D. A. Van Leirsburg and C. W. DeKock
134
Acknowledgments. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and to the Research Corporation for partial support of this research. The authors also thank Dr. J. Heinzer for a COPY of his computer program ESREXN.
References and Notes (1) J. R. Bolton and A. Carrington, Mol. Phys., 5, 161 (1962). (2) P. D. Sullivan, J. Amer. Chem. SOC.,89,4294 (1967). (3) P. D. Sullivan, J , Phys. Chem.. 75,2195 (1971).
(4) P. D. Sullivan and J. R. Bolton. Advan. Magn. Resonance, 4, 39 (1970). ~ phys,, 1 ,22, 167 (1971), (6) The splittings are assigned to the cis and trans isomers by comparison with the splittings in the 2,3- and 2,5-dimethylhydroquinone cation radicals. (7) In the course of remeasuring the spectra in A I C I ~ - C H ~ N O it ~was found that the temperatures reported previously were somewhat in error. (8) L. Jonkman and J. Kommandeur, Chem. Phys. Lett., 5, 579 (1970). (9) E. F. Caldin, Chem. Rev., 69, 135 (1969). (10) Note Added in Proof. In a recent publication A. I. Prokof'ev, N. N. Bubnov, S. P. Solodovnikov, and M. I. Kabachnik, Tetrahedron Left., 2479 (1973), have measured a kinetic'isotope effect in the intramolecular hydrogen migration within the 2-oxy-3.6-diteff-butylphenoxy radical. Their measured activation energies show ED E H = 1.6 kcal/mol, in good agreement with our results.
(5) J, Heinzer, ~
-
Interaction of Matrix-Isolated NiF2 and NiC12 with CO, N2, NO, and O2 and of CaF2, CrF2, MnF2, CuF2, and ZnF2 with CO in Argon Matrices' D. A. Van Leirsburg and C. W. DeKock* Department of Chernisfry. Oregon Stale University, Corvallis, Oregon 97337 (Received August 70, 1973)
The infrared spectra of matrix-isolated NiF2 and h'iCl2 with CO, N2, NO, and 0 2 and of CaF2, CrF2, MnF2, CuFz, and ZnFz with CO in argon matrices have been observed. Perturbation of the frequencies of both the small molecules and metal halides are found to occur and these have been tabulated. The perturbed CO frequencies are all above the gas-phase frequency. The high-frequency band is correlated to the strength of the electric field of the metal ion at the carbon nucleus. A good correlation is found for the isotropic metal ions while ligand field arguments are involved to explain the deviations for those ions with anisotropic electron distribution. Evidence for a Dzh planar fluoride bridged dimer structure for (NiF2)2 and (CrF& is also given.
Although the infrared spectra of a large number of matrix-isolated fourth period metal dihalides have been extensively studied,2-10 no work has appeared on the interaction of these coordinately unsaturated molecules with other reactive species. In this paper we report the interaction of matrix-isolated NiF2 and NiC12 with CO, Nz, NO, and 0 2 and of CaF2, CrF2, MnF2, CuF2, and ZnF2 with CO. Some of the transitions which we assign to the interacting species NiFzN2, NiFzCO, NiClZNz, and NiClzCO were noted by earlier workers2 but were not identified. A preliminary communication of part of this work has appeared.ll Experimental Section The matrix-isolation technique is well known and our experimental apparatus is described elsewhere.lZ The major change from the previous equipment is that cooling is now provided by a Model DE-202 Displex helium refrigerator made by Air Products and Chemicals, Inc., which allows deposits to be made a t 14°K. The argon was doped with CO, N2, NO, or 0 2 using a very low flow-rate valve into a tee in the argon line or in a few cases with CO by passing the reactive gas into the furnace chamber. Doping concentrations were as low as to as high as 1/50 depending on the experiment. The infrared spectra were recorded from 4000 to 200 cm-1 using a Perkin-Elmer 180 spectrophotometer. The abso-
-
The Journal of Physical Chemistry, Vol. 78, No. 2, 1974
lute uncertainty of the spectra is of the order of d~0.2 cm while the isotopic frequency differences are known to f O . l cm-'. Optimum resolution used for these experiments was 0.5 cm-I. The analytical reagent grade NiC12.6HzO (Mallinckrodt Corp.) was dehydrated at 400" under a stream of HC1 and then sublimed under HC1. The anhydrous NiF2 (A. D. McKay Corp.) and CrF2 (D. F. Goldsmith Co.) were further dried a t 800" under a stream of SiF4. The CuFz and MnF2 (both anhydrous 99%) were purchased from Research Inorganic-Research Organic C o . and were used without further purification. The anhydrous ZnFz was prepared by heating ZnCOa with 50% excess NH4HFz for 4 hr at 200" and then subliming off the excess NH4HF2 a t 250". The CaFz was B and A reagent grade and was used without further purification. Gases used were as follows: prepurified Nz and USP 0 2 from the NCG division of the Chemtron Corp.; chemically pure N O and CO and ultrapure Ar from the Matheson Co.; W O , enriched to 90% 13C, from the Monsanto Research Corp.; and 1 8 0 2 , enriched at 94% 1 8 0 , from Miles Laboratories. All samples were outgassed within 10" of the deposition temperature for at least 30 min prior to deposition. During the outgassing period the furnace walls were also heated with a heat gun to outgas the furnace chamber thoroughly. This effectively eliminated "impurity transitions" with the exception of HF. Deposits were made from nickel
