J. Phys. Chem. 1988, 92, 2745-2750 the clustering of H C N molecules. Although H C N molecules do display a preference for the direction of the C-N bond axis of CN-, the anisotropy of this anion is relatively small. Calculations with our highest level of theory suggest a variation in the binding energy of only 10% between the most and least attractive angles of approach. While the calculations identify a global minimum for the higher clusters, with n = 2 or 3, there are a number of alternate structures with competitive binding energies. Of particular note is the observation of no clear solvent shell structure. That is, for both CN- and C1-, clusters in which one H C N molecule interacts with the central anion indirectly through a second H C N are nearly equal in energy to those in which all HCN molecules are directly bonded to the anion. Binding energies for C1- tend to be slightly smaller and intermolecular separations a little shorter as compared to CN-. The ab initio results are consistent with the gas-phase clustering results, which also show a similarity in the clustering energetics of CN- and C1-, as well as Br-, with H C N , and a lack of thermochemical evidence for distinct shell structure. Furthermore, the similarities of the anisotropic CN- and isotropic C1- ions carry over into condensed-phase thermochemistry as a comparative
2745
review of the experimental data reveals. We note lastly that the behavior of CN- with respect to ligands other than H C N may be somewhat less akin to a pseudohalide, as may be seen for example in its clustering with H20,,0 although in this case there is no literature data for more than one H 2 0 complexing. Note Added in Proof. The similarity in H-bonding properties between CN- and C1- has recently been confirmed by pulsed ion cyclotron resonance spectroscopy: Larson, J. W.; McMahon, T. B. J. Am. Chem. SOC.1987, 109, 6230. Acknowledgment. Some of the calculations were performed on the SIU Theoretical Chemistry Computer, funded in part by a grant from the Harris Corp. This work was supported by grants to S.S. from the National Institutes of Health (GM29391, GM36912, and AM01059) and from the National Science Foundation (DMB-8612768). We thank Dr. Carol Deakyne for a number of helpful suggestions. Registry No. CN-, 57-12-5; HCN, 74-90-8. (30) Payzant, J. D.; Yamdagni, R.; Kebarle, P. Can. J . Chem. 1971, 49, 3308.
Spectroscopic Study of the Reaction of Silver Atoms with CO in a Rotating Cryostat‘ J. H. B. Chenier, C. A. Hampson,* J. A. Howard,* and B. Mile* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K l A OR9 (Received: July 27, 1987)
Reactions of Io7Agatoms with CO have been studied in inert hydrocarbon matrices in a rotating cryostat at cryogenic temperatures by EPR and FTIR spectroscopies. AgCO, bent Ag(CO)*, two conformers of Ag(CO),, and cluster carbonyls Agn(CO),, where n 2 5, are formed in adamantane. Magnetic parameters are reported for these silver carbonyls. IR spectra show the presence of a range of carbon monoxide complexes with single Ag atoms and Ag clusters, but the considerable overlapping of the bands from all these species precludes assignment other than that at 1952 cm-’ to planar Ag(CO)3. In cyclohexane unambiguous EPR or FTIR evidence for mononuclear Ag carbonyls is not obtained, but there are strong complex features in both spectra from a range of Ag cluster carbonyls Ag,,(CO), with n = 2-7.
Introduction An electron paramagnetic resonance (EPR), Fourier transform infrared (FTIR), and UV-visible spectroscopic study3 of the products given by sequential deposition of Cu atoms and C O on inert hydrocarbon surfaces at 77 K in a rotating cryostat has demonstrated that linear CuCO, planar ( D 3 J Cu(CO),, and dinuclear copper carbonyls Cu2(CO),, where n = 1-6, are formed, while Cu(CO)* has not been positively identified under these experimental conditions. EPR and IR studies in C O and rare gas matrices4s5have shown that CuCO, C U ( C O ) ~and , Cu(CO), are all formed at lower temperatures, and C U ( C O ) is~ believed to be EPR silent because it is a 211molecule. Dinuclear copper carbonyls are only formed on warm-up in rare gases4 Similar spectroscopic studies of the Ag atom-CO reaction in C O and rare gas matrices6,’ have shown that the analogous silver carbonyls AgCO, Ag(CO),, Ag(CO),, and Ag2(C0)6are formed (1) Issued as NRCC No. 28806.
