114
J . Phys. Chem. 1989, 93, 114-117
A Spectroscopic Study of the Reaction of Cu Atoms with CO in a Rotatlng Cryostat: Evidence for the Formation of CuCO, Cu(CO),, and Cu,(CO),’ J. H. B. Chenier, C. A. Hampson? J. A. Howard,* and B. Mile* Division of Chemistry, National Research Council, Ottawa, Ontario, Canada K l A OR9 (Received: March 30, 1988)
Reaction of 63Cuatoms with CO has been studied in inert hydrocarbon matrices in a rotating cryostat at 77 K by EPR, FTIR, and UV/visible spectroscopy. CuCO and CU(CO)~ have been identified by EPR spectroscopy. CuCO has the magnetic parameters a63 = 3961 MHz, aI3= 191 MHz, and g = 1.9966 and is a linear molecule with most of the unpaired spin located in an sp-hybridized orbital on Cu. It is unstable and disappears rapidly above 77 K. CU(CO)~ is a planar trigonal K radical with a 2 A F electronic ground state and a17= 11.2 MHz which indicates significant unpaired spin population on the ligands. It has three infrared bands at 1998, 1988, and 1978 cm-‘ which are assigned to the CO stretching mode of a D3,,molecule in three different trapping sites and is significantly more stable than CuCO. There are no bands in the FTIR spectra from Cu(CO)*. cu2(co)6 is formed in significant yields in adamantane and cyclohexane at 77 K and has a major CO stretching mode at 2035 cm-I. Optical spectra of CU(CO)~ and cu2(co)6 in adamantane have ‘Al’ and u* u electronic transitions at 565 and 405 nm, respectively.
-
Introduction There have been several spectroscopic studies of the interaction of copper atoms with C O in rare gas mat rice^.^-^ Thus Odgen3 found that Cu and C O at 12 K gave an IR spectrum that had two bands in the CO stretching region attributable to a copper carbonyl. In a similar study Huber, Kiindig, Moskovits, and Ozin4-’ assigned bands in the IR spectrum to CuCO, Cu(CO)’, Cu(CO),, and C U ~ ( C O )The ~ . formation of CuCO and C U ( C O ) ~ from Cu and C O in argon matrices at -4 K has been confirmed by EPR spectroscopy.* These studies suggested that CuCO is ~ trigonal planar linear with a 2Z+ ground state and C U ( C O ) is with a 2 A F electronic ground state in D3hsymmetry. We have previously reported an EPR study of C U ( C O ) ~in adamantane from 77 to 250 K99lothat confirmed the structure and electronic ground state of this binary carbonyl. There was, however, some disagreement in the nature of the semioccupied molecular orbital (SOMO). We concluded that most of the unpaired spin population (Cp)was located in the metal 4p, orbital while Kasai and Joness estimated that p(4p,) was 0.41. Theoretical calculations have been equally unsuccessful in predicting the composition of the SOMO. Thus nonrelativistic scattered wave calculations assigned the unpaired electron largely to the C O ligands” and an ab initio SCF calculationi2gave p(4p,) = 0.5 1. In a more recent study Arratia-Perez, Axe, and Marynick13 calculated by the partial retention of diatomic differential overlap (PRDDO) method that the most stable ground-state structure was trigonal planar. Magnetic parameters were calculated by the Dirac scattered wave (DSW) method and quasirelativistic spin unrestricted (QRU) methods and gave p(4p,) 0.25. We have recently performed a more thorough spectroscopic
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(1) Issued as NRCC No. 29518. (2) Department of Chemistry and Biochemistry, Liverpool Polytechnic, Liverpool, England L 3AF. (3) Ogden, J. S. J. Chem. SOC.,Chem. Commun. 1971, 978-979. (4) Huber, H.; Kiindig, E. P.; Moskovits, M.; Ozin, G. A. J . Am. Chem. SOC.1975, 97, 2097-2106. ( 5 ) Moskovits, M.; Ozin, G. A. Cryochemistry; Wiley-Interscience: New York. 1976: I) 342. (6jOzinl ‘G. A. Appl. Spectrosc. 1976, 30, 573-585. (7) Moskovits, M.; Hulse, J. E. J. Phys. Chem. 1977, 81, 2004-2009. (8) Kasai, P. H.; Jones, P. M. J . Am. Chem. Soc. 1985, 107, 8 13-8 18. (9) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. Chem. Phys. Lert. 1985, 117, 115-117. (10) Howard, J. A.; Mile, B.; Morton, J. R.; Preston, K. F.; Sutcliffe, R. J . Phys. Chem. 1986, 90, 1033-1036. (1 1) McIntosh, D F.; Ozin, G. A,, Messmer, R. P. Inorg. Chem. 1981,20, 3640-3650. (12) Tse,J. S. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 906-912. (13) Arratia-Perez, R.; Axe, F. U.; Marynick, D. S. J. Phys. Chem. 1987, 91, 5177-5183.