135
Interaction of Metal Dihalides with CO Knudsen cells or for CuFz and ZnFz from platinum cells and CaF2 from tantalum cells at temperatures corresponding to 10-4 t o 10-5 atm vapor pressure. Deposition times from as short as 0.5 hr to as long as 4 hr were employed. O
NIF2 In Ar
Results We assume that the interaction of the metal halides with small molecules is a diffusion-controlled process. In order to obtain reasonable concentrations of the species of interest one either allows diffusion to occur by warming the matrix or else uses a high concentration of reactive species in the matrix. The first method leads to severe complications for these molecules because the metal halides dimerize very readily when the matrix is warmed. The second method, used in this study, was carried out with a relatively high concentration of reactive small molecules (CO, N2, NO, or 0 2 (Yso)) and a rather low concentration of MX2. High concentrations of MX2 had to be avoided due to dimer formation. However, in many cases due to concentrations needed to observe complex formation some weak transitions attributable to dimers or higher polymer species were observed. An additional complication with this method is that the metal halide small molecule complex sometimes appears to be in more than one matrix site-apparently a result of the mixed matrix. If more than one frequency for the complex was observed, the higher frequency was always more intense and was taken to be the frequency in a pure argon matrix. These values are shown in Tables I-V with the additional sites shown in brackets. The nickel halide small molecule interactions were the first to be studied. Accordingly, they were studied in depth both with respect to different small molecules (Nz, 0 2 , CO, and NO) and as a function of the nickel halide concentration. These results will be discussed first. The remaining metal fluorides were studied only with CO because in the nickel halide work only the perturbed CO stretch was easily observed and CO perturbations appeared to give the most information concerning the nature of the interaction. In addition, no metal fluoride concentration studies were carried out with the other systems. Our interest in this paper is primarily centered upon the new transitions observed when a small molecule is doped into the argon matrix. In general, our undoped results are in very good agreement with literature values and therefore these referencesz-10 may be consulted for more elaboration on the spectra of the isolated metal halide monomer or polymers in pure argon. A . CO. Leroi, et al.,13 Davies and Hallam,l4 and Makil5 have deposited CO in Ar and all obtained three transitions at 2148, 2142, and 2138 cm-1. The 2148-cm-1 transition was assigned to monomeric CO in Ar while the 2142 and 2138 cm-l transitions were assigned to polymeric CO. It is also of interest to this study that when CO is codeposited with HCl in an argon matrix a new band appears a t 2155 cm-1 with a shoulder at 2157 cm-1 in addition to the usual CO transitions.16 E . Nickel Halides. 1. NiF2. The spectrum of pure NiF2 is shown in Figure l a . No absorptions other than those between 765 and 780 cm-1 are observed. These are readily assigned to u3 of NiF2 with the individual absorptions due to the various nickel isotopes (see Table I). This spectrum is in complete agreement with those previously publishedz-4 and confirms the assignment by Margrave, et al.,4 of a 152" bond angle for NiF2.
b NIF2 + C O In A r
C
N15
+ HZ In Ar
NlF2
+ NO In Ar
d
----Ifa NIF2 + 0, In Ar
I
Figure 1. Infrared absorption spectra of N i F 2 in (a) A r , ( b ) ArCO, (c) Ar-N2, (d) Ar-NO, and (e) A r - 0 2 matrices.
2. NiFZCO. Figure I b shows the spectrum of NiF2 in an Ar-CO matrix. New absorptions appear at 713 and 2200 cm-l together with the usual CO absorptions. (With high concentrations of CO an additional matrix site for these transitions is present a t 2195 and 694 cm-l; see Table I.) Figure l b clearly shows the strong similarity between the 713- and 780-cm-' absorptions and allows the 713-cm-l absorption to be assigned to the F-Ni-F asymmetric stretch of an NiFzCO complex. Table I shows the fit obtained for the 713-cm- transition using the isotope relation for the asymmetric stretch of free NiF2 with a 150" bond angle. The 2179-cm-1 absorption is assigned to an (NiF2)2 CO complex (vide infra). 3. NiF2N2. The spectrum of NiF2 isolated in Ar-NZ matrices is shown in Figure IC. A 736.2-cm-l absorption now grows in strongly which from its isotopic structure is assigned to an F-Ni-F asymmetric stretch of an NiFzN2 complex. A careful search in the 1900-2600-~m-~ region revealed no evidence of an N-N stretch. NiFz was also deposited in a pure nitrogen matrix in an attempt to observe the N-N stretch. A new very weak absorption was observed at 2327 cm-I which was not obThe Journel of Physical Chemistry, Vol. 78,No. 2, 1974
D. A. Van Leirsburg and C. W. DeKock
136 V
,
a NiC12 in A r
NiC12
+ CNg in A t
d NiCi2+N0 in A r
I:
- -\i-l/
I
! 2 dm
do0
" s'm
NiC12+
&
in Ar
,
I
400
500
I
FREQUENCY (CM-')
Figure 2. Infrared absorption spectra of NiC12
in (a) Ar, (b) Ar-
CO, (c) Ar-N*, (d) Ar-NO, and (e) A r - 0 2 matrices.