(2) Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L3 3AF. (3) Chenier, J. H. B.; Hampson, C. A,; Howard, J. A,; Mile, B. J. Phys. Chem., submitted for publication. (4) Huber, H.; Ktindig, E. P.; Moskovits, M.; Ozin, G.A. J . Am. Chem. SOC.1975, 97, 2097-2106. ( 5 ) Kasai, P. H.; Jones, P. M. J . Am. Chem. SOC.1985, 107, 813-818. (6) McIntosh, D.; Ozin, G. A. J . Am. Chem. SOC.1976, 98, 3167-3175. (7) Kasai, P. H.; Jones, P. M. J. Phys. Chem. 1985, 89, 1147-1151.
0022-3654/88/2092-2745$01.50/0
while we have demonstrated that Ag(C0)3 can have a pyramidal (2Al’) structure in adamantane.* In this paper we present the results of a more thorough spectroscopy study of the Ag/CO reaction on solid hydrocarbon surfaces at 77 K. This work demonstrates that a much wider range of carbonyls are formed from Ag than Cu. Linear AgCO and bent Ag(CO)* have been identified in adamantane but more importantly planar (2AF) and pyramidal (2Al’) Ag(CO), are both formed concurrently in this matrix. Several silver cluster carbonyls are also produced, but stoichiometric and structural assignment of these species has proved to be difficult.
Experimental Section The rotating cryostat and instruments used to record and calibrate EPR and FTIR spectra have been described previously.9310 The best IR spectra were obtained 2-5 min after metal deposition had commenced, that is, before there was significant scattering of the IR beam by the matrix. EPR and I R spectra were recorded from 4 to 250 K and 77 to 250 K, respectively, as (8) Chenier, J. H. B.; Hampson, C. A,; Howard, J. A,; Mile, B. J . Chem. SOC.,Chem. Commun. 1986, 730-732. (9) Buck, A. J.; Howard, J. A.; Mile, B. J. Am. Chem. Sor. 1983, 105,
3381-3387. (10) Howard, J. A.; Sutcliffe, R.; Hampson, C. A,; Mile, B. J. Phys. Chem. 1986, 90, 4268-4273.
Published 1988 by the American Chemical Society
Chenier et al.
2746 The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
a
I IC 3250 G
b
Figure 2. EPR spectrum from Io7Ag/CO/adamantaneat 243 K (a) and a computer-simulatedspectrum for a silver cluster carbonyl Ag,(CO), with the parameters given in the text (b).
I
I
3250 0
Figure 1. EPR spectra given by Io7Agand I2C0 (a) and I0'Ag and "CO (b) in adamantane at 7 7 K.
