J. Phys. Chem. 1985,89, 1147-1 15 1 systems as well as of ru and Fa. On the other hand, the situation is different when the NaBr concentration is 0.02 M or lower, where it was found that A+ < A and A > 0. Figure 5 shows the values of A, A+, and A as functions of C in the presence of 0.005 M NaBr. The values of A and A+ increase from a negative value at C = 0 and approach zero at high Cvalues. The value of A has a maximum at a certain C value. At low C values the behavior of A+ and A would be somewhat more complicated. As can be seen from Chart IA, the intrinsic adsorptions of D+ and Na+ are perturbed by the preferential adsorption of Br- over C1-. Then D+exhibits its strong adsorption power and penetrate into the surface, expelling some of Na+ from the surface. The preferential adsorptions of cations and anions interact with each
Downloaded by STOCKHOLM UNIV on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1985 | doi: 10.1021/j100253a019
Silver Carbonyls, Ag(C0) and Ag(CO),:
1147
other. The adsorption isotherms of four ionic species in the indifferent system can be quantitatively calculated by eq 21 or 20. By means of eq 28 we can further predict the surface excess densities of D+, Na+, and Br- in the common (Br-) system. Since D+ penetrates into the surface and C1- adsorbs less preferentially, we may expect that the adsorption isotherms of D+ and Br- in the common (Br-) system be higher than those in the indifferent system. We can furthermore predict the specific adsorptions of Na+, C1-, and Br- in the common (Na’) system by means of eq 25, which gives results of less interest. More detailed discussion based on the observed data of surface tension will be published soon.
Matrix Isolation ESR Study
Paul H.Kasai* and Paul M. Jones IBM Instruments, Inc., Orchard Park, Danbury, Connecticut 0681 0 (Received: October 9, 1984)
Silver carbonyls, Ag(C0) and Ag(CO),, were generated in argon matrices, and their ESR spectra were examined. The g tensors and the hyperfine coupling tensors to the Ag and I3C nuclei were determined for both complexes. For Ag(C0)
it is shown that the spin density in the Ag 5s orbital is 0.99, and the ”C hyperfine interaction is of the magnitude expected for “Ag atoms” substitutionally incorporated in the matrix separated from CO by the nearest-neighbor distance of the host lattice. Ag(C0)3 is a bona fide complex and is trigonal-planar. Its semifilled orbital represents the back-donation from the Ag 5p, orbital into the vacant r* orbitals of CO molecules.
Introduction
It is generally accepted that the bonding scheme of mononuclear transition-metal carbonyls such as Cr(C0)6 or Ni(C0)4 involves a a-type dative interaction between the lone-pair electrons of C O and the vacant orbitals of the metal atom and r-type back-donation from the filled d, orbitals of the metal atom to the vacant T* orbitals of CO molecules.’ Thermally stable carbonyls of the group 1 1 ” metal atoms (Cu, Ag, and Au), however, are not known. It is ascribed to the extra stability of the completed ndlo configuration of these atoms. Ogden2 and Ozin et al.,3-5however, showed that mononuclear carbonyls of group 11 metal atoms could be generated at cryogenic temperature (4--20 K) by co-condensation of the metal atoms and CO molecules in rare-gas matrices. Thorough analyses of the vibrational spectra by Ozin et al. rendered unequivocal proofs for the formation of Cu(CO), (n = 1,2, and 3) in the Cu/CO/Ar system3 and the formation of Ag(CO), ( n = 2 and 3) in the Ag/CO/Ar ~ y s t e m . The ~ analyses also showed that the dicarbonyls were linear, while the tricarbonyls were trigonal-planar. The dicarbonyls of linear geometry would have a 211 ground state, and their ESR spectra would hence be broadened beyond detection. The mono- and tricarbonyls of these atoms, on the other hand, should be amenable to ESR examination. Ozin et al. indeed showed ESR spectra of C U ( C O ) and ~ Ag(CO)3 in their report^.^,^ The analyses given, however, were cursory and erroneous in the case of Ag(C0)3. (1) See,for example: Cotton, F. A.; Wilkinson, G. “Advanced Inorganic Chemistrv”. 4th ed.: Wilev: New York. 1980 DO 82-86 and 1049-1079. (2) Ogden, J. S. J. Chim. SOC.,Chem. Commkn. 1971, 978. (3) Huber, H.; Kundig, E. P.; Moskovits, M.; Ozin, G. A. J. Am. Chem. SOC. _ . 1975. 97. 2097. (4) M&ntoshi D.; Ozin, G. A. J. Am. Chem. SOC.1976, 98, 3167. (5) McIntosh, D.; Ozin, G. A. Inorg. Chem. 1977, 16, 51. (6) Ozin, G. A. Appl. Spectrosc. 1976, 30, 573.
