848 J.C.S. DaltonSingle-crystal and Solution Near-ultraviolet-Visible Spectra of SodiumTetrachloroaurateBy Donald H. Brown and W. Ewen Smith,* Department of Pure and Applied Chemistry, University of Strath-Ternperature-dependent and polarised electronic spectra of a single crystal of sodium tetrachloroaurate dihydrateare reported and the l A 1 , j 1 A 4 ( d ~ ~ d = e _ , L ) , 1Alu+1B2,(dq+s), and lA1+l€o(d~-+dad,,) transitions assigned.Clyde, Glasgow G1 1XLTHE tetrachloroaurate ion and its substituted deriva-tives are often used as starting materials in studies ofgold-solution chemistry, and their near-u.v.-visiblespectra provide a simple method of monitoring the facilevalence state and co-ordination changes they undergo.The spectra are by no means understood.For example,apart from some differences in intensity, the ion [AuCl,-OH]- has a very similar spectrum to that of [AuCI,]-,~ inspite of the fact that both the geometry and electron-density distribution round the gold atom have beenchanged. We have found the same effect with a mono-substituted sulphur complex, AuCl,[SCH*C02H*CH2*CO,H]- (ref. 2) and it has also been reported for thespecies AuCI,-M~,SO.~ To facilitate our studies of theuse of gold complexes in the treatment of rheumatoidarthritis, we required a better understanding of thespectra of gold(xI1). As part of a study of the problem,we now report the results of a study of the single-crystalspectrum of sodium tetrachloroaurate dihydrate.EXPERIMENTALSodium tetrachloroaurate dihydrate crystals weregrown from solution as well-defined rhombs and two ofthese were oriented by use of precession and WeissenbergX-ray methods.The orthorhombic crystals, space groupPnma,4 have the b axis along the longest direction of therhomb. Under the optical microscope the crystals aredichroic, being colourless with polarised light propagatedalong the b axis and yellow with light propagated alongthe a or c axis. Sections with the a and c axes in theplane were yellow and showed no appreciable dichroism.The compound reacts with many organic compounds,such as wax, grease, and polishing fluids and we thereforehad some difficulty in polishing the soft crystals down tosufficiently thin sections for spectroscopic work. Thincrystals grown in situ on silica plates proved much moreB.I . Peshchevitskii, V. I . Belevatsev, and N. V. Kurbatova,Zhuv. neoyg. Khitn., 1971, 16, 1898.D. H. Brown and W. E. Smith, to be published.suitable and were used in the spectral studies. Theyhave a similar morphology to those described, and boththe morphology and dichroism indicate that the facestudied contains the b axis.Spectra were taken by use of a Displex closed-cyclerefrigerator system with a temperature range of 10-300K and a Pye Unicam SP 1800 spectrometer.300 LOO 500 x lnmI ~ X J R E 1 Spectrum of [-\uCl,]-. I\, Sodium tetrachloroauratedihydrate crystal: B, 3 x 10-4ar-sodium tetrachloroaurate in1 hi-sodium chloride at pH 4.00RESULTS .4ND DISCUSSIONThe unpolarised spectrum of a thin film crystal ofsodium tetrachloroaurate dihydrate (spectrum A, Figure1) is very similar to that of the tetrachloroaurate ion insolution (spectrum B, Figure 1 ; the solution was pre-pared with an excess of chloride ion to prevent hydro-lysis). A Raman study of the same system but withmore concentrated solutions gives spectra indicative of aion and which are almost identical in solid andsolution.We therefore consider that the sodium tetra-R. -4. Potts, J . Inorg. Nziclenv Chena., 1972, 34, 1749.H. Bonomico, G. Dessy, and A. Vaciago, Atti. Acad. naz.Liizcei, Rend. Classe Sci.fis. mat. Itat., 1965, 89, 6041976chloroaurate crystal is a reasonable model for the tetra-chloroaurate ion in solution. Dilution of the solutiondoes not appreciably affect the electronic spectrum ofthe tetrachloroaurate ion and we conclude, therefore,that it is basically due to a single ion, although wecannot rule out the possibility that some of the obviousintensity differences between solid and solution spectramay be due to interactions.The [AuClJ- units in the crystal are planar, with asymmetry very close to D4h and they are arranged inparallel planes so that the principal (2) axes are alignedalong the b axis of the c r y ~ t a l .