J . Phys. Chem. 1992,96, 9144-9148
9144
efficiently as acetylene so it is very likely that the (MACE],' ( n > 3) clusters are probably also covalently bonded molecular ions. The reaaon for methylacetylene's differing reactivity pattern may be associated with the prtsence of the methyl group; similar large changes in the CMS were observed upon changing from ethene to propene. It is possible that the bulk of the methyl group decreases the probability of forming larger polymer ions within the clusters by blocking reactive sites or by hindering isomerization reactions leading to the formation of kinetically stable products.
V. Conclmions Ionization of acetylene and methylacetylene cluster ions initiates intracluster polymerization reactions. The observation of magic numbers at n = 3 under conditions of efficient clustering is best explained in terms of formation of benzene and trimethylbenzene ions in acetylene and methylacetylene cluster ions, respectively. The formation of benzene ions, which are stable, relatively inert, cyclic mol& ions, represents a fairly efficient kinetic bottleneck in the intracluster polymerization reactions. The observation of further anomalous features, particularly in the acetylene cluster ion intensity distributions, is indicative of more extensive polymerization reactions.
Acknowledgment. We gratefully acknowledge the financial support of this work provided by the Office of Naval Research and the Alfred P. Sloan Foundation.
References and Notes (1) Coolbaugh, M. T.; Peifer, W. R.;Garvey, J. F. Chem. Phys. Lett. 1990. 168, 337. (2) Garvey, J. F.; Peifer, W. R.;Coolbaugh, M.T. Acc. Chem. Res. 1991, 24, 48.
(3) Coolbaugh, M.T.; Vaidyanathan, G.; Peifer, W. R.;Garvey, J. F. J. Phys. Chem. 1991, 95,8337. (4) Strictly speaking, the reactions observed in cluster ions would be better described as oligomerizationreactions since in most cases they appear to give rise to low molecular weight products containing two to five monomer units. (5) El-Shall, M.S.;Marks, C. J. Phys. Chem. 1991,95,4932. El-Shall, M. S.; Schriver, K. E. J . Chem. Phys. 1991.95, 3001. (6) Tsukuda, T.; Kondow, T. J . Chem. Phys. 1991, 95,6989. ( 7 ) Gerhardt, Ph.; Homann, K. H. J. Phys. Chem. 1990, 94, 5381. (8) Boyle. J.; Pfefferle, L. J . Phys. Chem. 1990, 94, 3336. (9) Peifer, W. R.;Coolbaugh, M.T.; Garvey, J. F.J . Chem. Phys. 1989, 91. 6684. (10) Whitney, S. G.; Coolbaugh, M. T.; Vaidyanathan, G.; Garvey, J. F. J . Phys. Chem. 1991, 95,9625. (11) Shinohara, H.; Sato, H.; Washida, N. J . Phys. Chem. 1990,94.6718. (12) Vaidyanathan, G.; Coolbaugh, M. T.; Garvey, J. F.J. Phys. Chem. 1992. 96. 1589. (13) Garrett, A. W.; Zwier, T. S. J . Chem. Phys. 1992, 96, 7259. (14) See,for example: Allcock, H. R.;Lampe, F. W. Contemporary Polymer Chemistry; Prenticc-Hall: Englewood Cliffs, NJ, 1981. (15) Lias, S. G.; Bartmes, J. E.; Liebman, J. F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data 1988, 17 Suppl. 1 . (16) Ono, Y.; Ng, C. Y. J. Am. Chem. Soc. 1%2, 104, 4752. (17) Booze, J. A.; Baer, T. J . Chem. Phys. 1992,96, 5541. (18) Bohme, D. K.; Wlodek, S.; Zimmerman, J. A.; Eyler, J. R. Inr. J . Mass Spectrom. Ion Proc. 1991, 109, 31. (19) The overall exothermicity of (2) is dependent on the exact structure of the trimethylbenzene ion, the heats of formation being 187, 190, and 192 kcalemol-' for the 1,2,4-, 1.33-, and 1,2,3-trimahylbemne ions, respectively. (20) Atlas of Mass Spectral Data; Stenghagen, E., Abrahamson, S., McLafferty, F. W., Eds.; Interdencc: New York, 1969; Vol. 1, p 19. (21) Ion-Molecule Reactions in the Gas Phase: Adv. Chem. Ser. No. 58:' Auslods, P., Ed.; American Chemical Society: Washington, DC, 1968. (22) Brill, F. W.; Eyler, J. R. J . Phys. Chem. 1981, 85, 1091. (23) Briggs, J. P.; Back, R.A. Can. J. Chem. 1971, 49, 3789 and references therecn. (24) Willis, C.; Back, R.A.; Morris, R. H. Can. J. Chem. 1977,55,3288 and references therein. (25) Schmieder, R.W. Radiar. Res. 1984,99,20 and references therein.