Accurate descriptions of the semifilled orbitals of these complexes, if obtained, should be extremely elucidative of the structures and bonding schemes involved. We have recently reported on ESR spectra of Cu(C0) and C U ( C O )observed ~ from the Cu/CO/Ar system.’ Reported in this paper are ESR spectra of Ag(C0) and Ag(CO)3 similarly generated in argon matrices. The g tensors and the hf (hyperfine) tensors of the Ag and I3C nuclei were determined for both complexes. The observed ESR spectrum of Ag(C0) is essentially that of “Ag atoms”. The spin density in the Ag 5s orbital is 0.99, and the 13Chf tensor is of the magnitude expected for Ag atoms substitutionally incorporated in the matrix separated from the I3C-labeled CO by the nearest-neighbor distance of the host lattice. Ag(C0)3 is a bona fide complex and is shown to be formed by the u-type dative interaction between the lone-pair electrons of C O and sp2 hybrid orbitals of the Ag atom and by the semifilled orbital representing the back-donation from the Ag 5p, orbital into the vacant r* orbitals of the CO moiety. Experimental Section
A liquid helium cryostat that would enable trapping of vaporized metal atoms in an inert-gas matrix and examination of the resulting matrix by ESR has been described earlier.* In the present series of experiments, silver atoms were generated from a resistively heated (-1400 “C) tantalum cell and were trapped in argon matrices containing controlled amounts of carbon monoxide ( 1-2076). The ESR spectrometer used was an IBM Model ER2OOD system, and a low-frequency (375 Hz)field modulation was used for the signal detection. All the spectra reported here were obtained while the matrix was maintained at -4 K. The specH.;Jones, P. M. J. Am. Chem. SOC.,in press. (8) Kasai, P. H.Acc. Chem. Res. 1971, 4 , 329. (7) Kasai, P.
0022-3654/85/2089-1147%01.50/00 1985 American Chemical Society
Kasai and Jones
1148 The Journal of Physical Chemistry, Vol. 89, No. 7 , 1985
C + l
Figure 3. Low-field and high-field components of signals A and B observed from the Ag/13CO(5%)/Ar system. The doublet splittings due to "C are indicated for the Io7Ag B signals.
Downloaded by STOCKHOLM UNIV on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1985 | doi: 10.1021/j100253a019
I
I lW0
I
I
I
1
I
1
asoo
I
7 0
Figure 1. ESR spectrum observed from the Ag/CO(S%)/Ar system. Signals A are due to isolated silver atoms; signals B and C are assigned to A g ( C 0 ) and Ag(C0)3,.respectively. Signals due to inadvertently formed formyl radicals, HCO, are also indicated.
of the individual spectra are presented below. ESR Spectra o f A g ( C 0 ) . As seen in Figure 1, the B signals are extremely similar to the A signals of isolated Ag atoms. The low- and high-field components of signals A and B observed from the Ag/CO( 1%)/Ar system are shown in Figure 2 in an expanded scale. In contrast to the A signals, the B signals are slightly askewed. The latter spectra were hence analyzed in terms of an axially symmetric spin Hamiltonian shown below."
A= gr1PeHrSr + g,Pe(HJx + Hysy) + A I I I A + A,(I$,
+ IyqJ
The following results were obtained for lo7Ag(CO): gll
= 1.9998 (6),
AIl(lo7Ag)= 1778 (1) MHz,
%.,O
,000
z,
$
t
1.JO l
'
'
'
"
,700 '
r
Figure 2. Low-field and high-field components of signals A and B ob-
served from the Ag/CO(l%)/Ar system (shown in an expanded scale). Weak signals indicated by arrows are due to unidentified species and are not directly associated with the B signals. trometer frequency locked to the sample cavity was 9.4280 GHz, and a typical microwave power level was -5 pW. Research-grade argon and CP-grade carbon monoxide were obtained from Matheson, and W-enriched (enrichment >90%) carbon monoxide was obtained from MSD Isotopes.
