Noble gas–metal chemical bonding : the microwave spectra, structuresand hyperÐne constants of Ar–AuF and Ar–AuBr¤

Corey J. Evans, Daryl S. Rubino† and Michael C. L. Gerry*

Department of Chemistry, T he University of British Columbia, 2036 Main Mall, V ancouver,British Columbia, Canada V 6T 1Z1. E-mail : mgerry=chem.ubc.ca

Received 30th May 2000, Accepted 21st July 2000Published on the Web 31st August 2000

The rotational spectra of the complexes ArÈAuF and ArÈAuBr have been observed in the frequency range7È22 GHz using a pulsed-jet cavity Fourier transform microwave spectrometer. Both complexes are linear andrather rigid in the ground vibrational state, with the ArÈAu stretching frequency estimated as D200 cm~1.Isotopic data have been used to calculate an structure for ArÈAuBr while for ArÈAuF only an estimation ofr0the geometry could be made. Ab initio calculations at the MP2 level of theory model the geometries andr0stretching frequencies well and predict an ArÈAu bond energy in ArÈAuF of D60 kJ mol~1. The Au nuclearquadrupole coupling constant changes signiÐcantly on complex formation, indicating extensive chargearrangement. This in conjunction with the large dissociation energy and ab initio results show that the ArÈAubonds in these complexes are weakly covalent in nature.

1 IntroductionThe vast majority of systems studied using cavity pulsed-jetFourier transform microwave (FTMW) spectrometers hasbeen van der Waals complexes. However, prior to 1999 onlytwo papers have appeared on FTMW spectroscopy of noblegas-metal containing complexes. Both systems studied, ArÈHg1 and ArÈNaCl,2 were found to be true van der Waalscomplexes, loosely bound and very Ñexible, with relativelylong ArÈmetal bonds.

Recently we have reported the pure rotational spectra of thecomplexes ArÈAgX,3 ArÈCuX4 (X\ F, Cl, Br), ArÈAuCl andKrÈAuCl.5 The noble gas (Ng)Èmetal bond lengths in thesesystems were found to be considerably shorter than those intypical van der Waals complexes. In addition, the NgÈmetalbond energies, were estimated using ab initio calculationsDe ,at the MP2 level of theory to be D23 kJ mol~1 for ArÈAgF,D47 kJ mol~1 for ArÈCuF and ArÈAuCl, and D71 kJ mol~1for KrÈAuCl. These are signiÐcantly larger than the corre-sponding value for ArÈNaCl, kJ mol~1.De(ArÈNa)D 10

For ArÈCuX4 and NgÈAuCl5 substantial changes in thenuclear quadrupole coupling constant (NQCC), eQq, of themetal were also found to occur on complex formation. Sincethe NQCC is dependent on the electric Ðeld gradient at thenucleus and is thus a good probe of its electronic environ-ment, any substantial electron rearrangement around thenucleus will be reÑected by changes in its NQCC. For thecopper complexes we found that the Cu NQCC almostdoubled on complex formation, with the di†erences goingabout one third of the way to the value in The goldCuCl2~.6complexes show however, abnormally large changes in the AuNQCC. For instance, for AuCl eQq(Au)\ ]9.63 MHz7 whilefor ArÈAuCl eQq(Au)\ [259.8 MHz.5 This is almost a factorof 25 di†erence, plus a change in sign. As with the ArÈCuXcomplexes, the change in the Au NQCC in ArÈAuCl is alsoabout one third of the way to the value in AnalysisAuCl2~.8of molecular orbital populations has shown that for the ArÈ

¤ Electronic Supplementary Information available. See http : //www.rsc.org/suppdata/cp/b0/b004352o/

CuX4 and ArÈAuCl5 complexes there is a net charge transferof D0.1 electrons from Ar to the metalÈhalide unit while forKrÈAuCl5 the net charge transfer is D0.2 electrons. There isthus strong evidence of noble gasÈmetal chemical bonding inthese complexes.