0022-3654/89/2093-0114$01.50/0
-
study on the carbonyl produced from Cu and C O in inert hydrocarbon matrices at 77 K in a rotating cryostat. In this work we have prepared CU(CO)~(C”O)and the magnetic parameters of this species give a more reliable empirical estimate of the contribution made by the atomic orbitals of each atom to the SOMO. In addition we have shown that two additional copper carbonyls, CuCO and C U ~ ( C O )are ~ , produced at 77 K.
Experimental Section The rotating cryostat and furnace used to vaporize Cu have been described previ0us1y.l~ The sequence of deposition on to the cold drum was matrix, Cu, and CO (after removal of all traces of 02,see below). EPR spectra were obtained on Varian E-4 and E-I2 instruments equipped with a Systron-Donner Model 6054 microwave frequency counter and a Brucker Model ER 035 proton magnetometer. Temperature control was achieved with an Oxford Instruments ESR 9 liquid helium cryostat and a cold nitrogen gas controller. IR spectra of deposits on the drum of the cryostat were obtained on a modified Mattson Sirius 100 FTIR spectrometer as described previo~sly.’~They were recorded initially at 77 K and then at higher temperatures by warming up the drum of the cryostat. If major changes in the spectrum occurred at higher temperatures the cryostat was recooled to 77 K and the spectrum recorded at this temperature. Spectra were the average of 256 scans at 4 cm-’ resolution from 4000 to 400 cm-* and the matrix absorption spectrum was usually subtracted from the matrix/Cu/CO spectrum. In situ UV/visible spectra were obtained by using an experimental technique that will be described in full elsewhere.16 In brief, light from a pulsed xenon lamp (10 Hz) monochromator combination was directed with a silica fiber optical on to the deposit at an angle of 10’ to the normal and the diffuse reflected beam was collected by another fiber optic (along the normal) and transmitted to a phase-locked photomultiplier detector. A reference spectrum was obtained by masking the lower half of the deposit so that only the top portion contained the metal species. A minicomputer with associated software was used to increment the wavelength output from the monochromator, store the detector output at each wavelength from the two halves of the deposit, and process the data so as to give an absorbance-wavelength curve. Reactions at 195 K were carried out by filling the drum with solid C 0 2 instead of liquid nitrogen. (14) Buck, A. J.; Howard, J. A.; Mile, B. J . Am. Chem. SOC.1983, 105, 3381-3387. (15) Howard, J. A.; Sutcliffe, R.; Hampson, C. A,; Mile, B. J . Phys. Chem. 1986, 90, 4268-4273. (16) Howard, J. A,; Mile, B.; Tomietto, M., unpublished results.