Results The ESR spectrum of Ag atoms (4d1° 5s') isolated in rare-gas matrices has already been a n a l y ~ e d . ~There are two naturally abundant Ag isotopes, lo7Ag(natural abundance = 5176, I = 1/2, p = -0.1 1308,) and lo9Ag (natural abundance = 49%, I = 1/2, p = -0.1299j3,). The ESR spectrum of Ag atoms thus comprises two sets of sharp doublets with the respective spacings of -650 and -750 G. Figure 1 shows the ESR spectrum observed from the Ag/ CO(5%)/Ar system. In addition to signals due to inadvertently formed formyl radicals,'O three types of signals, A, B, and C, were recognized as indicated. The A signals are the doublets of isolated Ag atoms discussed above. The B and C signals were observed only when the Ag atoms and C O molecules were co-condensed. In matrices with low C O concentration (lo%) the C signals dominated the spectrum. The color of the matrix with a given Ag concentration varied from pale to bright green with increasing C O concentration. On the basis of these observations and in cognizance of the C O stoichiometry determined in the IR study of Ozin et a1.: the B signals were assigned to Ag(C0) and the C signals to Ag(CO),. Analyses (9) Kasai, P. H.;McLeod, D., Jr. J . Chem. Phys. 1971, 55, 1566. (10) Adrian, F.J.; Cocharan, E. L.; Bowers, V.A. J . Chem. Phys. 1962, 36, 1661.
g, = 2.0003 (6)
A,(lo7Ag) = 1792 (1) MHz
As indicated by arrows in Figure 2, two weak additional signals were noted in association with each B signal. Their intensities relative to the B signals increased in proportion with the C O concentration and were 10% of the B signals in the Ag/CO(10%)/Ar system. The Ag hf coupling constants of these signals also increased with the C O concentration and approached the hf constants of the B signals. Clearly these signals are not directly related to the B signals. Their identities are yet to be determined. Figure 3 shows, in an expanded scale, the A and B signal regions of the spectrum observed from the Ag/"CO(S%)/Ar system. As it should be, signals A are not affected by the isotopic substitution. The doublet splittings of the 13C-labeledB signals centered about the nonlabeled B signals are clearly recognized as shown for the Io7Ag(l3CO)case. The 13C hf tensor of Ag(C0) was thus determined as follows.
-
All(13C)= 15 (1) G,
A,(13C) = 13 (1) G
Observation of the hf interaction with one 13Cnucleus constitutes a strong support for the assignment of the B signals to Ag(C0). ESR Spectra of Ag(CO)3. Figure 4a shows, in an expanded scale, the C signal region of the spectrum observed from the Ag/CO(S%)/Ar system. The weak, sharp quartet indicated by small arrows is due to inadvertently formed methyl radicals, CH,. The "envelope" of the remaining signals is essentially the same as the spectrum of Ag(CO), reported by Ozin et a14 The current spectrum is better resolved and shows additional details. The earlier analysis assigned the positive signal at 3355 G as the "parallel" component of the powder pattern expected from an axially symmetric system and the strong signal at 3385 G as the "perpendicular" component. The doublet splittings of these signals were then attributed to the hf structures due to the Ag n ~ c l e u s . ~ Several simulation attempts, however, made it immediately apparent that the upward intensity of the signal at 3385 G was too strong for a perpendicular component, and the additional signals at -3375 G could not be accounted for by this assignment. (1 1) For analyses of ESR powder patterns, see, for example: Ayscough, P. B. "Electron Spin Resonancc in Chemistry"; Methuen: London, 1967; pp
323-332.
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985 1149
Silver Carbonyls
TABLE I: g Tensors and lwAg and 13C Hyperfine Tensorsa of Ag(C0) and Ag(C0)3
Io7Ag complex Ag(CO) Ag(C0h
1 3 c
I l
I,
A II
AIb
A II
A,
1.9998 (6) 1.9988 (6)
2.0003 (6) 1.9948 (6) 1.9925 (6)
1778 (1)
1792 (1) -7.8 (8)
45 (3) +3 (1)
36 (3) -24 (3)
+78 (1)
-1.4 (6)
Downloaded by STOCKHOLM UNIV on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1985 | doi: 10.1021/j100253a019
"Given in MHz. For Ag(C0)3the perpendicular components of the g and Ag hf tensors were resolved into the x and y components.