Matrix Isolation EPR Study of the Reaction of Ag Atoms with PN, SiS, and GeOt James A. Howard,* Ruth Jones, John S. Tse, Mauro Tomietto, Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Canada KIA OR9
Peter L. Ti"s,*
and Andrew J. Seeley
School of Chemistry, University of Bristol. Bristol, England BS8 ITS (Received: May 6, 1992; In Final Form: August 4, 1992)
Reactions of Io7Agatoms with PN, SiS, and GeO,isoelectronic analogues of CO, in inert hydrocarbon matrices in a rotating cryostat at 77 K have been investigated by electron spin resonance spectroscopy. PN gives one readily recognizable product, the mononuclear monoligand silver(0) complex Ag(PN). The EPR spectrum of this species at 180 K consists of a doublet of doublets of triplets centered at g = 1.9987 with alcn(l)= 1 1 16 MHz, a3l(l) = 183 MHz, and a,,(l) = 14.7 MHz, indicating there is about 61% unpaired 5s spin population on the Ag nucleus. SiS and GeO both give complex spectra with evidence for several mononuclear Ag(0) complexes. There is, however, reasonable evidence for formation of Ag(SiS) and Ag(Ge0). Spin-polarized local density function calculations indicate that Ag adds to the P nucleus of PN, that the most stable geometry is bent with a Ag-P-N angle of 97.4', and that AgGeO and AgSiS have triangular structures. Calculated unpaired spin populations were found to be in good agreement with those obtained experimentally from the EPR spectra.
I.troduction From matrix isolation spectrogcopic (EPR and FTIR) studies it is known that silver atoms complex with CO to give Ag(CO), where x = 1-314, and with S i 0 to give Ag(Si0) and other monosilver complexes of S i 0 oligomers.s Complexation of silver Two atoms by PN has been studied by matrix IR ~pectrosoopy.6~~ species have been identifiad, AgpN and another identified initially as Ag2(p-PN)t but subsequently as Ag(P2N2).7 We now report Issued as NRCC No. 34225.
0022-3654/92/2096-9144$03.00/0
a detailed EPR spectroscopic study of the interaction of Io7Ag atoms with PN molecules in an adamantane matrix at 77 K in a rotating cryostats.9 and with SiS and GeO. This work has enabled the trends in bonding in a series of Ag atom-mono isoelectronic ligand species to be observed. Experimental Section The rotating cryostat technique has been thoroughly described el~ewhere.~*~ PN was prepared by decomposition of P3N5in a molyMcnum pouch at ca.1170 K (the P3N5was prepared by rapid
Published 1992 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9145
Reaction of Ag Atoms with PN, SiS, and GeO V - 9 4 5 1 MHr
TABLE I: EPR Parameters of the Major Products from Reacting lo7Ag Atoms with Si in Adamantaw at 77 K
species D
E F G
H
c L
’
I
IB
I
IA
L I
L
I
g
PSI
2.0013 1.9994 1.9993 2.0006 2.0003
0.81 0.67 0.53 0.39 0.19
by using a simple implementation of the BFGS procedures employing analytical gradients.16J7 The geometry was considered to be a minimum when all the gradients were less than 0.001 au. Vibrational frequencies were computed at the optimized geometry by using finite differences.
Result9
-
V 9355 MHz
200 0
a107/MHz 1599.2 1225.8 975.2 712 358
IB
Figure 1. EPR spectrum given by lo7Agand PN in adamantane at 77 K (a) and at 180 K (b). The insert in (a) is the simulated spectrum of AgPN produced by using the parameters given in the text.