Published 1989 by the American Chemical Society
Reaction of Cu Atoms with C O
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 115
, 100G ,
9109.7 MHz
530 G 1055 G 2548 G 5015 G Figure 2. EPR spectrum at 10 K of 63Cu13C0in adamantane.
IC 9123.2MHz
3255 G
Figure 1. EPR spectrum at 250 K of 63Cu(CO)3and 63Cu(C170)(CO)2 in adamantane. The stick diagram indicates sextets assigned to I7O hfi.
was obCopper oxide enriched to 99.89% in the isotope tained from Oak Ridge National Laboratory, TN. It was reduced to the metal by hydrogen at 500 OC. Carbon monoxide (99.99%), "CO enriched to 99.9 atom % in I3C, and C170 enriched to 36.8 atom % in I7O were obtained from Matheson; Merck, Sharpe, and Dohme Ltd., Montreal; and Prochem Isotopes, Summit, NJ, respectively. We found it imperative to pass the carbon monoxide through a bed of MnO on celite immediately before deposition so as to remove the last traces of O2 which could enter the gas stream through minute leaks in valves, pressure gauges, etc. The MnO reduced the concentration of O2to less than 1 ppb.I7 The inert matrices were adamantane, cyclohexane, and perdeuteriocyclohexane.
Results EPR. Cu atoms and CO in adamantane gave an EPR spectrum at 77 K that was dominated by four absorption shaped lines centered at g = 2.001 and an asymmetric triplet at g = 2.0028.9J0 These features were assigned to the powder spectrum of C U ( C O ) ~ and were consistent with a species that had (i) a( >> a,, (ii) a Cu hyperfine constant and a nuclear Zeeman term of similar magnitude, and (iii) a nuclear quadrupole term that had to be included in an axially symmetric spin Hamiltonian. This spectrum persisted at temperatures above 77 K and became "solution-like" above the plastic temperature of adamantane; Le., it resolved to give four symmetric first-derivative lines with a63 = 94 MHz and g = 2.0014 at 250 K. [XRD examination from 77 K to room temperature of adamantane deposited on the rotating cryostat and transferred a t 77 K showed it to be crystalline in the tetragonal state a t 77 K and to undergo a phase change to the cubic rotor phase at 209 K, the temperature at which many species trapped in adamantane give isotropic spectra, indicating that the guest molecules like the host molecules are rotating freely in their lattice positions.] The line widths were narrow enough that the M I = k 5 / 2 and f 3 / 2 ''0 hyperfine interactions associated with the Cu M I = f 1 / 2 lines were resolved in the spectrum from Cu atom and I70-enriched CO (Figure 1) and gave a17 = 11.2 MHz. This is only the second report of a I7O hfi in a metal carbonyl species and compares with a17 = 12.2 MHz for Al(C0)2 in adarnantane.l8 C U ( C O ) in ~ adamantane was persistent up to 250 K and only began to disappear irreversibly above this temperature. At 263 K it appeared to decay according to first-order kinetics with a half-life of 1.5 h. In addition to the spectrum from C U ( C O ) there ~ was a weak almost isotropic quartet a t 77 K with the magnetic parameters a63 = 3961 MHz and g = 1.9966 which disappeared immediately on warm-up to 100 K. These isotropic parameters are similar to those found for copper monocarbonyl, CuCO, in argon,s although in the rare gas matrix the powder spectrum resolved to
-
(17) McIlwrick, C R.; Phillips, C. S. G. J . Phys. E . Sei. Instrum. 1973, 6, 1208-1210.
(18) Chenier, J. H. B.;Hampson, C. A.; Howard, J. A.; Mile, B.; Sutcliffe, R.J . Phys. Chem. 1986, 90, 1524-1528.
40 G w
L-
tl' 324; G
li
/
Figure 3. EPR spectrum given by "Cu and CO in cyclohexane at 77 K. A, B, and C denote the three sets of parallel features.