The present paper reports an extension of the work on thegold complexes to ArÈAuF and ArÈAuBr. This is the Ðrstspectroscopic investigation of any kind for these species. Rota-tional and centrifugal distortion constants have been obtainedand used to determine the geometries, and to estimate theArÈAu stretching frequency using a pseudodiatomic approx-imation. The nuclear quadrupole coupling constants of Auand Br have been determined. Ab initio calculations have beenperformed using a relativistic e†ective core potential for theAu nucleus. Results from these calculations are discussed inrelation to the bond lengths, vibration frequencies and disso-ciation energies. Overall the results reinforce the picture ofnoble gasÈmetal chemical bonding.

2 Experimental procedureThe pure rotational spectra of ArÈAuF and ArÈAuBr weremeasured using the BalleÈFlygare type FTMW spectrometer,9that has been described in detail elsewhere.10 BrieÑy, thespectrometer cell consists of a FabryÈPerot cavity containingtwo spherical mirrors 28.0 cm in diameter, radius of curvature38.4 cm, held approximately 30 cm apart. One mirror is Ðxed,while the other can be moved to tune the cavity. A 4 mmdiameter glass rod wrapped with gold foil (D98% purity, 0.5mm thickness) was held near the center of the Ðxed mirror bya stainless steel nozzle cap 5 mm from the oriÐce of a GeneralValve series-9 pulsed nozzle.11 The gold foil was irradiatedwith the second harmonic (532 nm) of a Nd : YAG laser in thepresence of a gas mixture, which was then supersonicallyexpanded in to the cavity via a 5 mm diameter nozzle. Thisarrangement results in each line being split into two Dopplercomponents since the propagation of the microwaves is paral-lel to that of the supersonic beam. The gas mixtures used forthis work contained 0.1% or in Ar at backing pres-SF6 Br2sures of 6È7 atm.

DOI: 10.1039/b004352o Phys. Chem. Chem. Phys., 2000, 2, 3943È3948 3943

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Fig. 1 A portion of the hyperÐne structure of the J \ 3È2 transitionof ArÈAuF. Experimental conditions : 0.3 ls microwave pulse width,0.1% in Ar at 7È6 bar backing pressure, 300 averaging cycles, 8kSF6transform.

Table 1 Molecular constants calculated for ArÈAuF in MHza

Parametersb ArÈAuF

B0 1775.336 880(71)D

J] 104 5.0748(137)

eQq(Au) [323.3558(45)

a Numbers in parentheses are one standard deviation in units of thelast signiÐcant Ðgure. b Correlation coefficients are listed in the sup-plementary data.

Fig. 2 A portion of the hyperÐne structure of the J \ 6È5 transitionof ArÈAu79Br. Experimental conditions : 0.3 ls microwave pulsewidth, 0.1% in Ar at 7È6 bar backing pressure, 700 averagingBr2cycles, 8k transform.

Table 2 Molecular constants calculated for ArÈAu79Br and ArÈAu81Br in MHza

Parametersb ArÈAu79Br ArÈAu81Br

B0 775.313 231(41) 765.474 424(40)D

J] 105 6.5055(390) 6.2625(381)

CI(Br)] 104 c 5.42(190) 5.76(182)

eQq(Au) [216.7088(174) [216.7196(162)eQq(Br) 428.518(41) 358.019(45)

a Numbers in parentheses are one standard deviation in units of thelast signiÐcant Ðgure. b Correlation coefficients are listed in the sup-plementary data. c Nuclear spinÈrotation coupling constant.

The frequency range of the present experiments was 7È22GHz. Frequency measurements were referenced to a Loranfrequency standard accurate to 1 part in 1010. Observed line-widths were D7È10 kHz FWHM. The lines frequencies areestimated to be accurate to ^1 kHz.

3 Quantum chemical calculationsThe geometries of ArÈAuF and ArÈAuBr were optimized atthe second order (MP2)12 level of theory usingMÔllerÈPlessetthe GAUSSIAN 98 suite of programs.13 For Au a relativisticcore potential (RECP) was used. The basis set used for goldwas a (9s/7p/6d/3f ) Gaussian basis set contracted to (8s/4p/5d/3f ).14,15 For F we used the simple 6-311G** basis set and forBr we used the aug-cc-pVTZ basis.16 For Ar we used thecc-pVTZ basis set.16 All structures were constrained to alinear geometry.