heating of PzS,with NH4Cl followed by purification at 1050 K under vacuum for 24 h). GeO was made by heating an equimolar mixture of Ge and GeOZto ca. 1220 Kl0 and SiS was made by heating a mixture of Si and MoSz in a 2:l mole ratio to ca. 1170 K. Io7Agwas obtained from Oak Ridge National Laboratory, Oak Ridge, TN. Reactants and products were isolated in layers of adamantane on the drum of the cryostat. At the end of an experiment the deposit was scraped from the drum into a 3-mm0.d. Suprasil tube still at 77 K and under high vacuum. It was then examined by EPR spectroscopy (Bruker ESP 300) at 77 K in a liquid Nz Dewar or at temperatures from 97 to 300 K by using a variable-temperature controller. The magnetic field and microwave frequency were calibrated with a Varian NMR pussmeter and a Systron Donner frequency counter, respectively. Spin-polarized local density functional (LDF)I1 calculations employing the von Barth-Hedin exchangecorrelation potentialI2 were performed by using the program DMOL” to determine the geometries, stabilitica, and spin distributions of the PN, GeO, and SiS adducts of Ag. One of the advantages of the LDF method is that some electron correlation effects are already included in the Hamiltonian. A previous study on Ag(SiO)I4 has shown that the electron correlationeffect is important in the amect description of the electronic structure. Numerical basis sets of doubl*.f quality augmented with polarization functions computed from the ionic state (DNP) were used in the calculation.~5The electronic intergrals were evaluated numerically over a grid. For radial integrals the number of radial points was generated according to
NR = 14(2 + 2)‘13 where Z is the atomic number. The maximum distance for any function was 10 au. The angular integration points wcre generated at the NRradial points to form shells around each nucleus. Fitting functions with maximum angular momentum L = 3 were used for the evaluation of the charge density and Coulomb potential. Geometry optimizations were carried out in Cartesian coordinates
PN. Io7Agatoms (I = and PN (I of 31P = 100% abundance, and I of I4N = 1,99.63%) deposited in adamantane at 77 K gave a dark brown deposit. The EPR spectrum of this deposit at 77 K is shown in Figure la and consists mainly of a doublet labeled A with the magnetic parameters alo7= 1681 MHz and g = 2.0018, a doublet of doublet of triplets labeled B, and a central feature C centered at g = 2.008. The overall spectrum is more complex than the one from lo7Agatoms in adamantane, which gave a narrow doublet with (1107 = 1681 MHz and g = 2.0018 from Ag atoms in a substitutional site, a less intense broad doublet with (1107 = 1771 MHz and g = 2.0014 from Ag atoms in an interstitial site, and a broad anisotropic l i e with g = 2.009 from a Ag microcry~tallite.~ Doublet A can readily be assigned to trapped IMAgatoms, while the camer of spectrum B has two nuclei with Z = l/z and one with I = 1.0 and has the probable stoichiometry AgPN; Le., it is a monoligand complex of Ag(0). The relative intensities of the triplets in the spectrum were distorted at 77 K, indicating some hyperfine and g anisotropy. A satisfactory simulation of the powder spectrum (insert in Figure la) was obtained with the parameters u107= 1116 MHz, ~ ~ ( 3 =1 )192 MHz, ~ ~ ( 3 =1 178 ) MHz, (114 = 14.7 MHz, gll= 2 . h , and g, = 1.9980. The was no evidence for Ag or N anisotropy. The shape of the powder spectrum was quite sensitive to hfi and g tensors and the errors in P hfi and g tensors were f 2 MHz and f0.0002, respectively. On warming the sample in the cavity of the spectrometer to 180 K the relative intensities of these transitions approached 1:1, as is evident from the spectrum shown in Figure lb. Analysis of this spectrum gave a107 = 1116 f 2 MHz, a31 = 183 f 2 MHz, (114 = 14.7 f 1 MHz, and g = 1.9987 f 0.0002. Upon warming the central feature C resolved to give of a narrow line centered at g = 2.0045 and a broader line centered at g = 2.0002, neither of these lines carrying a resolved hyperfine interaction. At this temperature a doublet with a107 = 538 MHz appeared but did not carry P or N Mi and a doublet of doublets with 0107 = 204 MHz and ugl = 53 MHz. SiS. SiS deposited alone in adamantane at 77 K gave a brown deposit that turned orange-red on addition of lo7Agatoms. A final deposit containing a mole ratio of adamantane/Ag/SiS of ca. 100:0.005:0.004 gave the EPR spectrum (at a microwave power of 2 mW) shown in Figure 2a. It consists of two minor and three major almost isotropic doublets labeled D, E, F, G, and H in addition to Io7Agatoms in interstial and substitutional sites and an asymmetric central feature (I) centered at g = 2.010. The magnetic parameters of the mononuclear Ag(O)/SiS complexes D to H are listed in Table I. When the microwave power was increased to 100 mW the amplitude of spectrum H decreased dramatically while those of F and G increased. Upon warming the sample in the cavity of the spectrometer H was the last doublet to disappear and at 250 K H resolved into two doublets, H’and H”, with (1107 = 410 MHz and g = 2.0010 and 0107 = 376.5 MHz and g = 2.0010 as shown in Figure 2b. Many other lines developed in the center of the spectrum but have, as yet, defied analysis. When the mole ratio of adamantane/SiS/Ag was changed to 100:0.013:0.291 virtually the same spectrum was obtained except
Howard et al.