described previ~usly.~ Simulations of powder EPR spectra were carried out with the aid of a Belford program." The silver used for this work was isotopically enriched Io7Ag (98.22%) obtained from Oak Ridge National Laboratory, Oak Ridge, T N . Adamantane and cyclohexane were obtained from Aldrich. Natural C O was obtained from Matheson, I3CO (99.8 atom% I3C) and perdeuteriocyclohexane were obtained from Merck, Sharpe, and Dohme, Canada Ltd., and CI7O (36.8 atom % 170) was obtained from Prochem Isotopes, Summit, NJ. Traces of O2were removed from C O by passage through MnO on celite. Results and C O a t an inlet pressure of EPR. Io7Ag atoms ( I = 0.05 Torr in adamantane at 77 K give a multicolored deposit that exhibits the complex spectrum shown in Figure la. It consists of a doublet A with magnetic parameters (a107 = 1682 f 1.5 MHz and g = 2.0022 f 0.0003) that are slightly larger than those for Io7Agatoms isolated in the major trapping site I1 of adamantane9 (a107 = 1676.6 MHz and g = 2.0021). The lines are broader than the atom lines and are not accompanied by two spin-flip satellite lines.9 The doublet B with 8107 = 1586.7 f 1.5 MHz and g = 2.0010 f 0.0003 has been previouslys assigned to tricarbonylsilver with the unpaired electron located largely in the Ag 5s orbital, i.e., pyramidal (2AI') Ag(C0)3. In addition to these symmetric doublets there are two groups of lines C and central features D and E as well as several other minor components. The lines C are the outer lines of a species either with one large Ag hyperfine interaction (hfi) of -840 MHz or with two large and equivalent Ag hfi of -420 M H z and an M I = 0 line that is masked by the central features. Lines C carry small Ag hfi from at least three equivalent Ag nuclei with 8107 22.5 MHz and are assigned to Ag cluster carbonyl Ag,(CO), with n 3 5 . Features D and E are assigned to an anisotropic conduction electron spin resonance (CESR)I2+I3 from small particles of silver
-
(1 1) Belford, R. L.; Nilges, M. J. Computer Simulation of Powder Spectra, EPR Symposium, 21st Rocky Mountain Conference, Denver, CO, Aug 1979. (12) Ozin, G. A. J . A m . Chem. SOC.1980, 102, 3301-3303.
and part of the spectrum of trigonal planar tricarbonylsilver (see later), respectively. The EPR spectrum given by Ag and 13C0 (0.05 Torr) in adamantane at 77 K is shown in Figure 1b. The doublet A is still present, and B is a doublet of quartets with 8107 = 1587 f 1.5 MHz and aI3(3)= 53.8 f 1.5 MHz, confirming the stoichiometry of this tricarbonylsilver species. In addition to A and B there are two other lines F in this region of the spectrum. Upon annealing to 182 K it is clear that lines F are the outer lines of a doublet of doublets, implying that they can be assigned to a monocarbonylsilver, AgI3CO, with alo7= 1683 f 1.5 MHz, 8 1 3 = 30.5 f 1.5 MHz, and g = 2.0022 f 0.0003. Lines A in the Ag/I2CO system are, therefore, made up of overlapping lines from Ag atoms (A) and AgCO (F). There is no evidence in this experiment for a doublet of triplets that can be assigned to dicarbonylsilver, Ag(I3CO),. The resolution of spectrum C is lower with I3CO, which is further evidence that the carrier of the spectrum is a silver cluster carbonyl. Features D and E are still present, but there are extra lines in the central area region of the spectrum. As Ag/CO/adamantane codeposits are warmed, lines A, B, and F decrease slowly and are still present at 21 8 K but the lines from AgCO (F) and Ag(CO), (B) are relatively more intense than those from Ag atoms (A). The spectrum on further annealing to 243 K is shown in Figure 2a. At this temperature only lines C remain; the small Ag hfi are lost and the large Ag hfi is clearly anisotropic. Analysis of this spectrum, on the assumption that it has two magnetically equivalent Ag nuclei, gives the parameters a,, = 418.5 f 10 MHz, a , = 451.2 f 10 MHz, g,, = 1.9950 f 0.002, and g, = 1.9948 f 0.002. The spectrum that can be simulated from these parameters is shown in Figure 2b and is in good agreement with the experimental spectrum although the latter spectrum exhibits residual features from Ag microcrystallite CESR. All the lines disappeared on further annealing to 300 K, but on recooling to 100 K a three-lined spectrum with 8107 590 MHz and g = 2.085 developed which has been assigned to Ag514 but also has parameters similar to those for Ag7.15 It would, therefore, appear that the C O ligand(s) dissociates (dissociate) irreversibly from the Ag cluster at 300 K whose stoichiometry is either Ag,(CO), or Ag7(CO),, where x 3 1. Reaction of Ag atoms with I70-enriched CO gives a spectrum similar to Figure la, but I7O hfi are not resolved for any of the silver carbonyls; Le., 8 1 7 < 3 MHz. This contrasts with 8 1 7 = 11.2 MHz for C U ( C O ) ~ . ~ The low-field region (2910-3010 G) of the spectra resulting from reaction of Ag atoms with higher partial pressures of C O and I3CO (-1 Torr) are shown in parts a and b of Figure 3, respectively. Lines B from pyramidal Ag(C0)3 and Ag('3C0)3 now dominate the spectra, but there are also other features of interest. The line labeled G is the MI = transition of a doublet
-
(13) Andrews, M. P.;Ozin, G. A. J . Phys. Chem. 1986, 90, 2922-2928. (14) Howard, J. A,; Sutcliffe, R.; Mile, B. J . Phys. Chem. 1983, 87, 2268-221 1. (15) Bach, S. B. H.; Garland, D. A,; Van Zee, R. J.; Weltner, W., Jr. J . Chem. Phys. 1987, 87, 869-812.