4 Assigned spectra and analysisPrior to searching for ArÈAuF ab initio calculations werecarried out to obtain an estimate of the ArÈAu bond length.From a comparison of the results with the ab initio andexperimental results for ArÈAuCl,5 ArÈCuX4 and ArÈAgX3(X\ F, Cl, Br) the ArÈAu bond length in ArÈAuF was esti-mated to be 2.38È2.42 The AuÈF bond length was Ðxed toA� .the equilibrium bond length, of AuF.17 The prediction ofre ,the J \ 3È2 transition gave a search range of 400 MHz. Lineswere found around 10 635 MHz, well within the limits of theprediction. Further lines were found at intervals of 3550 MHzto both higher and lower frequencies indicating the lines werefrom a linear complex. Since 40Ar, 197Au and 19F all have100% natural abundance (or nearly so) there are no otherobservable isotopomers (given the current sensitivity of theinstrument) of this complex to help conÐrm our assignment.Instead we attributed the observed lines to ArÈAuF on thebasis of the following : (1) the rotational constant is in goodagreement with the ab initio result ; (2) the nuclear quadrupolecoupling constant of Au follows the trend observed in ArÈAuCl ; and (3) when either Au metal, Ar or was replacedSF6the lines were no longer observable. As was observed for ArÈAuCl, the gold NQCC has changed signiÐcantly from itsmonomer value. For AuF eQq(Au)\ [53.2 MHz,17 while forArÈAuF eQq(Au)[ 323.3558 MHz. In all, lines were measuredand assigned to the J \ 2È1 to J \ 6È5 transitions in theground vibrational state. Since no hyperÐne structure fromthe F nucleus was observed the lines were assigned using thecoupling scheme where Fig. 1 shows aIŒAu ] JŒ \ FŒ IAu\ 3/2.portion of the hyperÐne structure of the J \ 3È2 transition.The measured frequencies were Ðtted using PickettÏs globalleast-squares Ðtting program SPFIT.18 The Ðtted molecularconstants are listed in Table 1. The line frequencies and theirassignments are available as supplementary data.¤

The geometry of ArÈAuBr was estimated using the resultsfrom ArÈAuF and ArÈAuCl5 combined with the trendsobserved in the ArÈAgX3 and ArÈCuX4 (X\ F, Cl, Br) com-plexes. The AuÈBr bond length was Ðxed to of AuBr7 whilerethe ArÈAu bond length was estimated to range between 2.49È2.51 Rotational constants were derived from the predictedA� .geometry and a prediction of the spectrum made. Lines werefound around 9181 MHz. Further lines were found at 1530MHz intervals at both higher and lower frequencies indicatingonce again that they were from a linear complex. In contrastto the simple spectrum of ArÈAuF, the ArÈAuBr spectrum iscomplicated by the presence of two Br isotopes, along withthe additional hyperÐne structure from both Br nuclei (for79Br and 81Br, I\ 3/2). From initial Ðts the observed lineswere assigned to ArÈAu 81Br based on the value of the BrNQCC, which was similar to the value in Au 81Br (althoughslightly smaller). As with ArÈAuCl5 and ArÈAuF, the gold

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NQCC is radically di†erent from its monomer value : forAuBr eQq(Au)\ ]37.3 MHz,7 while for ArÈAuBreQq(Au)\ [216.7 MHz. ConÐrmation of the assignment wasfound with the prediction and observation of the lines fromthe ArÈAu 79Br isotopomer ; furthermore, the ratio betweenthe observed NQCC of 79Br and 81Br is in good agreementwith the ratio of their quadrupole moments. The observedlines were assigned using the coupling scheme IŒBr ] JŒ \ FŒ 1 ;

Fig. 2 shows a portion of the hyperÐne structureIŒAu ] FŒ 1 \ FŒ .of the J \ 5È4 transition of ArÈAu 79Br. In all, lines weremeasured and assigned to the J \ 5È4 to J \ 8È7 transitionsfor both ArÈAu 79Br and ArÈAu 81Br in the ground vibra-tional state. Molecular constants are listed in Table 2. Againthe line frequencies and their assignments are available as sup-plementary data.¤