served in the Ar-Nz matrix. However, this same weak transition is observed for NiClz in Nz which casts some doubt upon the N-N stretch assignment. Impurities, particularly a new site for COZ in nitrogen, may also be responsible. u3 for NiFz in nitrogen is at 706 cm-l. The stretching force constant, f r - f r ' , for NiFz in Nz and Ar is 3.46 and 4.22 mdyn/A, respectively, indicating that Nz is a strongly interacting matrix. 4. NiFzNO. Figure Id shows the spectrum of NiFz in an Ar-NO matrix. A new absorption observed at 727.7 cm-l and recorded in Table I may clearly be assigned to the asymmetric F-Ni-F stretch of a nickel fluoride-nitric oxide complex which will be written as NiFz(N0)x. It is not possible to characterize this complex more carefully since a number of transitions were observed in the N-0 stretching region which could be assigned to monomeric and dimeric NO (both cis and trans), as well as Nz0.17J8 The N20 was apparently an impurity in the NO. In addition a whole series of transitions which were in all cases considerably weaker than those assigned to cis or trans (NO)z or NO appeared between 1800 and 1850 cm-l even in the absence of NiFz. These are apparently higher polymers of NO arising from the relatively high concentration The Journal of Physical Chemistry, Vol. 78, No. 2, 1974
IB
iz
Interaction of Metal Dihalides with CO
137
TABLE 11: Observed and Calculated Frequencies for NiCl, and NiCl, Small Molecule Complexes NiCl?
NiCIIN?
NiCICO
- .
Obsd
Calcd
520.7 517.5 516.1 513.9 512.7 511.4 509.4 508.2 507.3 504.1
(520.7) 517.5 516.0 514.2 512.6 511.4 509.4 508.1 507.1 504.0
' Broad absorptiun centered at
Obsd
468.6 465.4 ... 461'.8
... 458.4 ... ...
...
2189.2 2140.2
Calcd
(468 .6) 465.7 464.3 462.8 461.5 460.2 458.4 457.2 456.4 453.4 (2189.2) 2140.5
483.1 480.0 478.7 476.9 475.7
...
472.9 472.0
... ...
NiChO? .
Calcd
(483.1) 480.2 478.6 477.1 475.8 474.4 472.7 471.6 470.5 467.5
Obsd
Calcd
474.9 472.2 471.1 468.4 466.8 466.8 464 . O 462.6
(474.9) 471.8 470.7 468.8 467.6 466.6 464.5 463.4 462.6 459.6
... ...
Calcd
Obsd
480.2 cm-1,
TABLE 111: Observed and Calculated Frequencies for P], of t h e Dimer (NiF,), and JCrFz)p Species
Obsd
NICI~NO
__
% Natural abundances
46.1 35.6 6.9
Obsd energy, cm - 1
670.8 669.1
Calcd energy, cm -1
(670.8) 669.3 667.9
667.6 668,l 666.9
5 .O 1.5 666.9 1.9 7.4 69.6 16 .O
631.8 630.1 629.3 (sh)
666.6 631.7 (630.1) 629.3
(y50) of NO needed for complex formation. Therefore, no
new N - 0 stretch was observed which could be assigned to an NiF2( NO)x complex; however, this is not surprising since it may well have been hidden under the existing (NO)xtransitions. 5. NiF202. The spectrum of NiFz isolated in an Ar-02 matrix is shown in Figure l e and is recorded in Table I. The additional stretching frequency observed beginning a t 748.6 cm-1 is assigned to the F-Ni-F asymmetric stretch of an NiFzOz complex. A very weak transition a t 1534.5 cm-1 grew in with the transition a t 748.6 cm-l. Deposition of NiF2 with Ar-1802 led to a weak transition at 1448.2 cm-I (calculated 1446.7) leading to its assignment as a n 0-0 stretch of the NiF202 complex. 6. NiC12. Figure 2a shows the spectrum of pure NiClz in an Ar matrix. The spectrum is identical with that previously reported for NiC122~3.6 except that the 483- and 468-cm-1 absorptions are now absent. Heavier deposits of NiC12 show an additional broad transition at 440 cm-1 which has previously been assigned to dimeric NiC12.213 Bond angle calculations for the monomer from the observed isotopic shifts indicate that a 180" model fits the observed frequencies very well, which is in good accord with molecular beam experiments of Buchler, Stauffer, and Klempererlg and electron diffraction experiments of Hedberg and Eddy.20 7. NiC12CO. Addition of NiCl2 to an Ar-CO matrix gives the spectrum shown in Figure 2b. A new transition is observed a t 468 cm-I which from its isotopic structure may clearly be assigned to a C1-Ni-Cl asymmetric stretch of an NiC12CO species. Formation of such a species is clearly indicated by the I 2 C 0 frequency observed a t 2189 cm-I which shifts to 2141 cm-I on addition of 13CO. Occasionally another I 2 C 0 frequency is observed at 2153
TABLE I V Observed and Calculated Isotopic Frequencies for Y~ of CaF,, CrF,, MnF,, CuF,, a n d ZnF, in Ar and CO-Ar Mixtures and W O and W O Frequencies MFzCO
MFz Obsd
A. WaF2
Calcd
'2CO
B. UCrF, 62CrF2 63CrF2 64CrF2 'TO
659.8 654.4 652.7 sh 649.5
659.8 (654.4) 652.6 649.1
13co
'TO
wo
'2CO
1 3 ~ 0
'2CO 1 3 ~ 0
'2CO
13co 1 3 ~ 0
'2CO
13co 699.4
D. " C U F ~ 743.5
W U F ~ 739.6 " C U F ~ [736.5 ] " C U F ~ [732.6] '2CO
637.8
...