'
200 G
'
v
e444
w;
3360 G
(a)
J r AD
I
&E
I
U
L 1
IG
OH
I
IK
F i p e 3. EPR spectrum given by Io7Agand GeO in adamantane at 77 K.
I
Figure 2. EPR spectrum given by Io7Agand SiS in adamantane at 77 K (a) and at 250 K (b). TABLE Ik EPR Panmeters of the Major products from Reacting '"Ag Atom with GeO in Adammnt8ne at 77 K species ai07/MHz g P% J 1599 2.00136 0.87 L 1516.7 2.0002 0.83 K 1095.6 1.9987 0.60
that D was relatively more intense and all of the isotropic doublets increased in amplitude relative to the central feature. In all spectra the l i e s of spectrum F were significantly broader than those of the other doublets and the high-field line of spectrum H had a larger amplitude than the low-field l i e . Ceo. Io7Agatoms and GeO deposited in adamantane at 77 K gave a green deposit that exhibited the EPR spectrum at 77 K shown in Figure 3. It consisted of a broad doublet with 0107 = 1777.3 MHz and g = 2.0009 and a narrow doublet with clearly visible spin-flip satellite lines from Ag atoms in interstitial and substitutional sites of adamantane. In addition there were two major doublets J and K and a minor doublet L with the magnetic parameters listed in Table I1 and a central asymmetric feature with g = 2.01 1. There was no evidence for satellite lines associated with any of these transitions from the 73Geisotope with I = 9/2 and natural abundance = 7.76%. Upon warming the sample the spectrum became even more complex between 3200 and 3600 G but we have not attempted spectral assignment because of lack of hyperfine information.
Discussion PN, SiS, GeO, and the previously studied S O 5 are all 10 valence electron diatomic molecules and, like CO,4 give very complex EPR spectra when rtacted with Ag atoms in adamantane in a rotating cryostat at 77 K, Le., under matrix isolation conditions. Ag and PN did, however, give one readily identifiable product, Ag(PN), and this will be discussed first. AgPN. Dividing the isotropic hyperfine interactions of AgPN, 0107 = 1116 MHz, (131 = 183 MHz, and a14 = 14.7 MHz, by A = 183 1,13 306, and 18 11 MHz, respectively, the parameters for
unit spin population in the Ag 5s, P 39, and N 2s orbitB18~*J9 giva the ns unpaired spin populations, p5, = 0.61, pSs= 0.014, and p2( = 0.008; i.e., the unpaired spin population is mostly on the silver. The unpaired 5s spin population on Ag is significantly lower than is found in other monoligand Ag complexes such as Ag(CO),4 Ag(C2H4),2OAg(HCN)?' Ag(C6H6).9and AgPFgZ2where p5# = 0.92,0.88,0.87,0.86, and 0.92, respectively. It is in fact similar to the unpaired 4s spin population in CuCO where p41is 0.66.23 An explanation of this observation is provided by local density function calculations (see below), which predict substantial transfer of unpaired spin population from the Ag to the N atom via the r* orbitals of the coordinated PN molecule. The value of p3, = 0.014 for P is smaller than the value of 0.023 of AgPF322but is larger than the value of pzs = 0.008 of AgC04 and is in the range found for other paramagnetic complexes with trivalent phosphorus ligands.24 There is some P anisotropy (all - a+ = 14 MHz), indicating a small contribution from the P 3p orbital to the SOMO. Dividing the experimental value of P = 11.7 MHz by the calculated P gives pD$(P)= 0.013, only onetenth the value of p3,(P) of Al[P(CH3)I2 and Al[P(OCH3)]2.25 There are three possible structw for AgPN, linear geometries with bonding through either P or N (structures I and 11) and a bent triangular structure with the Ag atom bonding either symmetrically or unsymmetrically to the P and N (structure 111). Ag--PEN
I
Ag+N=P
I1
P=N
PEN
\AB/
.) A9
111