~ If the second-nearest-neighbour environment round the gold atoms is takeninto account, the site symmetry of the gold is C, but weshall show that the spectra arise from a centrosymmetricsituation and, consequently, we interpret our results onthe basis of a tetragonal field.Since we were able toobtain sufficiently thin crystals in only one orientation,we were precluded from studying the effect of the ortho-rhombic field produced by the crystal. There was noevidence of orthorhombic or lower symmetry splittings inthe quite sharp powder Raman spectrum and no appreci-able dichroism in the ac plane. This latter piece ofevidence is weakened by the fact that there are twodifferent orientations of the [AuCl,]- units in the acplane and, consequently, the dichroic effect might beweak. Since the gold atoms do not lie a t symmetrypoints in the ac plane, the orthorhombic field would notbe centrosymmetric at the gold atoms and thus webelieve that, from a crystal as well as an ionic point ofview, the tetragonal approximation is reasonable.We carried out a CNDO calculation, including con-figuration interaction, using an established programand the results suggest a rather different ordering of thevirtual orbitals from that which is conventional (Figure2). It is the ordering of the excited states which isimportant from a spectroscopic viewpoint, and the calcu-lation consistently predicts the excited state arising froma d,+s transition to be one of the lowest-energyexcited states. Reasonable variations of orbital ex-ponents and geometry do not alter this result appreciably.The major contribution to all the molecular orbitalsinvolved in the lowest-lying states is from the atomicorbitals of the gold atom, but the ligand contribution canbe quite substantial (up to 20-30%).We label thetransitions, as others have done, by the gold metal orbitalinvolved but, since these are only symmetry representa-tions for the molecular orbitals, selection rules based onthe metal atom quantum numbers will not rigorouslyThe spectra of [PtC14Ja- and [PtBr4I2- have been moreextensively studied and we have considered the resultsobtained from these studies as well as the foregoing calcu-lation in our assignments. There are a number of dif-ferences between Au3+ and Pt2+ which must be taken intoapply.D. H. Brown, P. G. Perkins, and J. J. Stewart, J.C.S. Dalton,R. F. Kroening, R. M. Rush, D. S. Martin, and J. C. Clardy,1972, 1106 and refs. therein.Inorg. Chem., 1974, 13, 1366, and iefs.therein.account. They are: (i) the existence of lower-lyingunfilled s and p orbitals in gold as compared to platinumis indicated both by calculation and by chemistry. Ref.6 discusses and rejects the assignment of some bands inthe Pt2f spectrum to transitions from bonding orbitalsto the d z a - y a rather than from the dzy andd,,,d,,orbitals tohigher-energy antibonding orbitals. The rejected assign-ments for Pt2+ are even less likely for Au3+.(ii) The effect of the higher charge on the ion is to makethe d orbitals in Au3+ less available for bonding thanPr Py,. -.P . P"* Pr Pr"P.*MoIecular orbitals StatesFIGURE 2 Results of a CNDO calculation on [AuCIJ-. Atomicorbitals specified are the major contributors to the molecularorbitals in each case.Orbitals marked with asterisks arechloride orbitals, all others are gold. The vertical separationindicates the relative energies of the molecular orbitalsPt2+ and to increase the metal-ion content of the anti-bonding orbitals. The calculation suggests that thedza-ys orbital of Au3+ is almost a pure gold orbital. Thedw and d,,,d,, orbitals do contain appreciable ligandcharacter.(iii) The spectrum of [PtC1,]2- shows quite well-definedbands due to spin-forbidden transitions at lower energiesthan those due to spin-allowed transitions. In particu-lar, one of these transitions is to a doubly degenerateexcited state and in a magnetic circular dichroism(m.c.d.) study the A term is clearly seen.' There areno such spin-forbidden bands in the u.v.