Reaction of Ag Atoms with C O in Rotating Cryostat
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2747
a 9149 MHr
, 2 0 0 ,
6
Figure 5. Central feature from 107Ag/12CO/adamantanewith continuous photolysis during deposition at 77 K (a) and computer-simulated spectrum of planar Ag(CO), using the parameters given in the text (b). 9145.7 MHZ
2910 G
p,,
,
40 G
a
3010 G
Figure 3. Low-field lines from 107Ag/'2C0(a) and Io7Ag/"CO (b) in adamantane at 77 K at higher C O partial pressures. 9149MHr
IA
B
J/ 3100 G
1
3400 0
I,"
Figure 4. EPR spectrum from 107Ag/12CO/adamantane with continuous photolysis during deposition at 77 K.
with 8107 = 1646.3 f 1.5 MHz and g = 2.0010 f 0.0005 and becomes a doublet of triplets with a13(2) = 53 f 1 MHz with Ag/13C0. This spectrum can, therefore, be assigned to dicarbonylsilver Ag(C0)2. The carrier of the minor feature B' with aIo7= 1400 f 2 MHz and g = 2.000 f 0.0005 has not yet been positively identified. Figure 4 shows the EPR spectrum given by 107Ag/'2C0 in adamantane in which the Ag atoms are photolyzed before bombardment with CO. This spectrum has two doublets, one from overlapping doublets from Ag atoms (A) and AgCO (F) and the second from Ag(C0)3 (B) with similar intensities and a central feature that consists mainly of the feature E in Figure 1. Interestingly the lines from silver microcrystallites D and cluster carbonyl C are suppressed by photolysis. The central feature is shown in expanded scale in Figure 5a and is similar to the spectrum that has been assigned to planar trigonal AB(CO)~in solid argon prepared from natural Ag and C O by Kasai and Jones.7 This spectrum can be computer simulated by using the magnetic parameters lazz1= 78 MHz, g,, = 1.9988, lu,l = 7.8 MHz, g, = 1.9948, luy,l = 1.4 MHz, and gyy = 1.9925, Le., parameters identical with those reported by Kasai and Jones7 It should, however, be noted that a simulation with an axially symmetric = 8 MHz gives an equally good fit whereas Ag hfi, [uxxl= ~uy,,~ an orthorhombic g tensor is definitely required. This conformer of Ag(C0)3 has the same planar (D3h)structure as C U ( C O ) ~which , is isolobal with CH3. A significant fraction of the unpaired spin population (-0.5) is located in the metal 5p, orbital with the remainder distributed among the carbon and oxygen 2p2 orbitals of the ligands. Unfortunately, the spectrum, E, of Ag(C0)3 disappears, upon annealing, before the plastic temperature of adamantane is reached, and an isotropic spectrum of the quality given by C U ( C O ) and ~~~ Al(CO)217is not observed
Figure 6. Central feature from 107Ag/13CO/adamantanewith continuous photolysis during deposition at 77 K (a) and computer-simulated spectrum of planar Ag(13CO), using the parameters given in the text (b).