The EPR results are most consistent with either I or 111, but we must rely on local density function calculations to distinguish between these two possibilities. The Ag species formed on warm-up with alCn= 538 MHz and no other hfi and 0107 = 204 MHz and 431 = 53 MHz and no N hfi have not yet been identified, although the latter parameters suggest Ag(0) bonded to a PN polymer. SiS. A matrix isolation infrared spectroscopic study of vapordeposited Si has indicated that SB and Si$, are the major trapped species and that higher polymers are not formed.% We have previously reported that Ag(Si0) is formed in the Ag/SiO system and has 0107 = 1226 MHz, almost identical with that of the very minor s p i e s E in the Ag/SiS system. Of the major species in the Ag/SiS system the transitions of spectrum F were signifiitly koader (AH = 30 G) than any of the daublets given by Ag and Si0 where & was typically 5.3 G. Furthermore the values of 0107 of G an& were smaller than Mi assigned to Ag(Si303)and Ag(Si,O,), while Si3S3and Si,$, are not present in vapor-deposited SiS. The most intense spectrum in the Ag/SiS system H had 4107 = 358 MHz and began to resolved into two spectra H' and H" at 120 K with aIo7= 410 and 376 MHz. Ag hfi's of thB magnitude suggest an endon u-bonded complex, which could be either bent AgcSi=S or bent Agc-i. Perhaps both these species are formed in the Ag/SiS system. There were lines in the s p e c " of the right intensity for 29si (I= 1/2, natural abundance
The Journal of Physical Chemistry, Vol. 96, No. 23, 1992 9147
Reaction of Ag Atoms with PN, SiS, and GeO TABLE IIk LDF Cdculrtcd Propertied of AgXY Adducts' PN SiS SSi GeO r(XY)lA Calcd ObSd
PID calcd obsd v1cm-l calcd obsd r(AgX)lA r"/A r ( x W L(AgXY)/deg BE/kcal mol-' v I1cm-l calcd obsd v2/cm-I (calcd) v,/cm-l (calcd)
P,l%
calcd obsd
1.507 1.491
1.636 1.625
1.958 1.929
1.958 1.929
2.927 2.751
3.203 3.283
1.596 1.730
1.596 1.730
1338 1337 2.459 3.061 1.534 97.4 -18.1
979 986 2.597 2.440 1.684 65.5 -17.9
721 750 2.492 2.903 2.017 79.4 -23.1
721 750 2.447 3.717 2.011 115.5 -14.9
1206 1203 241 45
847
625
617
164 141
236 72
228 103
58 61
54 60
48
59
-
percentage of N 2p character in the SOMO might be expected to produce an anisotropic hyperfine tensor with ull - uL 60 MHz. The N hyperfine tensor was, however, isotropic down to 77 K. The Ag-GeO adduct has a triangular structure that is similar to that of AgSi0,SJ4where the Ag nucleus straddlm the Si0 bond, with a Ag-Ge-0 angle of 6 5 . 5 O . The Ag atom interacts with both the Ge and 0 atoms as reflected in the short Ag-Ge and Ag-0 bonds of 2.597 and 2.440 A, respectively. The Ge-0 bond has lengthened substantially from the free ligand value of 1.636 to 1.684 A. The stretching and bending vibrations of the adduct are 164 and 141 cm-l, respectively. The LDF binding energy is -18 kcal mol-'. The unpaired spin population is more delocalized in the Ag-GeO adduct with 0.6e,0.19e, and 0.21e on the Ag, Ge, and 0 nuclei, respectively. The unpaired Ag 5s spin population is approximately 54%, which is close to the value of pss = 0.60 for species K in the Ag/GeO system. The Ag-SiS adduct has a similar triangular structure to that of AgSiO and AgGeO. The Ag-Si-S angle is 79.4O, which is about 14O larger than that in Ag-GeO. The larger Ag-Si-S angle is suggestive of a weaker Ag-S interaction. This conjecture is supported by the observation that the Ag-Si bond of 2.492 A is closer to the normal covalent bond len h as compared to the rather long Ag-S bond length of 2.903 Nevertheless, the SiS distance in the adduct is elongated by 0.059 A from 1.958 to 2.017 A. The binding energy of the adduct is -23 kcal mol-'. The unpaired spin populations on the Ag, Si, and S nuclei are 0.53e, 0.18e, and 0.29e, respectively, with -0.48 of the Ag unpaired spin population in the 5s orbital. A second bent adduct with the Ag bonded exclusively to the S atom with a binding energy of 15 kcal mol-' is also located. The Ag-S-Si bond angle is 115S0 and the AgS bond distance is 2.445 A. The SiS bond distance of 2.01 1 A is only 0.006 A shorter than that of the more stable adduct but still significantly longer than that in the free ligand. As expected from the structure of the adduct, the majority of the unpaired spin is located on the Ag and Si nuclei with 0.61e on Ag and 0.41e on Si and a small amount of negative spin population on the S nucleus. The only species in the Ag/SiS system with pss close to these values is F with a value of 0.53. The nature of the Agadduct bonding derived from the molecular orbital calculations can be summarized as follows. In view of the large dipole moments, PN, GeO, and SiS are polar molecules. The most stable adduct structure is expected to be formed from the addition of the nucleophilic Ag atom to the positive end of the ligand. Thus the preferred addition sites are P in PN, Ge in GeO, and Si in SiS. Once the adduct