2188.4 2139 ,1 2186.6 2137.7 2184.9 2136.0 2183.7 2133.7 2181 . 7
643.2 (637 .8) 635.2 632.6 2139.8 2138.1 2136.4 2133.3
675.9 2183.2 2181 .O (743 .5) 739.3 [(736.5)1 [732.3]
1 3 ~ 0
E. e4ZnFz e6ZnF2 67ZnFz e8ZnF2 70ZnFz '2CO
Calcd
2131 .O 2167.8~ 2118 .O= 2119.7 2165.20 2114 .O 2117,l
'*CO
C. 5bMnF2 '2CO Z 'C0
Obsd
552.2 547.6 2178 .O
557.8
762.5 758 .O 756.2 754.4 750.2
(762.5) 758.3 756.2 754.2 750.4
704.2 700.2
(704.2) 700.2
2210.4 2160.9
2161.3
728.1 724.1
...
720.4
(728.1) 724.1 722.1 720.2 716.5
2185'.9
wo '2CO
. . .h
2137.4
. . .h
2135.5
2183 , 8
13co
' CO frequencies perturbed by
(CrFz)?. W O perturbed hidden by W O .
cm-I in addition to the usual I 2 C 0 absorptions at 2148, 2142, and 2138 cm-l. The 2153-cm-l absorption occurs only when some HC1 is present due to hydrolysis of NiC12. The Journal of Physical Chemistry, Voi. 78, No. 2, 1974
D. A. Van Leirsburg and C. W. DeKock
T %
v .
1
I
2200
2100
V
I 800
I
1
700
600
I
i
I
sm
400
xx)
FREOUENCY (CM4)
Figure 3.
Infrared absorption spectra of NiFz and
(NiF2)2
in (a) Ar and ( b ) Ar-12CO-13C0 matrices
TABLE V: Observed and Calculated Frequencies for u3 of CrF3 Assuming DJhS y m m e t r y Dr, Natural Species
30CrF3 32CrF3 33CrF3 54CrF3
abundance
4.4 83.5 9.6 2.6
Obed
Calcd
754.7 749.3 746.9 744.4
754,6 (749 . 3 ) 746.8 744.4
This transition is due to CO perturbed by HC1 as shown by Barnes, et al.16 8. NiCl2N2. Figure 2c shows the spectrum of NiClz in an Ar-Nz matrix. The absorption at 483 cm-1 is assigned to the C1-Ni-Cl asymmetric stretch of an NiC12N2 complex. No transitions are observed which may definitely be assigned to an N-N or a Ni-N stretch. When NiC12 is deposited in a Nz matrix it is found that the Cl-Ni-Cl absorption shifts to 428 cm-'. The very large shift in the force constant, f r - f r ' , for NiC12 in N2 and Ar, 1.78 and 2.63 mdynfA, shows that nitrogen interacts strongly with NiC12. A very weak transition at 2327 cm-1 was observed for NiClz in N2 which may be an N-N stretch (see (see NiF2N2). 9. NiC12NO. The spectrum of NiC12 in an Ar-NO matrix is shown in Figure 2d. A new absorption is observed a t 475 cm-1 which is assigned to a Cl-Ni-Cl asymmetric stretch of a N i c l ~ ( N 0 )species ~ from the isotopic structure. Because of the complexity of the NO stretching region no N - 0 stretches belonging to this species could be definitely assigned. 10. NiC1202. The spectrum of NiClz in an Ar-02 matrix is shown in Figure 2e. A new absorption grows in at 480 cm-I which may be assigned to the C1-Ni-C1 asymmetric stretch of NiC1202. No definite 0-0 or Ni-0 stretches were seen. 11. (NiF2)z. The infrared spectrum of monomer and dimer in the region 800-300 cm-1 is shown in Figure 3a. Dimer transitions are located at 671, 486, and 405 cm-I and grow in when the matrix is warmed or when concentrated deposits are made. The proposed dimer for MX2 species has invariably been the halogen bridged form, that is a four-membered The Journal ot Physical Chemistry, Vol. 78, No. 2, 1974
planar diamond-shaped ring and two additional halogen atoms located on the M-M axis outside the ring.21 The symmetry of the structure is D 2 h for which there are twelve normal vibrational frequencies. Of these, six are infrared active-2BlU(v7, US), 2B3"(vg, ~101,and 2 B 3 u ( ~ l ~ , ~ 1 2 ) .Following McNamee,22 the u7 and V g modes may be described as out-of-plane ring-bending motions; v10, as an outer-bond bending in the plane; v g and v11, as bridgebond stretching; and v 1 2 , as an outer-bond stretching. The stretching motions are expected to have higher energies than the bending motions, with the terminal stretch, ~ 1 2 , having the highest energy and therefore assigned to the 671-cm -1 transition, Excellent support for this assignment and structure comes from the agreement between the observed and calculated nickel isotopic frequencies of the 671-cm-l transition. The G matrix elements for both Bau modes (v11 and ~ 1 2 are ) p~~ C(F. The product rule requires23
+
ll. -U =
Vi2
cch'i
Vlll
.Y'n
W'Si
+ ccr + F'F
However, because no isotopic splitting could be resolved for either the 486- or 404-cm-I tran~itions,2~ the product rule was ignored and the isotopic frequency shifts were calculated from
'f=j
ccsi
/*2
P'Si
+ cc F +dF
This approximation is equivalent to assuming that the terminal stretch is independent of the bridge stretch. Good agreement is obtained as shown in Table IJI. The 486- and 405-cm-1 transitions are assigned to the bridge stretches, vg and v11. It is not possible from present information to make an unambiguous assignment of these two stretches. The bond bending modes are expected to lie below 200 cm-1, the limit of the instrument. Additional evidence for this model comes from a simplified normal coordinate analysis patterned after that by Buchler25 and McNamee.22 They assumed that the bridge bonds form a square structure and that the potential energy is completely specified by the force constant f r for the terminal stretch, the constant f d for the bridge-bond
Interaction of Metal Dihalides with CO
139
a
t T
t
x
T %
2
I
2100
0
V
7
I
70-3
800 FREOUENCY
4
ICM-'l
Figure 5. Infrared absorption spectra of CrF2 in (a) A r and (b) 2200
2100 600 FREOUENCY (CM-')
500
Figure 4. Infrared absorption spectra of CaF2 in (a) A r and ( b )
Ar-CO matrices.