for Ag(C0)3. This suggests that planar Ag(C0)3 is less stable to unimolecular decomposition than is planar C U ( C O ) ~ . Reaction of lo7Agwith l 3 C 0 under these conditions gives a spectrum, the central part of which is shown in Figure 6a. This spectrum is similar to that for planar Ag(13C0)3in argon7 and can be simulated (Figure 6b) by using Kasai and Jones' g tensors and Ag hfi of '07Ag(CO)3together with laill = 3 MHz and lull = 24 MHz from three equivalent I3C nuclei. 107Ag/'2CO/C6Dlz at 100 K give the spectrum shown in Figure 7a. There are at least five doublets (H, I, J, K, and L) with large Ag hfi (1521.3-1775 MHz) and g factors close to 2.0023. In the absence of CO only one doublet is formed from matrix-isolated Ag atoms with 8107 = 1723 MHz and g = 2.0021,9 parameters almost identical with those for species I. The rest of the spectrum is dominated by a single line N at g = 2.01 1 and six multiplets indicated by the stick diagram M. '07Ag/'3CO/C,D12 at 100 K gives a similar spectrum (Figure 7b) except that the relative line intensities are different, which is probably a function of experimental variability, the minor doublets H and L are absent, and a new doublet 0 with a,o7 = 1630 1.5 MHz and g = 2.0023 f 0.0003 is evident. Since none of the doublets from Ag/CO become doublets of multiplets with I3CO they cannot be assigned to monosilver carbonyls with significant unpaired spin population on CO ligands. The presence of five doublets apart from those from naked Ag atoms with large (16) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1033-1036. (17) Chenier, J. H. B.; Hampson, C. A,; Howard, J. A,; Mile, B.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1524-1528.
2748
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988
Chenier et al. TABLE I: EPR Parameters of Silver Carbonvls in Adamantane" Ag9
AgCO A%(C0)2 Ag(CO),C Ag(C0)3d
1676.6 1682 1646.3 1586.7 78 (z) -7.8' ( x ) -1.4' 01)
30.5 (1) 53 (2) 53.8 (3) 3 24 24
2.0021 2.0022 2.001 2.0009 1.9988 ( 2 ) 1.9948 ( x ) 1.9925 01)
"Hyperfine interactions in MHz, with errors of f 1 . 5 MHz except the anisotropic parameters of planar Ag(CO),, which have much larger errors. Number of equivalent carbon nuclei in parentheses. Pyramidal. dPlanar. These parameters have been chosen although we could not distinguish the orthorhombic parameters from the axially symmetric parameters laxl = la,l = lail = 8 MHz.
b '
IN
TABLE I 1 EPR Parameters of Io7Ag/CO in Cyclohexanea species a107 g ref Ag
H I
J K L
1723 1775 1724 1690 1640.6 1521.3
2.0021 2.0017 2.0020 2.0021 2.0010 2.0010
9 this this this this this
work work work work work
Hyperfine interactions in MHz f 1.5 MHz.
Figure 7. EPR spectra given by Io7Agand '*CO (a) and Io7Agand "CO (b) in cyclohexane at 77 K.
