., 81.3534 (19661. (21) Watan. J.H.,Bergcndahl,T. &and Iauis,S.,Chem. Commun., 631(1911). (22) Warr8n.L. P,nndHauthorne,M. P,LA m e l Chem Soc.. 90.4823 (1968). (231 A similar discupsion of this subject is ~ ~ n t a i n eind H u h e y , J. E.. "inorganic Chemistry: Principles of Structure and Reactiuity: Harper & Row, Publishers. New York, 1972, pp. 33G335. (24)See for instance Cotton, F. A,, and Wilkinson, G.. "Advanced Inorganic Chemistry," 2nd Ed., lnforneicnee Publishers, NPWYmk. 1967. pp. 89%W7. (261 Leauy, K.,end Bartlett,N., C h r m Commun., 903 (19721. (26) L o w , K., Zalkin. A,. and Banlett. N., Chem. Commun., 131 (19731.
