J . Phys. Chem. 1987, 91, 5218-5224
5218
Vibrational and Low-Lying Electronic Transitions in Tetraalkylammonium Salts of COB^,^-, COCI,~-,and CO(CNS),~-As Observed by Raman, Infrared, and Tunneling Spectroscopies K. W. Hipps* and Ursula Mazur Department of Chemistry and Chemical Physics Program, Washington State University, Pullman, Washington 99164-4630 (Received: April 6, 1987)
Inelastic electron tunneling (IET) spectra of tetraethylammonium salts of cobalt tetrabromide and cobalt tetrachloride and of tetramethylammonium cobalt tetrathiocyanate ions are reported. Spectra of simple tetraalkylammonium salts are also presented. IET spectra are compared with IR and Raman spectra of the same materials. The vibrational bands due to the tetraalkylammonium ions are prominent in all three types of spectra, but cobalt-ligand stretches are not observed in IETS. The lowest 4A2 4T2electronic transition is strong in IET spectra but unobserved in the IR. The electric dipole allowed 4A2 4T, electronic transition is observed in both IET and IR spectra. Structure on both electronic bands is observed in the case of the cobalt tetrabromide complex ion, but only the 4A2 4T, transition of the cobalt tetrachloride ion shows significant structure. Both electronic transitions of the cobalt tetrathiocyanate complex are unstructured.
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Introduction Tetrahedral complexes of cobalt(I1) have been the subject of continued interest for the past 25 years. The crystal structures of the tetraalkylammonium salts of cobalt tetrabromide and tetrachloride show that these complexes are nearly tetrahedral, with the X-Co-X angle varying from the tetrahedral angle by less than 4°.1-4 A number of studies of the cobalt halide vibrational modes have been p ~ b l i s h e d ,and ~ , ~the ~ vibrational modes of the halide complexes are fairly well known. Vibrational studies of the thiocyanate complex have been principally concerned with the N C S motion^.^-'^ Theoretical calculations of the positions and intensity of the lowest d-d transitions in tetrahedral cobalt(I1) have been performed by several Electronic spectra of tetrahedral cobalt halides and thiocyanate complexes in soluutilizing photon absorption t ion 16.1+21 and in the solid state8*18*20.Z2-31 ( I ) Wiesner, J. R.; Srivastava, R. C.; Kennard, C. H.; DiVara, K.; Lingafelter, E. C. Acta Crystallogr. 1967, 23, 565. (2) Stucky, G. D.; Folkers, J. B.: Kistenmacher, T. J . Acta Crystallogr. 1967, 23, 1064. (3) Dunsmuir, J . T.; Lane, A. P. J . Chem. SOC.A 1971, 2781. (4) Gerloch, M.; Lewis, J.; Rickards, R. J . Chem. Soc., Dalton Trans. 1972, 980. (5) Clark, R. J.; Dunn, T. M. J . Chem. SOC.1963, 1198. (6) Sabatini, A,; Sacconi, L. J . A m . Chem. SOC.1964, 86, 17. (7) (a) Dunsmuir, J. T.; Lane, A. P. J . Chem. SOC.A 1971, 404. (b) Edwards, H. G. M.; Woodward, L. A.; Gall, M. J.; Ware, M. J. Spectrochim. Acta, Part A 1970, 26A, 287. (8) Bird, B. D.; Cooke, E. A,; Day, P.;Orchard, A. F. Philos. Trans. R . SOC.London, A 1974, 276, 277. (9) Toeniskoetter, R. H.; Solomon, S. Inorg. Chem. 1968, 7 , 617. (10) Tsivadze, A.: Kharitonov, Ya.; Tsivadze, G . Russ. J . Phys. Chem. 1971, 45, 81 1 . ( I 1) Bailey, R. A.; Michelsen, T. W.; Mills, W. N. J. Inorg. Nucl. Chem. 1971, 33, 3206. (12) Zaitsev, B. E.; Santos, W.; Arynov, A. D.; Vasil'eva, N. P.: Akimov, V. K.; Molodkin, A. K. Z h . Neorg. Khim. 1979, 24, 414. (13) Ballhausen. C. J.: Jorgensen, C. K. Acta G e m . Scand. 1955, 9, 397. (14) Orgel, L. E. J . Chem. Phys. 1955, 23, 1004. (15) Ballhausen, C. J.; Liehr, A. D. J . Mol. Spectrosc. 1958, 2, 342. (16) Hamer, N. K. Mol. Phys. 1963, 6, 257. (17) Liehr, A. D. J . Phys. Chem. 1963, 67, 1314. (18) Gale, R.; Godfrey, R. E.; Mason, S. F. Chem. Pfiys. Lett. 1976, 38, 441. (19) Cotton, F. A.; Goodgame, M. J . A m . Chem. Soc. 1961, 83, 1777. (20) Cotton, F. A.; Goodgame, D. M.: Goodgame, M.; Sacco, A. J . A m . Chem. SOC.1961, 83, 4157. (21) Cotton, F. A.: Goodgame, D. M.; Goodgame, M. J . A m . Chem. Soc. 1961, 83, 4690. (22) Holm, R. H.; Cotton, F. A. J . Chem. Phys. 1959, 31, 788. (23) Ferguson, J. J . Chem. Phys. 1960, 32, 528. (24) Stephens, D. R.; Drickamer, H . G. J . Chem. Phys. 1961, 35,429. (25) Ferguson, J. J. Chem. Phys. 1963, 39, 116. (26) Harada, M.: Sakatsume, S.: Tsujikawa, I. J . Phys. Soc. Jpn. 1973, 35. 1234.