stretch, and the constant fa/r2 for the terminal bend. These assumptions give simple expressions for the B3" and Bzu representations. The additional assumptions they made are that f r is equal to the force constant derived from v 3 of the monomer and that f d = Yzf, and f a / r 2 = Y1ofr. These assumptions give the following values : v12, 717 cm-l; v11, 463 cm-1; v g , 454 cm-1. The observed values are 671, 486, and 405 cm-1. 12. (NiF2)2CO. Depositions were usually carried out with a low concentration of NiF2 to prevent dimer formation, and a reasonably high, YJO, concentration of the XY molecule. In a few instances involving CO both (NiFz)2 and the CO concentrations were reasonably high and frequencies attributable to dimer complexes were observed. Closely spaced absorptions at 2180 cm-1 are present when l 2 C 0 is deposited with dimeric NiFz. This transition grows in when a matrix containing NiF2, CO, and NiF2CO is warmed allowing diffusion to occur. These results are shown in Figure 3b. Lower frequencies which grow in with 2180 cm-1 are located a t 680, 622, 470.4, 440, and 404 cm-1. The 470.4-cm-l frequency is carbon dependent because substitution of I3CO shifts the absorption to 463.5 c m - l . The nature of this transition is unclear. It does not appear to be an Ni-C stretch because no Ni isotopic structure is observed and frequencies this high are not expected for these relatively weak interactions. C.CaF2. 1. CaFZ in Ar. The spectrum of CaF2 above 500 cm-I is shown in Figure 4a and recorded in Table IV. Transitions grow in at 557.8 and 529.2 cm-1 which following earlier workerss~gare assigned to v 3 of CaFz and dimeric CaFz, respectively. Additional weak transitions were also observed a t 484.8 and 368.1 cm-1 and are assigned to u1 and dimeric CaF2, respectively, following the previous workers. 2. CaF2CO. Figure 4b shows the spectrum of CaFz in an Ar-CO matrix. The new transitions observed here are recorded in Table IV. When Ar is doped with CO the 557.8 cm-1 transition broadens and shoulders grow in at 552.2 and 547.6 cm-l. A new broad CO transition at 2178 cm-1 grows in at the same rate as the 552.2- and 547.6-cm-1
Ar-CO matrices.
shoulders. No changes were observed for the v i mode or the polymer modes. D. 1. CrFz in Ar. The spectrum of CrF2 in argon above 600 cm-1 is shown in Figure 5a and the v3 region is recorded in Table IV. The isotopic structure of the intense transition at 654 cm- 1 is assigned to v 3 for CrFz with 180" bond angle. The isotopic structure of the weaker transition at 750 cm-1 is consistent with assignment v3 of CrF3 assuming 120" bond angle and is recorded in Table V. Apparently the CrFz was slightly contaminated with O F 3 . These assignments for both v 3 of CrFz and of CrF3 are the same as those made earlier by Linevsky.10 In addition, weak transitions grow in together at 630, 472, and 395 cm-1 which by the isotopic structure on the 630 cm-1 transition may be assigned to CrFz dimer. (See Table 111.) 2. CrF2CO. Figure 5b shows the spectrum of CrF2 in an Ar-CO matrix. The new transitions observed here are recorded in Table IV. Transitions in the 2180 and 2190 region grow in with a new transition at 637.8 cm-I. The 637.8-cm-1 transition is assigned to u3 of CrFz perturbed by CO. Isotopic proof of this assignment is lacking due to the small abundance of 50Cr, 53Cr, and s4Cr and overlap of the weak dimer transition a t 630 cm-1. A weak series of transitions also appear between 2160 and 2170 cm-1 which have 13CO counterparts between 2110 and 2120 cm-1 with no intermediate transitions. These are assigned to CO perturbed by CrFz dimer by analogy with (NiF2)z CO transitions. E . I . MnFz in Ar. Figure 6a shows the spectrum of MnF2 in the stretching region in argon and the results are recorded in Table IV. The intense transition a t 699.4 cm-1 is assigned to u 3 in agreement with Hastie, et al.26 In addition, with heavier deposits weak transitions are observed at 615, 602, 433, 426, and 376 cm-1 which are assigned to MnF2 dimer and polymer species. 2. MnF2CO. Figure 6b shows the spectrum of MnF2 in an Ar-CO matrix. The new transitions which appear here are recorded in Table IV. Two transitions at 2183.2 and 675.9 cm-1 grow in simultaneously and the 675.9-cm-1 transition is assigned to u 3 of MnF2 perturbed by CO. The transition observed at 2159 cm-1 is CO perturbed by HF (vide infra). F. I. CuF2 in Ar. The spectrum of CuFz in the stretching region is shown in Figure 7a and is recorded in Table IV. The doublet observed at 743.5 and 739.6 cm-I is assigned to u 3 of 63CuFz and 65CuFz. respectively, in agreeThe Journal of Physical Chemistry, Voi. 78, No. 2, 1974
D. A. Van Leirsburg and C. W. DeKock
140 1
1
I
I
Figure 6. Infrared absorption spectra of MnF2 in (a) Ar and ( b ) Ar-CO matrices.