5 Structures of the complexesa. Geometries

The rotational constants, in Tables 1 and 2, all of whichB0 ,are well determined, have been used to obtain geometricalinformation for the two complexes. Since only one isotopomerof ArÈAuF was observed, the geometry of this complex wasderived by Ðxing the AuÈF bond length to of the monomerreAuF.17 This approximation is justiÐed since for ArÈAuCl5 itwas found that the ground state e†ective bondr0(AuÈCl)length is very close to the of the monomer AuCl (B0.001re A�di†erence).7 Use of this approximation results in an ArÈAubond distance in ArÈAuF of 2.391 with an estimated uncer-A� ,tainty of D^0.01 This is a decrease of 0.16 from thatA� . A�observed for ArÈAgF. This di†erence is the result of thedecrease in the kinetic repulsion energy of Au because of rela-tivistic e†ects.19

Since two isotopomers were observed for ArÈAuBr aground state e†ective geometry was obtained. The results(r0)are listed in Table 3. As with ArÈAuCl, there is only a smalldi†erence between the AuÈBr distance of the complex and reof monomeric AuBr (B0.002 The ArÈAu bond length hasA� ).increased from 2.47 in ArÈAuCl15 to 2.50 which isA� A� ,Table 3 Geometries of ArÈAuF and ArÈAuBr

PropertyBond lengths/A� ArÈAuF ArÈAuBr

Expt. (AuÈX) 1.918a 2.316bMP2(AuÈX) 1.9488 2.3319Monomer re(AuÈX) 1.918 449c 2.318 410dExpt. (ArÈAu) 2.391a 2.502bMP2(ArÈAu) 2.3948 2.4933

a AuÈF bond length Ðxed to of AuF (ref. 17). structure. c Ref.re b r017. d Ref. 7.

similar to the increase observed for both ArÈAgX3 and ArÈCuX4 (see Table 4). The ArÈAu bond length, however, is 0.13

less than that observed for ArÈAgBr. Also listed in Table 3A�are the ab initio geometries for both ArÈAuF and ArÈAuBr.As with the complexes studied earlier there is good agreementbetween the experimental and MP2 geometries.

b. Vibration frequencies and dissociation energy

The complexes previously studied have all been found to haveremarkably small centrifugal distortion constants, andD

J,

high NgÈM stretching frequencies, indicating that they arerelatively rigid. ArÈAuF and ArÈAuBr Ðt the same pattern.The ArÈAu stretching frequencies, have been estimatedue ,using the pseudodiatomic approximation

ue \ (4B03/DJ)1@2 (1)

The resulting values are 221 and 178 cm~1 for ArÈAuF andArÈAuBr, respectively. These values are considerably higherthan those typically found for van der Waals complexes (D20cm~1), and are much closer to those of chemically bondedspecies. They are presented in Table 4 in comparison with theab initio values from the MP2 calculations ; clearly there isexcellent agreement. This agreement, plus the fact that theAuÈhalide frequencies (also in Table 4) are signiÐcantly higherjustiÐes use of the pseudodiatomic approximation.

Table 4 also contains a comparison of the ArÈmetal vibra-tion frequencies of all the ArÈcoinage metal halides studied sofar. The values for ArÈAuF and ArÈAuBr clearly Ðt the samepattern. To remove any mass dependence, and thus obtain abetter indication of the rigidity of the bonds, the stretchingforce constants k have been obtained, and are also listed inTable 4. For each metal there is a clear, monotonic decreasein k in going from the Ñuoride to the bromide, which parallelsan increase in the ArÈM bond length. The ArÈAu stretchingconstant in ArÈAuF is clearly the largest.