0022-3654/87/2091-5218$01.50/0
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or scattering have been reported. The lowest electronic states of tetrahedral cobalt(I1) complexes of halides and thiocyanate can be found by considering the free ion 4F term split by the effects of a tetrahedral crystal field in the weak field limit. The ground term is a 4A2,and the first two excited terms are 4T2and 4T,. The 4T2and 4T, terms are expected to lie A and 1.8A above the ground term, respectively. The 4A2 4T2transition is electric dipole forbidden but magnetic dipole allowed, and the optical absorbance is expected to be 50-100 times weaker than the electric dipole allowed 4A2 4T1transition. While the electric dipole allowed transition is ~ e l l - k n o w n , * J ~ ~ ' ~ - ~ ' the position of the magnetic dipole allowed transition for halide and thiocyanate complexes was not reliably determined by optical ~pectroscopy.~'Very recently, we have reported the use of inelastic electron tunneling spectroscopy to observe both the 4A2 4T2 and 4A2 4T1transitions in cobalt(I1) t e t r a ~ h l o r i d e . ~That ~ report emphasized the utility of IETS for the study of electronic transitions in complexes and described in detail the innovations in data analysis utilized to obtain high-quality spectra. Our objectives in this article are (1) to present vibrational and electronic tunneling spectra taken from three different tetrahedral cobalt(I1) complexes, (2) to analyze this data in terms of the electronic structure of the complexes studied, and (3) to compare IR, Raman, and tunneling data to expose the relative strengths and weaknesses of IETS.
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Experimental Section Tunneling Instrumentation. The spectrometer used was recently described in the l i t e r a t ~ r eand , ~ ~ the data-acquisition programs were modified as described in ref 32 in order to obtain normalized tunneling spectra. Thus, the spectra are presented as (d21/ d p ) / ( d I / d V ) vs. V plots. In the case of electronic tunneling spectra, one or more of three postacquisition data manipulation programs were used. TUNLFIT provided a least-squares fit of a polynomial background function to selected regions of the spectrum, and then the polynomial function was subtracted from the entire data set. DIPFIX was used with to replace the quantum size effect (QSE) structure dip34,35 (27) Harada, M.; Tsujikawa, I . J . Phys. Sor. Jpn. 1974, 37, 1353. (28) Harada, M.; Tsujikawa, I. J . Phys. SOC.Jpn. 1974, 37, 1359. (29) Gale, R.; Godfrey, R. E.: Mason, S. F. Chem. Phys. Lett. 1976, 38, 446. (30) Tsujikawa, I.; Harada, M. Reu. Roum. Chim. 1977, 22, 1305. (31) Jha, N. K.; Saxena, A. Inorg. Chim. Acta 1977, 22, L21. (32) Hipps, K. W.: Mazur, U. J . Amer. Chem. Soc. 1987, 109, 3861. (33) Hipps, K. W. Rec. Sci. Instrum. 1987, 58, 265. (34) Jaklevic, R . C.; Lambe, J.; Kirtley. J.: Hansma, P. K. Phys. Rec. B : Solid Stare 1977, 15, 4103.