-
-
-
2200
I
2100 800 FREQUENCY (CM-')
7 0
Figure 8. Infrared absorption spectra of ZnF? in (a) Ar and (b) Ar-CO matrices.
1
2h20
.
1
2100 800 FREOUENCY (CM-')
700
Figure 7. Infrared absorption spectra of CuFp in (a) Ar and (b)
Ar-CO matrices.
ment with earlier workers.4 A weaker doublet with the same isotopic structure at 736.5 and 732.6 cm-I is assigned to v 3 of CuFz in a different matrix site. In addition, weaker transitions were observed at 660, 487, and 395 cm-I and are assigned to CuFz dimer by analogy with the other difluorides studied here. 2. CUFZCO.Figure 7b shows the spectrum of CuFz in an Ar-CO matrix. The new transitions observed here are recorded in Table IV. A new doublet grows in at 704.2 and 700.2 cm-1 together with a CO transition a t 2210.4 cm-l. The isotopic structure of the doublet is consistent with its assignment as v 3 for CuFz perturbed by CO. The weak transition at 2159 cm-1 is again CO perturbed by HF. G. 1. ZnFp in Ar. Figure 8a shows the spectrum of ZnFz in the stretching region in Ar and the results are recorded in Table IV. A series of transitions are observed beginning at 762.5 which in accordance with earlier workers4.7 may be assigned to v 3 of ZnF2 with 165" bond angle. In addition, previously assigned dimer modes were also observed at 661,458, and 400 cm-l. 2. ZnFzCO. Figure 8b shows the spectrum of ZnFz in an Ar-CO matrix and the new transition a t 2185.9 cm-l grows in together with a new series of transitions beginning at 728.1 c m - l . The isotopic structure of the latter The Journal of Physical Chemistry, Vol. 78, No. 2, 1974
transitions is consistent with v 3 of ZnFz perturbed by CO. The 2159-cm-1 transition is due to CO perturbed by HF . H. 13C0 Experiments. For CrFz, NiFz, NiClz, CuF2, and ZnFz experiments using isotopically enriched 13CO were done. In all cases both for 9O:lO and 50:50, W O / l 2 C 0 enrichment only the pure 12CO or 13CO frequencies were observed. No mixed frequencies were ever observed proving the absence of species of the type M X Z ( C O ) ~ where x is 2 or greater. 1. HF and H F Perturbed Transitions. The most common impurity observed in these experiments was HF. It was often observed in the spectra of MnF2, NiF2, CuF2, and ZnFz and apparently resulted from the hydrolysis of the MF2 compounds by traces of water present in the samples. In addition to the usual HF transitions in the 3800-4000 cm-1 region, transitions were observed at 681, 560, and 445 cm-1 apparently caused by HF because these transitions were also observed when only HF waa deposited in the matrix. H F also complicated the CO spectrum giving rise to a transition at 2159 cm-I which is observed for MnF2, CuFz, and ZnFz. This transition is present only when both CO and HF are present and is assigned to CO perturbed by H F in argon. This is close to 2155 and 2157 cm-1 which have been observed for CO perturbed by HC1.16
Discussion All the perturbed CO frequencies observed in this study are above those of free CO. There are a number of studies in the literature in which this effect is observed.27 In general these are systems in which CO is physically adsorbed on solids, particularly metal oxides. Of direct interest to the present work is a study by Angel1 and SchaffeP on CO adsorbed to zeolites in which the unipositive cation was replaced by dipositive cations. They report CO stretches up to 2217 and 2214 cm-1 for Niz+ and Zn2f cation exchanged zeolites, respectively. Most significantly, they were able to correlate the frequency of these CO absorptions with the strength of the electric field near the cation. Thus they showed unambiguously that the high-
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Interaction of Metal Dihalides with CO frequency absorption was due to CO adsorbed to the metal ion. The present study also indicates that the CO is definitely attached to the metal ion. This conclusion may be drawn because the isotopic structure of the CO perturbed u3 stretch is of exactly the same form as for free MX2. If the CO coordinated to the halogen, the two M-X bonds would be nonequivalent and two M X frequencies would be observed. Also the coupling between the two MX bonds would be destroyed and the isotopic structure of these species would be different than that expected for uncoordinated MX2 or MX2 coordinated to CO through the metal. In order to understand the CO perturbed frequencies it is necessary to establish the nature of the MX2 molecules. CuF2 has been shown to be ionic by Kasai, et al.,29 from an esr study of CuF2 in argon. This conclusion is based on the fact that there was very little coupling between the unpaired electron on the copper and the magnetic moment on the 19F nucleus. It is assumed that the other metal dihalides used here are also ionic. Turning now to the model used by Angel1 and Schaffer,28 we have correlated the perturbed CO frequency with the electric field of the cation a t the carbon nucleus and this correlation is shown in Figure 9. It is assumed here that the CO is attached to the metal ion through the carbon nucleus, the negative end of the dipole. The electric field strength at the carbon nucleus was calculated from the equation