Ag hfi does, however, indicate perturbation of the unpaired spin population on matrix-isolated Ag atoms by neighboring CO molecules in cyclohexane trapping sites. The central feature N is similar in C O and 13C0experiments and is similar to the central feature in the absence of C O that has been assigned to the conduction EPR spectrum of silver microcry~tallites.~J~J~ The multiplets M are relatively more intense from Ag/13C0, and the resolution is similar to that from Ag/IZCO. The carrier(s) of this spectrum is (are) still unknown although the relative line intensities suggest that it is not a species with five equivalent Ag nuclei. (It is certainly not adventitious Mn or Mn2+ ( I = 5 / 2 ) which in adamantane produce sharp almost isotropic spectra with aMn= 73.1 M H z and g = 2.0010 and aMn= 256.3 MHz and g = 2.003, respectively.) Carbon monoxide does, however, appear to be essential for its production. FTIR. IR spectra of Io7Agatoms and CO, in adamantane at 77 K in the CO stretching region (2200-1700 cm-I) give a broad band at 1952 cm-' and a broad envelope of absorptions from 2160 to 2030 cm-' centered at -2100 cm-I, Le., -30 cm-I below the band for matrix-isolated C O (2134 cm-I). A similar pattern of bands is obtained from I3CO with all the bands shifted to lower frequency showing that they can all be assigned to silver carbonyls. The spectrum from Ag and a 1:1 mixture of I2C0and 13C0 has a broad band centered at 2050 cm-' and weak bands at 1952 and 1910 cm-l with tentative evidence for two even weaker bands between 1952 and 1910 cm-I. When the matrix is changed to cyclohexane, only a broad band at 2075 cm-' with a width at half-height of -45 cm-I is observed. There is no evidence for a band at -1950 cm-' from planar Ag(C0)3. Discussion EPR parameters of mononuclear silver carbonyls produced by reaction of Ag atoms with C O on adamantane and cyclohexane a t 77 K in a rotating cryostat are summarized in Tables I and 11. There are clearly fundamental differences between these two inert hydrocarbon matrices, and the results are discussed separately.
Adamantane Matrix. Ag and C O give a plethora of silver carbonyls which include the mononuclear mono-, di-, and tricarbonyls AgCO, Ag(CO),, and Ag(CO),, silver cluster carbonyls, and microcrystallites which may or may not have associated C O ligands. Perhaps the most intriguing aspect of this system is, however, the formation of two conformers of Ag(CO), in the same matrix. We have previously rationalized18 the existence of planar 2A2/1Ag(CO), in argon and pyramidal 2Al' Ag(CO), in adamantane in terms of matrix effects. Thus we have concluded that the 2Al' state represents the geometry of the free molecule, while in argon the reduced size of the substitutional sites imposes a planar configuration on the molecule. The trapping of two conformers in adamantane could, therefore, be attributed to two distinct trapping sites, but it should be noted that only the 'Ai' state of Cu(CO), has been observed in adamantane.I6,I9 It is interesting that the formation of the 2A2/1state of Ag(C0)3 appears to be enhanced by photolysis during deposition of the reactants. This may indicate that additional energy is required to bring about sp2 hybridization of the orbitals of Ag relative to those of Cu, but this is not entirely consistent with a smaller s p promotional energy for Ag than CU.~O The composition of the singly occupied molecular orbital (SOMO) of monosilver carbonyls can be estimated from the experimental hyperfine interactions and the calculated atomic parameters A and P for unit spin population in s and p orbitals respectively in the usual way.21 Thus we find that AgCO, Ag(CO),, and pyramidal Ag(CO), have 5s unpaired spin populations ( p s , ) = 1.003, 0.98, and 0.94, respectively, indicating that the S O M O s of these carbonyls have almost entirely metal s character. These carbonyls do carry 13Chfi which are virtually isotropic, and there is little C 2p contribution to the S O M O while the C 2s contributions (p2,) are 0.008, 0.014, and 0.014 for AgCO, Ag(CO),, and Ag(C0)3, respectively, which probably arises by bond polarization. None of these silver carbonyls exhibits a resolved 170hfi. The magnetic parameters of CuCO, AgCO, C U ( C O ) ~and , planar Ag(C0)3 in argon and adamantane are compared in Table 111; bent Ag(CO), is not detected in argon and linear Ag(C0)2 is not detected by EPR spectroscopy in argon or adamantane. The
-
(18) Howard, J. A,; Mile, B.; Morton, J. R.; Preston, K. F. J. Phys. Chem. 1986, 90, 2027-2029. (19) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. Chem. Phys. Lett. 1985, 117, 115-117. (20) Moore, C . E. Atomic Energy Leoels; NBS 3 5 ; U S . Government Printing Office: Washineton. DC. 1971: Parts 1-111. (21 jMorton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577-582.