is formed the Ag atom may migrate toward the negative end of the ligand to accommodate the electron through overlap with the empty Ag 5p orbital. The stability gained from this secondary interaction is governed by the efficiency of the overlap and the energy matching between the empty Ag 5p and the bonding r orbitals of the ligand. In the case of PN, a combination of a strong a-bond and the low availability of nonbonding electrons on the N atom makes AgPN stable with almost no Ag-N interaction. However, S i 0 is more polar with a weaker a-bond and more electron density on the 0 atom, and SiS has weaker 2 p 3 p bonds with more diffuse 3p orbitals on S,all factors favoring triangular structures with Ag-0 and Ag-S bonds.
f
" X Y = PN, GeO, and SiS.
= 4.7%) satellites, but the two components necessary to determine were not located. It is unlikely that 33S(I = natural abundance = 0.76%) satellites would have been resolved. Consequently, we can not unambiguously assign spectrum H or for that matter spectra F and G. 0. The mass spectrum of the vapor of GeO heated to 10oO K showed the presence of GeO, Ge202,and Ge303in the ratio 3:l:l , l o and matrix isolation infrared spectroscopy of vapor-deposited GeO indicated the formation of GeO,WO,,Ge303,and Ge404. Two major products were formed on reaction with Ag, a species J with the same uloTas species D from the Ag/SiS system but in higher yield. Species D has not been identified and in the absence of 73Ge(I 5 9/2, natural abundance = 7.76%) satellites unambiguous assignment of J cannot be made. The second major species K in the Ag/GeO system has pss= 0.6, which is perhaps close enough to that of AgPN to suggest that the carrier of spectrum K is Ag(GeO), but again 73Gesatellites were not observed. Doublet spectra with Ag hfi that could be assigned to a-bonded complexes were not detected in the Ag/GeO system. Locrl Density Function Calculations. A summary of the optimized geometries, vibrational frequencies, and Ag 5s spin populations for PN, SiS, and GeO and their most stable adduct to the Ag atom is given in Table 111. The LDF calculated geometries, dipole moments, and vibrational frquencies of the free ligands arc in good agreement with experimental values. The bond distances are accurate to 0.024.03 A, the dipole moments are within 6%, and the stretching frequencies are within 4% of the observed values. Ag was found to bond exclusively to the P nucleus of PN in a bent geometry with a Ag-P-N angle of 97.4O. The Ag-P bond distance of 2.459 A indicates fairly strong covalent bonding. The strong Ag-P interaction is at the expense of a slightly weakened P-N bond, which lengthened by 0.027 to 1.534 A. The low vibration frequency (45 cm-I) of the Ag-P-N bending mode shows that the potential energy surface is quite shallow. The calculated binding energy is -18 kcal mol-'. This value probably represents an overestimate since no gradient correctionswere made.16 The unpaired spin population is concentrated on both the Ag (0.59e of 98% 5s character) and the N (0.36e of 100% 2p character). Transfer of electron density from the Ag to the N causes weakening of the PN bond and probably accounts for the observed fall of 120 cm-'in the P-N stretching frequency of AgPN compared with free PN reported by Atkins and Ti"s.6 The linear Ag-PN geometry was found to be an unstable adduct (one imaginary frequency) of higher energy. No stable adduct was found with Ag bonded to the N nucleus. It should be noted that such a high 029
-
Acknowledgment. We gratefully acknowledge support of this research by a NATO collaborative research grant (to J.A.H., A.J.S., and P.L.T.), by an SERC grant and studentship (to P.L.T. and A.J.S.), and by the BP Venture Research Fund (to R.J. and J.A.H.). We thank a referee for several useful comments and Dr. P. H.Kasai (I.B.M., San Jose) for making his powder spectrum simulation program available to us. References and Notes (1) McIntosh, D.; Ozin, G. A. J . Am. Chem. Soc. 1976,98,3167-3175. (2) Kasai, P. H.; Jones, P. M. J . Phys. Chem. 1985, 89, 1147-1151. (3) Chenier, J. H. B.; Hampaon, C. A.; Howard, J. A.; Mile, B. J . Chem. Sm., Chem. Commun. 1986, 73G732. (4) Chenier, J. H. B.; Hampaon, C. A.; Howard, J. A.; Mile, B. J. Phys. Chem. 1988, 92, 2745-2750.