where c is the electric field strength at the carbon nucleus, 91 is the charge on the cation, assumed to be +2 in all cases, r M is the Pauling ionic radius of the cation, rc = 1.5 A was chosen to be the van der Waals radius of carbon on CO, and t o is the permittivity of the medium assumed to be that of a vacuum. F is the coulombic force of attraction between the ion and the dipole and q2 is the charge on the carbon. The correlation shown in Figure 9 shows that a straight line may be drawn through the points for CaF2, MnF2, and ZnF2 and that the intercept occurs a t 2148 cm-l, the frequency of free monomeric CO in argon. These results may be easily understood from the fact that electron density is being removed from the 50 orbital of CO by the positive metal ion. H u o , ~ "from a b initio calculations, has shown that the 5u orbital of CO is weakly antibonding and located principally on the carbon atom. This removal leads to an increase in bond strength and the observed frequency increase. This effect is also observed from the stretching frequency of CO+.s1 Removal of a 5u electron from the CO molecule raises the stretching frequency (from 2143 cm-l for CO(g) to 2184 cm-I in CO+ (g)). Figure 9 exhibits a double-humped curve strongly suggestive of ligand field effects. It is just those ions, Cr2+, N?+, and C U ~ +which , have anisotropic d electron configurations which do not lie on the line. Furthermore, it may be observed that the CO frequencies for NiF2 and CuF2 deviate farther from the correlation than CrF2. A simple crystal field model for the M2+ ion in MF2 compounds places the energy levels in the order 6g < Kg < ug+ for a linear MF2 compound.29 Qualitatively then, it is expected that the ug+ is the least filled orbital and it has the correct symmetry to overlap with the 5ug orbital of CO. Therefore it is just these ions, C++, Ni2+ and Cu2+,
co
2'90-
FREOUErCY 21802170-
Stretching frequency of CO vs. electric field at carbon nucleus due to various fourth-period dipositive metal ions.
Figure 9.
which would appear to the CO to have a higher positive charge than those with an isotopic distribution of electrons leading to the observed extra frequency increase. In addition, Cu2+ and Ni2+ are expected to have a greater effect than Cr2+ because as the nuclear charge increases the 3d orbitals become more stable across the series. Calculations show that the 5u of CO is below the 3d orbitals of the transitions metal ions.32 Therefore as the 3d level drops across the series a greater interaction is expected between the 5u of CO and the ug of the transition metal ion yielding the observed results. The model also accounts for the frequency difference of CO adsorbed to NiF2 u s . NiClz (2200 us. 2189 cm-1). NiClz is expected to be more covalent than NiF2 leading to a smaller electric field strength for Ni2+ in NiCl2 than NiF2 and a consequent decrease in CO frequency. Our results are also consistent with recent semiempirical calculations of Politzer and Kasten33 on the interaction of CO with NiO, N i t , Ni2+, and Ni3+. Their findings for CO adsorbed to Nin+ species show that the CO is bound to metal or metal ion through the carbon and that the CO bond strength increases as n increases due to a greater u charge transfer as n increases. Their calculations also show K back bonding but this is relatively small for Ni2+ and Ni3+ species and their results clearly show that the u donation contributes to the observed CO frequency increase. Little can be said about the CO perturbed by the dimer species in (NiF2)2 and (CrF2)2 other than to note that the lower CO frequency compared to the monomer species is probably due to the lower metal ion charge in the dimers. We now turn to the results of Nix2 with N2, 0 2 , and NO. No perturbed N-N or N - 0 stretch was observed for either of these molecules with NiX2. A weak 0-0 stretch was observed for NiF2 at 1535 cm-I. Hatzenbuhler and Andrew+ report the stretching frequency of 0 2 in Ar as 1551 cm-1 by Raman spectroscopy and of 0 2 - as 1074 cm-1 as in Li02. Herzberg3l reports the stretching frequency of Oz+(g) a t 1835 c m - l . Therefore the 0 2 frequency observed here is essentially like molecular oxygen. Because 0 2 is more electronegative than CO it is not unreasonable to assume it to be a poorer u donor and better K acceptor than CO. These two effects combined could lead to the observed result. All that may be definitely said for NO is that the perturbed frequency is not in the NO+ region, that is above The Journal of Physical Chemistry, Vol. 78, No. 2, 1974
D. A. Van Leirsburg and C. W. DeKock
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1900 cm-1, because this region was clear in our spectra. Because NO is a better A acceptor than CO the (I donation from the nitrogen may be offset by the back A bonding to the NO from the metal again resulting in an NOlike frequency. This is consistent with Politzer and Kasten's33 calculations which show that even NiZ+ and Ni3+ do back bond to CO and therefore may be expected to do so to an even greater extent with NO. The lowering of the asymmetric X-Ni-X stretch frequency upon interaction with the small molecule is also consistent with this model. It is well known that the stretching force constant increases with increasing oxidation state of the metal. For both nickel halides the asymmetric stretch decreases in the order 0 2 > N2 > (NO)x > CO suggesting an order for the electron donor ability of these molecules.