0 1987 American Chemical Society
Electronic Transitions in Cobalt(I1)
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
5219
a short curved line segment. SUBTUNL was used extensively for interactive data subtraction. A reference spectrum was interactively subtracted from a sample spectrum to provide the best cancellation of common bands. SUBTUNL was also used to obtain difference spectra in the case of a few of the vibrational tunneling spectra. For the electronic tunneling spectra reported, the modulation voltage was set at 8 mV near 30-mV bias. Since constant modulation current was used, the modulation voltage typically fell to about 3 mV at 1-V bias. In the case of vibrational tunneling spectra, 1.6-mV modulation was employed. A 0.3-s linear time constant with 0.1 s sampling time was used. Typically one point was collected every 7 cm-’ (8.066 cm-’/mV) for electronic spectra and every 3 cm-’ for vibrational data. Data were collected with the aluminum electrode biased negatively, and all measurements were made at 4.2 K. In order for spectral subtraction to be effectively employed, several high-quality spectra differing in surface coverage for each type of junction were collected. Typical junction resistances ranged from 90 to 900 ohm. The spectra have been corrected for the shift induced by the superconductivity of the Pb top metal. Tunnel Junction Fabrication. All the junctions (tunnel diodes) reported here were of the form AI-Al0,-sample-Pb. Standard resistive deposition techniques were used to provide 1-“-wide metal strips. The oxide was grown by ac discharge in 100 bm of oxygen. The species of spectroscopic interest was incorporated into the M-I-M device by placing a solution containing that species on the freshly grown insulator and then spinning the substrate at several hundred rpm. This process is known as “spin doping”36and will be referred to as “doping” in what follows. A lead-top electrode was then deposited to complete the M-I-M structure. Electrical contact was made with indium solder. Tetraethylammonium cobalt(I1) tetrachloride [ (Et,N)2CoC14] and cobalt(I1) tetrabromide [ (Et4N),CoBr4] were doped from ethanol with solution concentrations of 5 and 8 g/L, respectively. In the case of electronic tunneling spectra, the reference junctions for the above were doped with a mixture of 0.3 g/L ZnClz and 1.2 g/L (Et,N)CI in ethanol and with a mixture of 0.5 g/L ZnBr2 and 1.5 g / L (Et,N)Br in ethanol. Tetramethylammonium cobalt(I1) thiocyanate [(M~,N),CO(NCS)~] junctions were doped with 3 g/L of the complex in acetone. Reference junctions for use in obtaining electronic difference spectra were doped with a mixture of 1.5 g/L (Me,N)NCS and 0.75 g/L KSCN in acetone. Vibrational tunneling spectral differences utilized references composed of the appropriate tetraalkylammonium salt prepared as described below. Tunneling spectra of (Et,N)CI and (Me,N)NCS were obtained from junctions doped with 1 g/L in ethanol and acetone, respectively. The choice of solvent was made by trading off the desire for low dielectric constant and thus high adsorption against the need for sufficient solubility. In some cases, SmCo magnets were used to quench the superconductivity of the top lead film. Because the tunneling current decreases exponentially with barrier thickness and because IETS signals do not increase significantly when the coverage exceeds one monolayer,36the spectra presented here are probably due to an average coverage of one monolayer and are certainly due to an average coverage of less than three monolayers. FT-ZR Data. Diffuse reflectance data of pure solid powders were obtained on an IBM IR-98 Fourier transform instrument. A tungsten lamp source, germanium-coated KBr or quartz beam splitter (depending on spectral range), and a cooled InSb detector were employed in the region above 2800 cm-I. Data were acquired at 8-cm-I resolution and are the result of adding 1024 scans. The FT-IR reflectance data presented in this paper are in the Kubelka-Monk format3’ with KBr providing the reference. All spectra were obtained at room temperature. Some of the spectra
are the result of subtracting a fractional multiple of the spectrum of the appropriate tetraethylammonium halide from the spectrum of the halide complex in a manner identical with that used for the subtracted tunneling spectra. Absorbance spectra of the halide salts and complexes were obtained by pressing the appropriate compound into the matching cesium halide. Absorbance spectra of the thiocyanate salts and complexes were obtained in KBr pellets. Mid- and far-IR spectra were obtained at 4-cm-] resolution Mylar beam splitter, by using a helicoil source, Ge-KBr or 6 - ~ m and MCT or TGS detectors. Raman Spectra. Raman spectra were collected from the neat materials as powders. The 4880-8, line of a Ar ion laser was used for excitation. No Raman spectra were obtained from the deeply colored cobalt thiocyanate complex. Data was collected with a 4-cm-’ step size and 2-s integration time. The spectra presented are the result of the addition of 4-10 scans. Materials. Cobalt complexes were prepared by the methods given by Cotton et a1.20s2’Solvents were of reagent grade and used as purchased. Aluminum and lead metals were >99.995% pure.
(35) Hipps, K. W.; Susla, B. P.; Dunkle, E.J . Phys. Chem. 1986, 90, 3898. (36) Hansma, P. K., Ed. Tunneling Spectroscopy; Plenum: New York, 1982. (37) Wendlandt, W . W. Modern Aspects of Reflectance Spectroscopy; Plenum: New York, 1968.
Results Figure 1 portrays the Raman, IR, and IET spectra (the sum of 14 scans) obtained from (Me,N)NCS. The positions of the principal peaks are listed in Table I. It is interesting to note that
t >
c H cn Z
W t-
z H
1600 ENERGY (cm-1)
0
3200
Figure 1. Vibrational spectra of (Me,N)NCS. Tunneling (a), IR (b), and Raman (c) spectra are shown
I
I1
t >t-
H
v,
z
W
t-
z
H
0
1600 ENERGY ( c m - I )
3200
Figure 2. Tunneling spectra of (Me4N)2Co(NCS),.Trace a is the direct spectrum and trace b is the difference spectrum using Figure la as a reference.