I
I
I
I
All I
I
I
I
3400
3350
I
1
0
Figure 4. (a) The C signal region of the spectrum observed from the
Ag/CO(S%)/Ar system. Arrows indicate the quartet due to inadvertently formed methyl radicals. (b) Computer-simulatedspectrum based upon the parameters given in the text. The spectra due to the Io7Agand IwAg species were superimposed. It was noted that the doublet structures observed at 3355 and 3385 G represented respective spacings of 28 and 32 G, if paired as indicated. The ratio of these spacings is exactly that of the nuclear magnetic moments of lo7Ag and "Ag. The signals at -3375 G were then realized as the perpendicular component with a hf structure of -3 G. The optimum agreement between the observed and simulated spectra was achieved when a slight departure from axial symmetry was allowed for in both the g tensor and the hf tensor with the Ag nucleus. The g tensor and the Ag hf tensor of 107Ag(C0)3were thus determined as follows. g tensor A('"Ag)
z
X
Y
1.9988 ( 6 ) 27.7 ( 5 ) G
1.9948 ( 6 ) 2.8 (3) G
1.9925 ( 6 ) 0.5 (2) G
Here the original symmetry axis is identified with the z axis. Figure 4b shows the computer-simulated spectrum based on these parameters; the spectra due to both the lo7Agand '"Ag species were considered and superimposed. Figure 5a shows the ESR spectrum (the C region) observed from the Ag/13C(10%)/Ar system. The most peculiar aspect of the spectrum is that, in spite of radical changes caused by the
I
I
I
3350
1
I
I
I
I 3400
I
I 0
Figure 5. (a) The C signal region of the spectrum observed from the Ag/I3CO(10%)/Ar system. (b) Computer-simulated spectrum based upon the parameters used for Figure 4b and the interactions with three equivalent I3C nuclei with the hf tensor given in the text.
isotopic substitution, the quartet pattern expected from the hf interaction with three equivalent I3Cnuclei is not apparent anywhere in the spectrum. Although a slight departure from axial symmetry is predicted in the g tensor and the Ag hf tensor determined above, the axial symmetry of the complex is basically upheld, and the three carbonyl groups are expected to be (nearly) equivalent. If it is realized that the signal at 3355 G is totally due to the parallel component, the magnitude of A,,(I3C)may be assessed from the increase of the line width of the signal. Two downward signals at 3381 and 3396 G in Figure 5a are at higher fields than any signal observed from the normal complex (Figure 4a). Let us hence assert that these signals represent the upper two components of the expected quartet; the spacing between these signals is then A1(13C). The 13Chf tensor of Ag(CO), was thus assessed as follows. IA,(L3C)I= 1.0 (2) G, &(13C)l N IAy(13C)(= 8.7 (2) G Figure 5b is a computer-simulated spectrum based upon the g tensor and the Ag hf tensor determined earlier and the interaction with three equivalent 13C nuclei with the hf tensor given above. The agreement between the observed and simulated spectra is gratifying. It not only substantiates the assignment of the C signals
Kasai and Jones
1150 The Journal of Physical Chemistry, Vol. 89, NO.7, 1985 to Ag(C0)3 but gives credence to the analysis of the non-I3Clabeled spectrum.