Conclusions The present work indicates that the matrix isolation technique may be used to study the interaction of small molecules such as CO, NZ, 0 2 , and NO with ionic compounds. The advantage of the technique is that the ionic species is precisely known in contrast to the infrared studies of adsorbed molecules on various other solid supports in which more than one type of active center may be present. This study also shows that small concentrations of these molecules often lead to unexplained transitions in matrix isolation studies and special care must be taken to exclude such effects. For example, it may be that these types of interactions are responsible for some of the unclassified transitions in the infrared spectra of the matrix isolated mercuric halides.35 Acknowledgments. We wish to thank the U. S. Atomic Energy Commission for support of this research and the National Science Foundation for providing funds for the purchase of the Perkin-Elmer 180 infrared spectrophotometer. References and Notes (1) (a) Presented in part at the 27th Northwest Regional Meeting of the American Chemical Society, June 15, 1972. (b) Based on the thesis presented by D . A Van Leirsburg to the Graduate College of Oregon State University, Corvallis. Ore.. in partial fulfillment of the requirement for the Ph.D. degree, 1973.
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(2) D. E. Milligan, M. E. Jacox, and J. D. McKinley, J. Chem. Phys., 42, 902 (1965). (3) K. R. Thompson and K. D . Carlson, J. Chem. Phys.. 49, 4379 (1968). (4) J. W. Hastie, R. H. Hauge, and J. L. Margrave, High Temp. Sci., 1, 76 (19691. (5) M. E. Jacox and D. E. Milligan, J. Chem. Phys., 51,4143 (1969). (6) J. W. Hastie, R. H. Hauge, and J. L. Margrave, High Temp. Sci., 3, 257 (1971). A. Loewenschuss, A. Ron, and 0. Schnepp, J. Chem. Phys., 49, 272 (1968). V. Calder, D. E. Mann. K. S. Seshadri, M . Allavena, and D. White, J. Chem. Phys., 51, 2093 (1969). A. Snelson, J. Phys. Chem., 70, 3208 (1966). M. J. Linevsky, U. S. Government Research and Development Report AD-670-626 (1968) C. W. DeKock and D. A. Van Leirsburg, J. Amer. Chem. Soc., 94, 3235 (1972). (12) R . D . W e s i y and C. W. DeKock,J. Chem. Phys., 55,3866 (1971). (13) G. E. Leroi, G. E. Ewing, and G. C. Pimentel, J. Chem. Phys., 40, 2298 (1964). (14) J. B. Davies and H . E. Hallam, J. Chem. SOC., Faraday Trans. /I. 509 (1972). (15) A. G. Maki,J. Chem. Phys., 35,931 (1961) (16) A. J. Barnes, H. E. Hallam. and G. F. Scrimshaw, Trans. Faraday Soc., 65, 3172 (1969). (17) W. G. Fateley, H . A. Bent, and B. Crawford, Jr.. J. Chem. Phys., 31, 204 (1959). (18) W. A. Guilloryand C. E. Hunter,J. Chem. Phys.. 50, 3516 (1969). (19) A. Buchler, J. L. Stauffer, and W. E. Klemperer, J. Chem. Phys.. 42, 902 (1964). (20) K. Hedberg and L. Eddy, private communication. See, for example, ref 3 and 7 and references therein. R. W. McNamee, Jr.. Ph.D. Dissertation. University of California at Berkeley, 1962; also issued as Univ. of Calif. Radiation Lab. Dept. UCRL-10451. See also ref 3 for agreement of this model with a number of 3d transition metal dichloride dimers. E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, "Molecular Vibrations," McGraw-Hill Book Co.. Inc.. New York, N Y., 1955. The maximum resolution attainable in the 400-cm-' region was 1 c m - The calculated splitting between (58NiF2)2and 58Ni6aNiF4is 1.2 c m for the 4 8 6 - c m - ' transition. This transition is sufficiently broad to encompass all the isotopic structure. A. Buchler, Ph.D. Dissertation, Harvard University, 1960. J. W. Hastie. R. Hauge, and J. L. Margrave, Chem. Commun., 1452 (1969). L. H. Little, "Infrared Spectra of Adsorbed Species," Academic Press, London, 1966, pp 67-74. C. L. Angel1 and P. C. Schaffer, J. Phys. Chem., 70, 1413 (1966). P. H. Kasai, E. B. Whipple, and W. Weltner, Jr., J. Chem. Phys., 44, 2581 (1966). W. M. H u o , J. Chem. Phys.. 43, 624 (1965). G. Herzberg, "Spectra of Diatomic Molecules," D. Van Nostrand Co., Inc.. Princeton, N. J., 1950. M. B. Hall and R. F. Fenske, lnorg. Chem.. 11, 1619 (1972). P. Politzer and S. D. Kasten, Surface Sci., 36, 186 (1973). D. A. Hatzenbuhler and L. Andrews, J. Chem. Phys., 56, 3398 (19721. A. Loewenschuss, A. Ron, and 0 Schnepp, J Chem. Phys, 50; 2502 (1969).
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