-visible spectrumof gold8 and the m.c.d.spectrum shows no low-energyA term.' It seems likely that these spin-forbiddentransitions lie close to, or higher in energy than, thecorresponding spin-allowed transitions in gold(II1).The selection rules are straightforward. The ground7 A. J. McCaffery, P. N. Schatz, and P. J. Stevens, J . Amev.8 W. R. Mason and H. B. Gray, Inov-g. Chem., 1968, 7, 56.Chem. SOC., 1968, 90, 5730850 J.C.S. Daltonstate is ~ A I , . A Laporte-allowed transition to an orbital-singlet excited state will be allowed in xy polarisationonly, whereas to an orbital doublet it will be allowed inboth xy and x polarisations. The introduction of avibronic mechanism based on an E, allowing vibrationleads to exactly the same polarisation rules for Laporte-forbidden transitions.Polarised spectra (Figure 3)show two singlet-to-singlet transitions (bands A and B).The assignment of band B to the 1Alg+1E,,(dz.,d9z+is clearly in-correct. The xy spectrum cannot be accurately re-corded to the peak of band C but this spectrum has beenobtained a number of times, using different samples,polars, sources, and sample alignments and we aresatisfied that band C has appreciable intensity in the xydirection. We consequently assign band C to a singlet-to-doublet transition.The x polarised spectrum shows no bands at lowerenergy than band C. We have checked this point withsolution-grown crystals which are sufficiently thick forband A to have an absorbance >2 and have been ableto identify no extra ban&.This result is in agreementwith comment (iii) that the spin-forbidden transitionsevident in [PtC1,I2- at lower energy than the correspond-ing spin-allowed ones are at higher energy than the spin-allowed ones in [AuClJ-, and suggests that bands A andB are both due to spin-allowed transitions.transition by previous workersbtIiILOO 500X /rimFIGURE 3 Room-temperature polarised spectra of sodiumtetrachloroaurate dihydrateA temperature-dependent study in xy polarisationshows that band B is vibronic (Figure 4). The spectraare in agreement with current theories of vibronic inten-sities showing a shift in peak position to higher energies asthe temperature falls and a drop in oscillator strength inagreement with the f = focoth(hv/2kT) expressi~n.~*~~It establishes band B as a Laporte-forbidden electricdipole transition of a centrosymmetric ion and is themain evidence in favour of our tetragonal approximation.Band C is too intense to allow such an accurate assess-ment of its temperature dependence, but it also appears400 50 0X /nmFIGURE 4 Temperature-dependent spectra of sodium tetrachloro-aurate dihydrate; temperatures A 294, B 260, C 200, D 150,E 100, F 50, and G 9 Kto be vibronic, and we consider band A to be so also,although in this case the evidence is not conclusive.We assign the bands as follows:At first sight, the assignment of band A to a strictlyforbidden transition is strange, particularly since apossible alternative (dzs-+dzn-ys) is available.Bothcalculation and intuitive reasoning, however, place thedze orbital at a lower energy in the Au3+ than in the Pt2+case, and the dzs+dz~-y~ transition is already considerablyhigher in energy than the dxZ,dgg+dz~-p transition in[PtC1J2-. The forbidden nature of the dW+s tran-sition is purely formal, since both metal orbitals mix withdifferent combinations of the same s and p ligand orbitdsand, consequently, the metal-ion quantum numbers arenot good quantum numbers for the complex. (In atypical calculation the total orbital contribution of eachchloride was 15 and 2% for the dxy and s molecular orbitalsrespectively.) We therefore consider our assignment tobe the most reasonable.These transitions are of very high intensity to beassigned to d+d and d+s transitions of Au3+ but, again,the molecular orbitals described here as dzy and dd,dvzcontain a considerable amount of s and$ ligand character,whereas the dxs-yl orbital does not. Consequently, anyA. D. Liehr and C. J. Ballhausen. Phys. Rev., 1957,106,1161.10 J. Brynestad, H. L. Yakel, and G. P. Smith, J . Chem. Phys.,1966, 45, 46621976 851distortion, including a vibronic one, away from a centro-symmetric situation will give rise to a transition involvingan appreciable figand L to metal d orbital componentand, hence, a reasonably intense transition.We thank Professor P. G. Perkins for the use of hisprogram, the Royal Society for a grant-in aid for the[a1957 Received, 20th May, 19761Displex equipment, and A. Hunter for experimental help