Discussion The g tensors, the Io7Aghf tensors, and the I3C hf tensors of Ag(C0) and Ag(C0)3 determined in the present study are compiled in Table I. The hf tensor elements determined in gauss were converted to those in megahertz by multiplication by go,. The signs of the hf elements of Ag(C0)3 were chosen on the basis of rationale presented later in this section. Ag(CO), if formed, should be linear (Walsh’s rule) and should rely on a bonding scheme similar to that established for CU(CO).~ It would thus invoke, as illustrated below,
a dative interaction between the lone-pair electrons of C O and a vacant u orbital of Ag and the back-donation from the filled d, orbital of Ag into the a* orbital of CO. The semifilled orbital of the complex would be a hybrid sp, orbital of Ag pointing away from the ligand. An LCAO description of the orbital is given in eq 1. Here the term involving the carbon orbital, &(sp,), is Downloaded by STOCKHOLM UNIV on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1985 | doi: 10.1021/j100253a019
@
+ &Ag(SPo) - ck(sPu)
=
(1)
included in order to make the orbital orthogonal to the orbital of the CO lone-pair electrons. It has been shown that the principal elements A,, and A , of an axially symmetric hf tensor are related to the isotropic term Aisoand the anisotropic term Adjp as follows12
+ 2Adip
(2a)
A , = Ais0 - Adip
(2b)
Ai, = gePegnP,p,(8a/3) I@(O) I’
(2c)
All
=
where
Adip
= gd&n8.(
3 cos2 a - 1 2r3
)
Here 1@(0)12represents the spin density at the nucleus, r the separation between the nucleus and the electron, and a the angle between r and the symmetry axis. Only the spin density in an s orbital contributes to Aisoand that in a non-s orbital to Adjp. Analysis of the Ag hf tensor determined for Ag(C0) in terms of eq 2 yields the following: Aiso(Io7Ag)= 1787 M H z
The atomic values Abo and Adipoexpected for a unit spin density in the C 2s and C 2p orbitals, respectively, have been computed theoreti~a1ly.l~They are Aho(l3C) = 3780 MHz, and A&p0(13C) = 107 MHz. The spin density distribution in the C orbitals of Ag(C0) was thus assessed as ~ ( 2 s =) 0.01 ~ and p(2p)c = 0.02. Solid argon has a face-centered cubic lattice with a unit lattice dimension of 5.31 A. We suggest that the “Ag(C0)” detected here represents the situation where substitutionally incorporated Ag atoms and CO molecules attain a linear Ag-CO arrangement with the Ag-C distance equal (or close) to the nearest-neighbor separation of the lattice (3.8 A). For eq 1 we have already shown that u2 zi 1.O and b2 0.0. If there is to be no “active interaction” between the Ag 5s orbital and the sp, orbital of the lone-pair electrons, the semifilled orbital of “Ag(C0)” may now be given by eq 3. Here the overlap integral (&&s)( $c(sp,)) is for the @
($Ag(5S) - (~A~(SS)~&(SPU) )&(Spu)
indicated orbitals at the nearest-neighbor separation. Assuming that &(sp,) = (1/2lI2) [&-(2s) - ~ # ~ ~ ( 2 pand , ) ] using the Slater-type orbitals, the overlap integral could be estimated; the result obtained for the Ag-CO separation of 3.8 8, was 0.17. This then predicts the spin-density distribution of 0.014 each in the C 2s and C 2p, orbitals. It is thus shown that the observed I3C hf tensor is of the magnitude expected for Ag and C O separated by the nearest-neighbor distance of the host lattice. The corresponding overlap integral between Cu and C O for the same separation is smaller (0.13). The observed I3C hf coupling constant of Cu(C0) is 5 times larger, h ~ w e v e r . ~ Ozin et aL4 concluded “tentatively” that the C O stretch of Ag(C0) in argon occurred at 1958 cm-l, accidentally degenerate with the CO stretch of Ag(CO),. This assignment is questionable; according to the analysis presented above, the C O stretch of “Ag(C0)” should be very close to (or perhaps indistinguishable from) the stretching frequency of isolated C O molecules at 2140 cm-l. Significantly the same authors reported that it was impossible to observe the silver monocarbonyl signal free from those of di- and tricarbonyl~.~This is in sharp contrast with their experiences with other group 11 metal atoms. Monocarbonyls of Cu and Au atoms were clearly formed preferentially in matrices with dilute C O c ~ n c e n t r a t i o n . ~ , ~ The trigonal-planar structure of Ag(CO)3 has been firmly established by Ozin et al.4 The small but extremely anisotropic Ag hf tensor determined here indicates a substantial unpaired electron density in the Ag 5p orbital and no “direct occupation” of the 5s orbital. The complex must rely on the bonding scheme similar to that established for C U ( C O ) (see ~ be lo^).^
Adjp(Io7Ag)= -4.7 M H z The hf coupling constant of Io7Agatoms (4dI05s1)isolated in an argon matrix is 1810 MHz? The spin density in the Ag 5s orbital in Ag(C0) is thus assessed as follows: p(5S)Ag = 0.99. It can be shown readily that Adipgiven by eq 2d is a positive quantity for a spin density in a p orbital. The negative Adjp determined above hence states p(5p,)A, z 0.0. The negative Adip is best explained by admixture of 4d, orbital in the form of $AB(%) 6+,,(4d,); the electron-density distribution is thus made slightly oblate. The result obtained above is extremely interesting when compared with the result obtained earlier for Cu(CO).’ The Cu hf tensor of C u ( C 0 ) revealed that ~ ( 4 s =) 0.67 ~ ~ and p(4~,,)~,,= 0.08. Clearly “Ag(C0)” detected here could hardly be a bona fide complex homologous to Cu(C0). However, the presence of one C O molecule in the Ag(C0) entity is firmly established by its I3C hf structure. Analysis of the observed I3C hf tensor in terms of eq 2 yields the following: Ais,,(13C) = 38 M H z Adi,(I3C) = 2 MHz (12) Smith, W. V.; Sorokin,P. P.; Gelles, I. L.; Lasher, 1959, 115, 1546.