give parallel and perpendicular copper hyperfine interactions and g anisotropy. The stoichiometry of this molecule was confirmed by the observation that 63Cu and 13C0gave an isotropic quartet of doublets with al3(l) = 191 MHz (Figure 2). Cu and C O in cyclohexane a t 77 K gave a spectrum similar to the one in adamantane except that three sets of parallel features, A, B and C were evident (Figure 3). The spacings of the lines in these quartets gave the following magnetic parameters: (A) all = 226.4 MHz and gll = 1.9998, (B) all = 199.8 M H z and gll = 1.997, and (C) all = 193.1 M H z and gll = 1.9996 at 77 K. The value all for A is close to the value of 224 M H z for Cu(CO), in adamantane at 77 K while B and C have significantly lower values. The three quartets could be associated with the tricarbonyl in three magnetically different trapping sites or conceivably one of them could be from copper dicarbonyl, CU(CO)~. This species is believed to be linear with a zIIground state and However, if the degeneracy of the to be EPR silent at 77 p orbitals is lifted by a matrix perturbation, Cu(CO), would give a powder EPR spectrum that might be difficult to distinguish from C U ( C O ) ~because of similar 4p, contributions to the SOMO. The influence of temperature on the Cu hfi is a diagnostic test for Cu(CO), because it has been shown to increase with temperature above 100 K as a result of an out-of-plane vibration that modulates the isotropic or average Cu hfi.1° Of the three quartets observed in cyclohexane all for B and C increased by -3 MHz in the temperature range 77-137 K whereas all for A actually decreased by -3 MHz. On this evidence alone one might be tempted to suggest that the carrier of spectrum A was C U ( C O ) ~ . There was, however, no IR evidence for this carbonyl (see later). FTZR. FTIR spectrum from 63Cu atoms and I 2 C 0 in adamantane at 77 K after 2 min deposition is shown in Figure 4a. It consists of two major absorptions, a broad band (H) at 2035 cm-' with a shoulder at 2041 cm-I, and three narrow bands (I) at 1998,1988, and 1978 cm-' and a weak band (J) at 2014 cm-'. At lower pressures of CO band (H) resolved into two bands at 2041 and 2035 cm-I. These bands increased in intensity relative to bands I as the deposition time increased and upon annealing to 170 K. Further annealing to 230 K resulted in the development of a broad band at 2073 cm-l and a slow loss in intensity of the H and I bands until eventually only the 2035- and 1988-cm-' absorptions remained. The same pattern of lines shifted by -40 cm-' to lower frequencies (H at 1993 and 1986 cm-I, I at 1952, 1945, and 1932 cm-I, and J at 1966.7 cm-I) and showing the same annealing and K.438
Chenier et al.
116 The Journal of Physical Chemistry, Vol. 93, No. 1, 1989
_ _I 1
0 136r
TABLE I: Magnetic Parameters of 63CuC0and QCu(CO)pin Argon and Adamantane'
argonb
IY!
0.09061
g11 g, g
~ll(M) a,(M)
I . , *
2m a 4:
a
aM a11(C)
al(C)
0.00 2200
2100
2000
1900
1800
1700
aC 00
1.998 1.995 1.996c 4174 4126 4142d 182 182 182'
CuCO adamantane 1.9966 1.9966 1.9966 3961 3961 3961 191 191 191
CU(W3 argonb adamantane 2.0008 2.0002 2.0004' 233.5 -10 71.2d 6 -3 1 -18.7'
2.0010 2.0029 2.0023 225 0 94 --lo .--30 -22 11.2
"Hyperfine interactionsin MHz. -4.2 K. 'Calculated from dCalculated from (gll + 2g,)/3.
(all
+
2a,)/3.
orbitals of CO. The semioccupied orbital is the other sp orbital pointing away from CO, i.e., 0 113t
2200
2100
2000
1900
1800
1700
WAVENUMBERS / cm-'
Figure 4. IR spectra of 63Cu/C0 (a) and 63Cu/'3C0 (b) at 77 K in adamantane (after subtraction of the adamantane spectrum).