The ArÈAu dissociation energy for ArÈAuF has been cal-Deculated ab initio. It, too, is given in Table 4 along with ourpreviously reported values for ArÈCuF, ArÈAgF and ArÈAuCl. Again, a relatively large value, considerably bigger thanthose typical of van der Waals bonds (\10 kJ mol~1) isfound, and provides further evidence for weak ArÈmetalchemical bonding. There is furthermore a rough, but clear,correlation of the dissociation energy with the force constants.A similar correlation was found in the ab initio calculations ofthe properties of OCÈMCl (M \ Cu, Ag, Au) complexes byAntes et al.20 The Ag derivatives are considerably moreweakly bound than those of Cu or Au, with those of Au mar-ginally the most strongly bound. A further comparison ismade with the complex ArAuAr` (Table 4), which is valenceisoelectronic with ArÈAuX, and whose properties have been

Table 4 Stretching frequencies and force constants, ArÈmetal bond lengths and calculated dissociation energies, of argon-coinage metals halides

u(ArÈM)a,b/ r(ArÈM)/ k(ArÈM)b,c/ De a/Complex u(MX)/cm~1 cm~1 A� mdyn A� ~1 kJ mol~1

Monomer ComplexaArÈCuF 621d 674 224(228) 2.22 0.794 47ArÈCuCl 418e 456 197(190) 2.27 0.648ArÈCuBr 313f 350 170(164) 2.29 0.534ArÈAgF 513g 541 141(127) 2.56 0.356 23ArÈAgCl 344h 357 135(120) 2.61 0.335ArÈAgBr 247h 124 2.64 0.298ArÈAuF 500i 583 221(214) 2.39 0.970 59ArÈAuCl 383j 413 198(184) 2.47 0.785 47ArÈAuBr 264j 286 178(165) 2.50 0.652ArAuAr`k (201)l 2.54 44k

a Ab initio values. ArÈCuF from ref. 4. ArÈAgF from ref. 3. ArÈAuCl from ref. 5. ArÈAuF and ArÈAuBr from present work. b Evaluated fromcentrifugal distortion constants in a diatomic approximation from Values in parentheses are ab initio values at the MP2 levelue \ [(4B03)/DJ

]1@2.of theory. c Evaluated from (ArÈM) using a diatomic approximation. d Ref. 26. e Ref. 27. f Ref. 28. g Ref. 29. h Ref. 30. i Ref. 17. j Ref. 7. k Ref.ue21. The value of is one half the atomization energy given in ref. 21. l Asymmetric stretchDe u(ru).

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Table 5 Comparison of NQCC (MHz) of Au(I) and Cu(I) monohalides and their complexes

X AuX ArÈAuX [XÈAuÈX]~ CuX ArÈCuX [XÈCuÈX]~

F [53.2344a [323.3558 21.9562e 38.0545e35Cl eQq(Au) 9.6331b [259.8352c ([)765d eQq(63Cu) 16.1691f 33.1859e 61.4g

([)802d79Br

g37.2669b [216.709 ([)790d

g12.8510f 29.923e 57.7g

35Cl eQq(Cl) [61.9969b [54.0502c ([)35.2d [32.1273f [28.0318e ([)19.2g79Br eQq(Br) 492.3271b 428.518 202.3d 261.1799f 225.554e 152.8g

a Ref. 17. b Ref. 7. c Ref. 5. d Ref. 8, 24, 25. e Ref. 4. f Ref. 23. g Ref. 6.

calculated by The ion has a predicted ArÈAu bondPyykko� .21distance and dissociation energy comparable to that inArÈAuX.

c. Nuclear quadrupole coupling

As with ArÈAuCl and KrÈAuCl, the Au NQCCs change con-siderably on complex formation. For linear, weakly-bondedvan der Waals complexes such changes are usually interpretedin terms of large amplitude bending vibrations using the fol-lowing equation22

eQq \ [eQq0] *(eQq)]T3 cos2 b [ 1

2

U, (2)

where eQq and are the NQCC of the complex andeQq0monomer, respectively ; *(eQq) is the coupling constantchange caused by electron rearrangement on complex forma-tion ; b is the Eckart angle between the AuX unit and the a-inertial axis ; and the averaging is over the ground vibrationalwavefunction. Since the complexes studied in this workhowever are relatively rigid, vibrational averaging e†ects willlikely be small and the observed changes in NQCC can beattributed chieÑy to extensive electron rearrangement.