(3)
O==C:oAg
c@o . 0:
o:, %o
Here dative interactions occur between the lone-pair electrons of C O and vacant hybrid sp2 orbitals of Ag, and the back-donation occurs from the semifilled Ag 5p, orbital into the a* orbitals of CO. The semifilled orbital of Ag(C0)3 is thus given as follows. @ = aq5Ag(5p,)
+
b(Tl*
+ a2* + .3*)
(4)
Here a,* represents the vacant a* orbital of the indicated C O molecule. The g tensor and the Ag hf tensor of Ag(CO), determined here show slight deviation of the complex from axial symmetry. However, apparently the deviation is not large enough to remove the equivalency of the I3C hf tensors of the three carbonyl groups. The Ag hf tensor may hence be analyzed setting A,(Ag) = (1/2)(Ax A J . As for the I3C hf tensor of the complex, it is determined essentially by the C 2p, part of eq 4. Hence it should also be axially symmetric about the z axis, as observed. We stated earlier that the Adip (eq 2d) for an unpaired electron in a p orbital
+
G.J. Phys. Reu (13) Morton, J. R.; Preston, K. F. J . Magn. Reson. 1978, 30, 577
J. Phys. Chem. 1985,89, 115 1-1 154 is positive. It follows that, for both the Ag and the I3C hf tensors of Ag(C0)3, All > A,. The lo7Ag and 13C hf tensors of axial symmetry (with signs) were thus deduced as follows: A,l(i07Ag)= +78 MHz,
AL(lo7Ag) = f 5 M H z A,(13C) = -24 MHz
All(i3C) = f 3 MHz,
Let us assume the negative sign for A,(.107Ag) and the positive sign for A,(l3C), since they lead to a slightly more reasonable spin-density distribution. Analyses of these tensors in terms of eq 2 then yield the following: Aiso('O7Ag) = +23 MHz, Ai,(13C) = -15 MHz,
Adip(Io7Ag)= +28 M H z Adip(13C)= +9 MHz
Downloaded by STOCKHOLM UNIV on September 1, 2015 | http://pubs.acs.org Publication Date: March 1, 1985 | doi: 10.1021/j100253a019
The atomic values Aim0(lo7Ag),Ah0(I3C), and Adip0(13C) expected for a unit spin density in the Ag 5s, C 2s, and C 2p orbitals, respectively, have been stated already. The atomic value Adip0(Io7Ag)for the Ag 5p orbital was estimated to be 52 M H z from the known hf splitting term a ( j = 1/2) of the li51n atoms ( 5 2 5 ~ ~ The ) . ~spin-density ~ distribution in the silver and carbon orbitals in Ag(CO)3 were thus assessed as follows. p(5s)Ag = +0.013, p ( 2 ~ ) c= -0.004,
p(5p,)Ag = +0.53 p(2p,)c = +0.084
The balance of the distribution is presumably at the oxygen 2p, orbitals (-0.08 each). The,extremely small spin densities computed above for the Ag 5s and C 2s orbitals should not be taken (14) Kopfermann, H. "Nuclear Moments"; Academic Press: New York, 1958; pp 123-138.