recooling characteristics was observed when 13C0was substituted for l2CO (Figure 4b). Bands H , I, and J are, therefore, from copper carbonyls and have frequencies similar to those that have been assigned to cu2(co)6, CU(CO)~, and cuco in rare gases."' There was no band at 1890 cm-' from Cu(C0)z. The IR spectrum in the v(C0) region form 63Cuatoms and CO in cyclohexane at 77 K after -5 min deposition consisted of a broad band at 2035 cm-I and two less intense bands at 2002 and 1988 cm-', These bands were all shifted to lower frequency (1988, 1957, and 1938 cm-I) when I3CO was used. Again there was no band that could be assigned to CU(CO)~. Reaction of the Cu atoms with CO at 195 K gave only a broad band at 2069 cm-' in the CO stretching region. UV/Visible. The purple-colored deposit from Cu atoms and CO in adamantane at 77 K, which had intense CO stretching exhibited a strong visible modes from cu2(co)6 and CU(CO)~, absorbance at 405 nm and a weaker band at 565 nm.
-
Discussion EPR. The magnetic parameters of CuCO and CU(CO)~ in argon and adamantane are compared in Table I. CuCO. The magnetic parameters of monocarbonylcopper are similar in the two distinct matrices although a63 is slightly smaller and a13 slightly larger in the inert hydrocarbon matrix. Dividing a63by A = 5995 MHz19 for 63Cuyields a value of 0.66 for the atomic 4s orbital contribution to the SOMO which is only slightly smaller than the values that have been found for monobenzenecopper (p(4s) = 0.69-0.72), monoacetylenecopper20,2' (p(4s) = 0.71), monoethylenecopperZo (p(4s) = 0.7) and mono(hydrogen cyanide)coppeP (p(4s) = 0.68473). Kasai and Jones* concluded that bonding in CuCO involves a a-type dative interaction in which the 5a electrons of CO are donated to an empty sp-hybrid orbital of Cu while *-type back-donation occurs from the Cu d orbitals of the correct symmetry into the empty 2a* (19) Morton, J. R.; Preson, K. F. J . Magn. Reson. 1978, 30, 557-582. (20) Kasai, P. H.; McLeod, D., Jr.; Watanabe, T. J . Am. Chem. Soc. 1980, 102. 179-190.
(21) Chenier, J. H. B.; Howard, J. A.; Mile, B.; Sutcliffe, R.J. Am. Chem. SOC.1983,105, 788-791.
(22) Howard, J. A.; Sutcliffe, R.; Mile, B. J . Phys. Chem. 1984, 88, 5 155-51 57.
The unpaired spin populations are only qualitatively consistent with this description since p(4s) is significantly > O S and p(4p) is too small to produce measurable hyperfine anisotropy in our spectrum. Furthermore, the small degree of hyperfine anisotropy in Kasai and Jones' spectrum* only gives p(4p) = 0.08. The I3Chfi of 191 MHz indicates a small amount of unpaired s spin population on carbon (p(2s) = 0.05) but information is not available on the C 2p orital contribution to the SOMO because of the isotropic nature of the spectrum. The unpaired spin population on C arises by a spin polarization mechanism and is negative. The spin count in CuCO is thus too low by at least 0.26. It is possible that there are contributions to the SOMO from the filled 3d orbitals of Cu as well as from the vacant 4p orbital. Since the sign of the Ad, term will be opposite for these two contributions its resultant value will be low and will not be a good measure of unpaired 4p spin population. TseI2 concluded from ab initio molecular orbital calculation that linear CuCO has a zZ+ground state but the potential energy surface does not have a minimum even at the calculated Cu-C bond distance of 1.9 A. Thus at this separation CuCO is unbound with respect to 2S Cu and a free CO. The unpaired electron was found to reside predominantly in the Cu 4s orbital with a small contribution from the 4p orbital polarized away from the CO ligand. The calculated unpaired spin populations are p(4s) = 0.79, ) ~0.01 and little spin p(4p) = 0.09, p(2s)C = -0.02, p ( 2 ~ = population on oxygen, values that are in reasonable agreement with the experimental p values in adamantane and argon. The major stabilizing force in CuCO is believed to involve d x backdonation from Cu to the x* orbital of CO. Theoretical calculationsi2also indicate that CuCO has a lowlying zII state which arises from bonding of CO to a *PCu atom. The potential energy surface for this state does have a minimum at a Cu-C distance of 1.9 8,and the 211state is bound with respect to 2P Cu and lZ+CO by -57.5 kJ mol-'. However, at the potential minimum the 211 state is about 118 kJ mol-' above the 2Z+ state. The low g for CuCO can be attributed to configuration interaction between the 28+and zII states. The theoretical calculations suggest that the CuCO observed in matrices is not a bona fide molecule but rather a caged pair complex in which the Cu and CO perturb each other because of their forced proximity in the matrix trapping site. A more recent CAS SCF c a l c ~ l a t i o non~ ~22+CuCO again did not yield a bound state while the binding energy of zrI CuCO was estimated to be 124.7 kJ mol-'. The detection of CuCO at 100 K does, however, indicate a binding energy of 20-30 kJ mol-'. Cu(CO),. Although the magnetic parameters of CU(CO)~ are similar in argon and adamantane, there is still some controversy (23) Broomfield, K.; Lambert, R. M. Chem. Phys. Lett. 1987, 139, 267-270.