Listed in Table 5 are the NQCC of Au, Cu, Cl and Br forthe gold(I) and copper(I) monohalides, the complexes ArÈMX,and for the ion complexes [XÈMÈX]~ (X\ Cl, Br). The ioncomplexes [XÈMÈX]~ which are isoelectronic with ArÈMXare clearly chemically bonded and generally stable in the solidphase. For instance, (ref. 8) and (ref. 24) haveAuCl2~ AuBr2~been isolated and their Au NQCC determined using

spectroscopy. On the other hand has notMo� ssbauer AuF2~been observed in either solution or solid.The shifts in the Au and Cu NQCC from the monohalides

to the ArÈMX complexes are very large. This is especially thecase for Au, at least on an absolute scale ; for the chloride andbromide there is even a change of sign. Interestingly, though,the changes in MHz are roughly the same for each Aucomplex, namely [250 to [270 MHz. For the chloride andbromide the changes are roughly one third of the change ongoing from AuX to [XÈAuÈX]~. There is clearly a very signiÐ-cant change in the electron arrangement near the Au nucleuson complex formation, providing yet another indication ofprobable ArÈAu chemical bonding. The fractional changes,furthermore, parallel those of Cu in its corresponding com-plexes, leading to the conclusion there is some form of chemi-cal bonding there as well. These changes can in no way beaccounted for with eqn. (2) assuming *(eQq)\ 0 ; they are notdue to vibrational averaging, for they go in the wrong direc-tion. Vibrational averaging can only decrease the absolutevalue of the coupling constants.

The halogen coupling constants, on the other hand, do alldecrease on complex formation, so eqn. (2) with *(eQq) \ 0could possibly apply in these cases. If this were so, then forArÈAuBr the angle b would become 17¡ and the angle of bendwould be 18.8¡. Given the rigidity of the complex, this wouldseem excessively large. (This rigidity extends to the bendingmodes, as well as the stretching modes : the ab initio calcu-

lations predict bending frequencies of 121 cm~1 and 64 cm~1for ArÈAuF and ArÈAuBr, respectively.) A more likely sce-nario would have Ar acting as a weak Lewis base, donatingelectrons to AuX and making the electron distribution roundX more nearly spherical. This too is entirely compatible withthe observed changes.

Corroborative evidence for ArÈAu chemical bonding(indeed, covalent bonding) comes from the MP2 ab initio cal-culations for ArÈAuF. Fig. 3 shows electron density contourplots of two of its valence molecular orbitals, 9r and 6p.31Both orbitals have two electrons, and both, particularly 9r,show considerable overlap of Ar and Au orbitals. The Mulli-ken populations from the calculations are given in Table 6.For this complex there is signiÐcant electron r-donation

Fig. 3 Contours of electron density of the 9r and 6p orbitals ofArÈAuF; values of contours (a) 9r: value\ 0.04n, n \ 1,6 (b) 6p :value\ 0.02n, n \ 1,12. Dotted line indicates a negative value of thewavefunction.

Table 6 Mulliken orbital populations n for AuF and ArÈAuFcomplex

Ar ] AuF ArÈAuF

nsF 2.00 2.00npsF 1.69 1.69

nppF 3.84 3.84

nsAu 0.41 0.59npsAu 0.07 0.12

nppAu 0.12 0.12

ndsAu 1.83 1.72

ndpAu 4.00 4.00

nddAu 4.00 4.00

nsAr 2.00 2.00npsAr 2.00 1.88

nppAr 4.00 4.00

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([0.1 e~) from the Ar to Au. The other main changes are inthe 6s and 5dr orbitals of Au. In this case the halogen orbitalpopulations do not change appreciably on complex formation.

6 Discussion and conclusionsWith the results reported in this paper microwave rotationalspectra have been measured for all nine argon-coinage metalhalides ArÈMX, with X\ F, Cl, Br. Once the initially unex-pected lines of the Ag derivative had been identiÐed thespectra have been reasonably easy to Ðnd and assign. Of thenine complexes, perhaps the most remarkable has beenArÈAuF. The very existence of the monomer AuF has beenopen to question, and it is only recently that its existence hasbeen conÐrmed.17 Detection of the complex, and measure-ment of its structural properties, is further evidence that AuFis a real compound. As an aside, it should also be noted thatthe ion [FÈAuÈF]~ is yet unknown; the ease of generation ofArÈAuF in the current laser ablation/supersonic jet systemshould give encouragement for the detection of the ion, andperhaps also neutral using other spectroscopic tech-AuF2 ,niques.