1151
literally. Small determined here are the sum attributes of the spin polarization in all the filled s orbitals of the respective atoms. However, Ai, of the I3C nucleus induced by spin polarization by an unpaired electron in the 2p orbital of the same atom is usually positive.I5 A negative spin density determined above for the C 2s orbital is hence particularly interesting. It suggests spin polarization of electrons in the u dative bond by the large positive spin density in the Ag 5p, orbital. The semifilled orbital of Ag(CO)3 thus delineated above is extremely similar to that determined for CU(CO)~.The situation is in sharp contrast to the difference noted between Cu(C0) and Ag(C0). The interaction between the Ag atom and CO in the latter complex is probably best described as the van der Waals type. The energy levels of the Cu 3d, 4s, and 4p atomic orbitals are -9.1, -7.7, and -3.9 eV, respectively, while those of the Ag 4d, 5s, and 5p orbitals are -1 1.3, -7.6, and -3.9 eV, respectively.I6 We note that while the energy levels of the s and p orbitals are virtually identical, those of the d orbitals are quite different. An essential difference between the bonding schemes shown above for the mono- and tricarbonyls is that the back-donation occurs from the d, orbital in the monocarbonyl scheme while the same occurs from the semifilled p, orbital in the tricarbonyl scheme. Failure to form a bona fide Ag(C0) complex is hence attributed to the extra stability of the Ag 4d orbital. Registry NO. Ag(CO), 59751-30-3; Ag(CO)3, 58832-57-8. (1 5) See,for example, a review article by: Morton, J. R. Chem. Rar. 1964, 64, 453. (16) Moore, C. E. Natl. Bur. Stand. (US.)Circ. No. 467, 1949, 1; 1952, 2; 1958, 3. (17) The group notation is being changed in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is being eliminated because of wide confusion. Group I becomes groups 1 and 1 1 , group I1 becomes groups 2 and 12, group 111 becomes groups 3 and 13, etc.
Effect of Tltanla on the Chemlsorptlon and Reaction Properties of Pt R. A. Demmin, C. S. KO,and R. J. Gorte* Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 (Received: October 15, 1984)
The unusual properties of titania-supported Pt catalysts for CO and H2 chemisorption and reaction have been duplicated on a Pt foil with a titania overlayer. The titania overlayer completely suppressed CO and H2 chemisorption, but, at 400 torr I$ and 100 torr CO, the methanation activity was high and consistent with the rate reported for titania-supported Pt. A change in activation energy from 30.2 to 18.9 kcal/mol was also observed in going from the clean to the titania-covered surface, with the clean surface exhibiting lower methanation rates over most of the temperature range studied. On the clean sample, Auger electron spectroscopy ( A B ) indicated no impurities, including carbon, following reaction. The titania overlayer was also unaffected by reaction conditions below 850 K. Above 850 K, the titania was reduced and the titanium diffused into the bulk Pt. Even though we observed no impurities, rates were dependent on the sample pretreatment. Initial rates measured on the clean foil varied by more than an order of magnitude depending on the pretreatment conditions. This is interpreted as indirect evidence that methanation may be structure sensitive on Pt. These results give strong evidence that titania-support effects are due to the migration of a titania species onto the metal crystallites. This titania prevents CO and H2 chemisorption by blocking the surface but does not prevent the methanation reaction from occurring.
Introduction Metals supported on titania have been shown to have very unusual catalytic properties. When these catalysts are heated in H2 above 750 K, they lower their ability to adsorb CO or H2 a t room temperature,'S2 yet these catalysts remain active for converting CO and H2to methane and other hydrocarbons.' Methanation rates are actually higher for metals supported on titania than for those supported on silica or alumina. Several possible (1) Tauster, S.J.; F'ung, S.C.; Garten, R. L. J. Am. Chem. Soc. 1978,100. 170. ( 2 ) Tauster, S.J.; Fung, S.C. J . Catal. 1978, 55, 29. (3) Vannice, M. A. J. Catal. 1982, 74, 199.
mechanisms have been presented to explain the cause of these catalytic properties, including charge transfer between the metal and the oxide: active sites between the oxide and the metal particle^,^,^ particle morphology changes>* and decoration of the metal surface with titania.g-" (4) Kao, C. C.; Tsai, S.C.; Bahl, M. K.; Chung, Y. W.; Lo, W.J. Surf. Sci. 1980, 95, 1 . (5) Burch, R.; Flambard, A. R. J . Catal. 1982, 7,, 389. (6) Vannice, M. A.; Sudhakar, C. J . Phys. Chem. 1984, 88, 2429. (7) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L. J . Catal. 1979, 59, 293. (8) Vannice, M. A,; Twu, C. C. J . Cutal. 1983, 82, 213. '
0022-3654/85/2089-1151%01.50/0 0 1985 American Chemical Society