The Journal of Physical Chemistry, Vol. 93, No. 1, 1989 117
Reaction of Cu Atoms with C O
-
-
regarding the nature of the SOMO; thus we concludedi0 that p(4p2) 1.1 while Kasai and Jones* estimated p(4p2) 0.41. This difference arises principally because we compared the dipolar hyperfine interaction (Adip) with an estimated value of 171 MHz for Po (the atomic parameter for an unpaired electron in a Cu 4p orbital) while Kasai and Jones8 used Po 500 MHz. A more recent calculation by Lindsay and K a ~ agave i ~ ~Po = 217.5 MHz which gives p(4p) = 0.86, a value still significantly larger than given by theoretical M O calculations."-i3 A large unpaired spin population in the Cu 4p, orbital would induce negative unpaired spin population at the carbon nuclei by spin polarization which would have its maximum value in the perpendicular direction, as is observed. The observation of an isotropic I7Oof 11.2 MHz does, however, indicate direct transfer of unpaired spin to the ligands. Unfortunately it was not possible to estimate A d , p for oxygen from the powder spectrum. However, the McConnell relationship ai7= -14p(2p2),, which related the isotropic 170hfi to the unpaired spin population in the 2p, orbital on oxygen25indicates that p(2p), 0.1 and the total unpaired p spin population on all the oxygens is 0.30. This suggests that Cp(2p)c is at least 0.3 because the lobes of the C O 277 orbital are larger at carbon than they are at oxygen. We therefore conclude that -60% of the unpaired spin population is located on the ligands and -40% at the metal center which is more in line with theoretical calculations than our previous estimate.I0 It should however be emphasized that estimates of unpaired spin populations for C U ( C O )are ~ at the best qualitative because the atomic Po value for Cu(4p) may be perturbed in C U ( C O ) and ~ the McConnell relationship may not apply to this transition metal complex. It is perhaps more informative to compare empirical I3C and I7O hfi with those calculated by theoretical methods. The QRU methodI3 gave a13 = -23 M H z and a17 = 15.7 MHz which are in good agreement with the empirical values of -22 and 11.2 MHz, respectively. FTIR. This spectroscopic technique confirms the formation of C U ( C O ) ~as the major paramagnetic mononuclear copper carbonyl in inert hydrocarbon matrices along with minor amounts of CuCO; C U ( C O ) is~ not detected. In addition to these mononuclear carbonyls the diamagnetic dinuclear carbonyl c u * ( c 0 ) 6 is formed a t 77 K, indicating mobility of Cu atoms on the solid hydrocarbon surface. This species is most likely produced by reaction of Cu(CO), with a mobile Cu atom followed by further reaction with CO, reaction 1, rather than dimerization of Cu(C0)3, reaction 2.