The microwave spectra of all nine complexes have revealedthree particularly notable features : (1) They have shown theArÈmetal bonds to be remarkably short, in the range 2.22È2.64 distances are very much less than the sums of theA� .Thesevan der Waals radii of the neutral atoms and even signiÐ-cantly less than the sum of the Ar van der Waals radius andmetal ionic radius. The shortest bonds are found for the Cuderivatives and the longest for the Ag derivatives, with the Auderivatives in between. For all three metals the ArÈM dis-tances increase in the sequence X\ F, Cl, Br.

(2) All these complexes have very small centrifugal distor-tion constants indicating that they are rather rigid. TheseD

J,

distortion constants in turn indicate remarkably high ArÈMstretching frequencies, ranging from D120È225 cm~1. Thesein turn give ArÈM stretching force constants in the range 0.3È1.0 mdyn In this case, for a given halogen the Au com-A� ~1.plexes are the most rigid, with the Cu complexes a little lessso, and the Ag complexes relatively Ñexible, though muchmore rigid than would be found for a van der Waals bond.For a given metal, the force constant decreases with increasingbond length.

(3) There are large changes in the nuclear quadrupole coup-ling constants on complex formation. The changes in thehalogen coupling constants, while moderately large, couldnonetheless be attributed to vibrational averaging over a largeamplitude bend, were there no evidence to the contrary. Therigidity of the complexes described above make such a conclu-sion doubtful, however. The changes in the Cu and Au coup-ling constants are very large indeed, and go roughly one thirdof the way from the values of the MX monomer to those ofthe [XÈMÈX]~ ions, which are valence isoelectronic with theArÈMX complexes. A second look at the Cl and Br quadrupo-le coupling constants reveals the same phenomenon; theirchanges also go one third of the way from those of MX tothose of [XÈMÈX]~. Since the ions are undoubtedly chemi-cally bonded, the changes in the coupling constants, togetherwith the short ArÈM bonds and the remarkable rigidity of thecomplexes, strongly suggest the presence of some ArÈmetalchemical bonding.

In parallel with the experiments, ab initio calculations of thegeometries and other properties of the complexes have beenperformed. At the MP2 level of theory the bond distances andstretching frequencies are well reproduced. Dissociation ener-gies have also been calculated for some complexes, and haveranged from 23 kJ mol~1 for ArÈAgF to 59 kJ mol~1 forArÈAuF. These values are also considerably larger than thosetypically found for van der Waals bonds (D10 kJ mol~1).Furthermore, electron density contour plots of some valence

molecular orbitals show signiÐcant overlap between Ar andthe metal, particularly for the Ar(pr)ÈM(dr) bonding orbital.Mulliken orbital populations indicate signiÐcant electron don-ation, up to D0.11 electrons from Ar to MX. Overall the cal-culations corroborate the view that there is indeed signiÐcantArÈmetal chemical bonding, especially for Cu and Au.

If ArÈmetal chemical bonding is signiÐcant, it should alsobe observable for other noble gases, especially Kr and Xe.This is indeed the case. As was indicated in the Introduction,we have already reported the spectrum of KrÈAuCl,5 forwhich we have found greater rigidity, a greater change in theAu NQCC, and a greater dissociation energy than for ArÈAuCl. Further studies are underway on other complexes. Aparticularly intriguing aim is to measure a nuclear quadrupolecoupling constant for 83Kr (I\ 9/2) or for 131Xe (I\ 3/2),which would give a direct measure of the electron rearrange-ment at the noble gas. Though this has thus far been elusivethe experiments are continuing.

AcknowledgementsThis research has been supported by the Natural Sciences andEngineering Research Council of Canada (NSERC), and bythe Petroleum Research Fund, administered by the AmericanChemical Society.

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