J. Phys. Chem. 1992, 96,9148-9158
9148
(5) Chenier, J. H. B.; Howard, J. A,; Joly, H. A.; Mile, B.; Timms, P. L.
J. Chem. Soc., Chem. Commun. 1990,581.
(6) Atlrins, P. M.; Timms, P. L. Inorg. Nucl. Chem. Lett. 1978,14, 113. (7) Ahlrichs, R.; Bar, M.; Plitt, H. S.; Schnockel, H. Chem. Phys. Lett. 1989, 161, 179. (8) Bennett, J. E.; Mile, B.; Thomas, A.; Ward, B. Adu. Phys. Org. Chem. 1970, 8, 1-77. (9) Buck, A. J.; Mile, B.; Howard, J. A. J . Am. Chem. Soc. 1983, 105, 3381-3387. (10) Ogden, J. S.; Ricks, M. J. J . Chem. Phys. 1970.52, 352. (1 1) Density functionul methods in chemistry; Labanowski, J. K., Anderson, J. W., Eds.; Springer-Verlag: New York, 1991. (12) Hedin, L.; Lunqvist, B. J. J . Phys. C 1971, 2064. (13) Program DMOL, Version 2.0, Biosym. Technologies, San Diego, CA, 92121. (14) Tse, J. S. J . Chem. Soc., Chem. Commun. 1990, 1179. (15) Dclley, B. J . Chem. Phys. 1990, 92, 508. (16) Delley, B. J . Chem. Phys. 1991, 94, 7245.
(17) Press, W. H.; Flannery, B. P.; Teukolsky, S. A.; Vetterling, W. T. Numericul Recipes, the Art of Scientific Computing, Cambridge University Press: New York, 1986. (18) Morton, J. R.; Preston, K. F. J . Mugn. Reson. 1978. 30, 577. (19) We have sometimes uscd ulo7= 1681 MHz for a lo7Agtrapped in a substitutional site of adamantane to calculate pssfor Ag(0) complexes. This would of course given slightly larger values than calculations using A. (20) Howard, J. A.; Joly, H. A.; Mile, B. J . Phys. Chem. 1990, 94, 6627-663 1 . (21) Howard, J. A.; Sutcliffe, R.; Mile, B. J . Phys. Chem. 1984,88, 5155. (22) Histed, M.; Howard, J. A.; Jones, R.; Tomietto, M.; Joly, H. A. J . Phvs. Chem. 1992. 96. 1144. 123) Chenier, J.' H.B.; Hampson, C. A.; Howard, J. A,; Mile, B. J . Phys. Chem. 1989, 93, 114-117. (24) Baird, M. C. Chem. Reu. 1988, 88, 1217. (25) Histed, M.; Howard, J. A.; Jolv. H. A.; Mile. B. Chem. Phvs. Lett. 1989, 161, 122. (26) Atkins, R. M.; Timms, P. L. Spectrochim. Acta 1977, H A , 853.