TABLE II: Infrared Absorptions of the Products of Reaction of Cu Atoms with CO at Cryogenic Temperatures species
-
-
cu(co)3
+cu
-+
cu2(co)3
co
cu2(co)6
(1)
The increase in concentration of c u z ( m ) 6 relative to C U ( C O ) ~ with time at 77 K is ascribed to an increase in temperature of the hydrocarbon surface as rt moves away from the cold surface of the drum and closer to the furnace. The formation of a carbonyl with vco = 2073-2069 cm-' upon annealing deposits containing mono- and dinuclear carbonyls to 200 K is probably associated with further clustering to give Cu,(CO), where x > 2. There is a monotonic red shift of vco from 21 16 to 2094 cm-' for the cluster carbonyls Cu,CO as n (24) Lindsay, D. M.; Kasai, P. H. J . Magn. Reson. 1985, 64, 278-283. (25) Melamud, E.; Silver, B. L. J . Phys. Chem. 1973, 77, 1896-1900.
matrix
vcn/cm-'
ref
argon adamantane argon
2010/2008.3 2014 1890.8/ 1876.1 1990/ 1976.8 1985 1998/ 1988/1978 2002/1988 2039/2003 204 1/2035 2035 21 16 2102 2094 2069/2073 2082
4 this work 4 3, 4 4 this work this work 4 this work this work
co cu2(c0)6
argon adamantane cyclohexane argon adamantane cyclohexane argon argon argon adamantane cyclohexane
" /
7 7 this work 27
increases from 2 to 4.7 This suggests that in our cluster carbonyl > 5 . This receives support from the preliminary observation that Cu7(CO), prepared from Cu7 and C O in the rotating cryostat2, has vco = 2082 cm-'. The C O stretching frequencies of copper carbonyls prepared by metal vapour deposition techniques are summarized in Table 11. The agreement between the values of vco in the two distinct types of matrix is satisfactory. The observation of three vibrational frequencies for C U ( C O )in ~ adamantane and two in cyclohexane arises because of different trapping sites and not to a distortion of the molecule from D3,,symmetry. Similarly cu2(co)6 occurs in two sites in adamantane and one in cyclohexane. UV/Visible. FTIR spectra indicate that C U ( C O ) and ~ Cu2(CO), are the major binary copper carbonyls given by Cu atoms and CO in adamantane at 77 K. The optical spectrum is consistent with this conclusion since the bands at 405 and 565 nm have u transition of c u 2 ( c o ) 6 (417 frequencies similar to the u* ~ nm) in CO! nm) and the 2Al' 2A2//transition of C U ( C O ) (562 The latter wavelength translates to 2.2 eV, in good agreement with the theoretical prediction.I2 x
-
-
Conclusions EPR, FTIR, and UV/visible spectroscopic studies on the same sample have shown that Cu atoms and C O react in solid hydrocarbon matrices at 77 K to give one major (Cu(CO),) and one minor (CuCO) mononuclear carbonyl and a dinuclear carbonyl, C U ~ ( C O )Thus ~ . the products are similar to those formed in rare gas matrices at lower temperatures. CuCO is a genuine complex and is much less stable than the 17-electron C U ( C O ) ~ .
Acknowledgment. C.A.H. thanks SERC for a studentship. B.M. acknowledges the financial help from SERC for the purchase of equipment and J.A.H. and B.M. thank NATO for a collaborative research grant (No. 442/82). We also thank M. Tomietto for help in developing the UV/visible reflectance spectroscopy unit and Dr. J. S. Tse for undertaking the XRD studies of adamantane at low temperatures. Registry NO. CUCO, 55979-21-0; CU(CO),, 55979-19-6; CU2(C0)6r 56174-61-9; 63C~(CO)3, 117341-02-3; 63C~(C'70)(CO)2, 117341-03-4; 63Cu13C0,117341-04-5; 63CuC0,117341-05-6; adamantane, 281-23-2; cyclohexane, 1 10-82-7; perdeuteriocyclohexane, 1735-17-7. (26) Howard, J. A.; Mile, B., unpublished results.