Reaction of Ag Atoms with CO in Rotating Cryostat
The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2749
TABLE III: Magnetic Parameters of 63Cuand IMAg Mono- and Tricarbonyls4
CUCO
AgCO
argonb Cu(C0)j
1.998 1.995 1.996 4174 4126 4142 182 182 182
1.9998 2.0003 2.0001 1778 1792 1787 4s 36 39
2.0008 2.0002 2.0004 233.5 -10 71.2 6 -3 1 -18.7
Hyperfine interactions in MHz.
AWO),
CuCO
AgCO
1.9988 1.9948, 1.9925 1.9954 78 -7.8, -1.4 22.9
1.9966 1.9966 1.9966 3961 3961 3961 191 191 191
2.002 2.002 2.002 1682 1682 1682 30.5 30.5 30.5
3 -24 -15
-4.2 K, calculated isotropic parameters.
parameters for all these group 11 carbonyls are similar in the two quite different matrices, suggesting that structures must be close to those expected in the gas phase. The value of pSsfor AgCO is closer to the atom value than p4s for CuCO, which is linear with a zZ+ ground state with the unpaired electron in a sp hybridized orbital pointing away from Kasai and Jones7 have suggested that this the CO ligand.3~5~z2 is not the structure of AgCO but that Ag and CO occupy substitutional sites in face-centered-cubic solid argon with the Ag-C distance close to the nearest-neighbor separation of the lattice. An a b initio study on AgCOz2 has shown that the bonding is similar to that in CuCO but that the interaction between Ag and CO is much weaker and the calculated unpaired spin populations are p(5s)Ag = 0.94 and p(5p)A = 0.05. The total Ag-C atomic overlap is slightly negative and A back-donation from the metal to the 27r* orbitals of C O is less important than it is for CuCO. Interestingly we find that AgCO persists in adamantane to much higher temperatures than CuCO, contrary to what is predicted by theory. The theoretical work of Tsezz indicates formation of planar Ag(CO), rather than the pyramidal form with the Z A Fstate more stable by 106-112.5 kJ mol-I. The metal-CO ligand interaction in the zAl' state is strongly repulsive, and he suggests that the molecule is best described as an Ag atom and three CO molecules held together by van der Waals forces. Furthermore, Tse has argued that the unpaired spin population will never be predominantly Ag 5s for the pyramidal geometry. It is unlikely that the three silver carbonyls AgCO, Ag(CO)z, and pyramidal Ag(C0)3 with their well-resolved EPR spectra are merely statistical assemblies of Ag with one, two, or three C O molecules held together only by enclosing molecules of the trapping site. Rather it seems more reasonable that they are bona fide chemically bound complexes in which each of the empty 5p orbitals of Ag progressively accepts electron pairs from the 5u orbitals of up to three C O ligands. This would give linear AgCO, bent Ag(CO)z with a OCAgCO angle of 90°, and pyramidal Ag(CO), with the unpaired electron in the Ag 5s orbital. Our observation of both planar and pyramidal Ag(C0)3 in adamantane suggests an energy difference significantly less than the theoretical estimate of -100 kJ mol-'. McIntosh and Ozin6 originally suggested that Ag(CO), has a trigonal-planar structure and EPR spectra in argon7 and adamantane are consistent with this structure for one conformer. The unpaired electron is essentially localized in the Ag 5p, atomic orbital with the three C O ligands donating their 5u electrons to three sp2 hybrid orbitals io the x y plane; Le., the molecule has D3h symmetry and is isostructural with Ag(CN)33-.18 The dipolar hyperfine interaction Adlp calculated from
is 27.5 MHz if (al) = (-7.8 - 1.4)/2. Dividing Adip by a P ('A'di,,), where a,the angular factor for a p orbital, is 0.4 and P , the atomic parameter for unit spin population in a p orbital, is 52.7 MHz,I8 gives p5p= 1.3. This is significantly larger than (22) Tse, J. S . Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 906-912.
adamantanec Cu(CO)% 2.0010 2.0029 2.0023 225
0 94 -10 -30 -22 11.2
AdCO), 1.9988 1.9948, 1.9925 1.9954 78 -7.8, -1.4 22.9 3 -24 -1 5