Raman, Infrared, and UV-Visible Studles of Matrix-Isolated NiOComplexes with Ethylene, Propylene, and Butadiene Th4rhe Merle-Mejean,? Colette Cosse-Mertens, a d Bouchareb: Florence Calan, Jdlle Mascetti,* and Michel Tranquille* Laboratoire de Spectroscopie Molkulaire et Cristalline, URA 124 CNRS,UniversitC de Bordeaux I , 351 cows de la LibPration, 33405 Talence Cedex. France (Received: May 7, 1992; In Final Form: August 4, 1992)
The cocondensation products of nickel atoms with pure ethylene or ethyleneargon mixtures trapped at 12 K were reinvestigated by IR and UV-visible spectroscopies. The first Raman spectra were obtained. Isotopic H/D experiments and comparison of IR and Raman spectra indicate unambiguously that three complexes are formed, Ni(C2H4)3,Ni(C2H4)2,and Ni(C2H4), with a normal order of increasing perturbation of ethylene vibration modes from the 1:3 to the 1:l stoichiometries. As expected, the metal-ligand vibrations follow the reverse order. The UV-visible spectra have been reassigned on the basis of dual IR-UV experiments on the same matrix. Preliminary results on cocondensation of nickel atoms with butadiene and propylene in argon matrices at 12 K are also presented. With propylene, the complexes observed are similar to those obtained for ethylene. In the case of butadiene, three species are formed, all ligands being coordinated to the nickel in a trans monodentate fashion.
Introduction The metal-olefin bond is of fundamental importance both in organometallic chemistry and in chemisorption studies on metal surfaces. For example, nickel atoms introduced in an olefin polymerization catalyst may completely modify its specificity in leading to the exclusive formation of oligomers: this is the so-called "nickel effect". In the case of 1,3-butadiene, dimer or trimer cyclooligomers are obtained, depending on the nature of the ligands. These reactions have important industrial applications; their mechanisms have been extensively studied but the first steps of the chemical pathway have never been elucidated.' This prompted us to study the different stoichiometries and structures of the binary complexes formed between nickel and butadiene. But, besida the mechanistic interest described above, this study should lead to a better knowledge of the metal-olefin r bond. In case of transition-metal complexation, the mechanism of Chatt-Dewat-Duncan2 involving olefin T molecular orbital metal d empty orbital electron donation and metal d electron olefin T* molecular orbital back-donation is proposed. The vibrational spectroscopy of metal-bonded olefins has produced an abundant literature,'" but there are actually few complete vibrational spectra of such systems, even in the case of ethylene, whereas they are necmary to measure the force and evaluate the structure of coordination bonds of zerovalent transition metals. In all cases, discussions on the structures and properties of
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*To whom correspondence should be addressed. 'Present address: Laboratoire de Spectrom6trie I.R. (URA320), Facult€ des Sciences, 123 rue Albert Thomas, 87060 Limoges Cedex, France. *Present address: ENS F b , BP A34, F b , Morocco.
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metal-bonded ethylene species hinge on the detection of the inphase CH2scissoring + C-C stretching modes, which are thus IR activated. Matrix isolation techniques constitute a convenient way to study metalO-olefm complexes. Some metal atoms have been demonstrated to react with ethylene (Et) when trapped in cryogenic conditions; the resulting complexes have been previously studied by W-visible, IR, and EPR techniques.'-I9 Previous studies on the Ni/Et/Ar system have been mainly reported by Ozin et a1.,7.20,21whereas theoretical studies were performed by numerous authors.22-28 Ozin et al., using IR and UV-visible absorption techniques, demonstrated the presence of three binary complexes Ni(Et), (n = 1, 2, 3), their abundance depending on the ligand dilution in argon. The identification of species was mainly done on the basis of the position and isotopic shifts of ligand internal modes: u(C=C), 6(CH2), and w(CH2). The use of Raman spectrcwcopy has been demonstrated by us as powerful in the case of Cu(Et), c o m p l e ~ e s . ~ ~ . ' ~ We have thus undertaken a new vibrational study on the Ni/ethylene system in order to determine definitely the stoichiometry of the complexes by looking at the metal-ligand stretching vibrations. Mixed isotopic substitutions (C2H4/C2D4) and annealing experiments were performed in order to characteh the different species by IR and Raman spectroscopies. In the case of nickel compounds, the u(MC) modes were easily detected by both spectroscopies. After the description of the spectra obtained for the Ni/Et/Ar systems, we propose assignments of the vibrational modes of the Ni(Et), ( n = 1, 2, 3) complexes. Then, we present first results 0 1992 American Chemical Society