5220
Hipps and Mazur
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
t t kH
ffl Z
W F Z H
Raman I
0
1600 ENERGY (cm- 1)
3200
0
1600 ENERGY
Figure 3. Vibrational spectra of (Et,N)CI.
3200
(cm-1)
Figure 5. Comparison of Raman spectra obtained from cobalt(I1) tetrachloride and tetrabromide complex ions.
t
t
t IH
t.
I-
cn
H
cn z
Z
w
i-
W
H
z
n
k
z
H
I
I
0
1600 ENERGY (cm-1)
3200
0
000 (cm-1)
1600
ENERGY
Figure 4. Tunneling (A) and tunneling difference (B) spectra obtained from (Et,N)2CoC14.
Figure 6. Comparison of tunneling spectra obtained from cobalt(I1) tetrachloride (a) and cobalt(I1) tetrabromide (b) complex ions.
while the C N stretch near 2100 cm-' dominates the IR and Raman spectra, it is not the strongest band in the tunneling spectrum. Further, note that it is split into two bands in the tunneling spectrum. The upper trace of Figure 2 is the tunneling spectrum of (Me,N)$o(NCS), (28 scans summed). The peak positions observed for the thiocyanate complex in IET and IR spectra are also provided in Table I. The lower trace is the difference spectrum resulting from subtracting the tunneling spectrum shown in Figure 1 from that shown in Figure 2. The plotting scale is the same for parts a and b of Figure 2, but Figure 2a is shifted slightly for ease of viewing. The extent of subtraction was chosen (by eye) to minimize the net variations about the base line. Note that the only significant positive features occur at 2077 cm-I, the C N stretch, and near 470 cm-I, the NCS bend. This latter band is differential in form in the difference spectrum, indicating a significant increase in frequency of the band in the cobalt complex relative to the salt. Note also that the C N stretch is not split as in Figure l a . Figure 3 contrasts the Raman, IR, and IET spectra obtained from (Et,N)CI. The upper trace is the tunneling spectrum and reflects the addition of 14 scans taken at 1.6" modulation. The most obvious difference between IET and Raman or IR spectra is the lack of structure in the C H stretching band observed in the tunneling spectrum. Peak positions for all three spectra are given in Table I. The upper trace in Figure 4 shows the result of adding 129 spectra of (Et4N),CoCl4 taken with 1.6-mV modulation at 4.2 K. Peak positions of the (Et4N)2C~C14 bands observed in
IETS, Raman, and IR are also given in Table I. The lower trace is the result of subtracting (a multiple of) the spectrum of (Et,N)CI shown in Figure 3 from that of (Et,N),CoCl4 shown in Figure 4A. The plotting scale is the same for parts A and B of Figure 4, but Figure 4A is shifted slightly for ease of viewing. Note that there is near-perfect removal of all adsorbate bands below 1800 cm-l, resulting in a spectrum similar to that of the alumina substrate. Figure 5 allows comparison 4f the Raman spectra obtained from (Et4N)2C~C14 and (Et,N),CoBr,. Note that they are essentially identical above 300 cm-' but differ significantly in the region below 300 cm-I. The differing band occurs at 270 cm-] in the chloride complex and at 166 cm-' in the bromide complex. The IR spectra of these complexes are also nearly identical above 500 cm-I. A single strong band in the 150-500-cm-' region is found at 297 and 230 cm-' for chloride and bromide complexes, respectively. Figure 6 displays the tunneling spectra obtained from (Et4N),CoCI4 and (Et4N),CoBr4 in the presence of a magnetic field of sufficient strength to quench the superconductivity of the top lead film. The spectra were taken with a modulation amplitude of 1.8 mV. Aside from a broad low-energy tail present in the chloride complex spectrum and a slight difference in the relative intensity of the broad aluminum oxide band near 900 cm-]. the two spectra appear identical. Figure 7 presents tunneling (solid line) and IR diffuse reflectance (broken line) difference spectra of (Et,N),CoBr, obtained by interactive subtraction of the tunneling spectrum of (Et,N)-
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5221
Electronic Transitions in Cobalt(I1) TABLE I: Principal Vibrational Peak Positions (cm-')O
(Me,N),CoIETS
(Me,N)NCS Raman
(NCS)4
IR
IETS
IR
IETS
(Et4N)CI Raman
IR
IETS
(Et4N)zC0C14 Raman
IR
270 303 377 465
154
454
397 457
380 470
752
741
757 839 947 1066
942
949 1069
947 1069 1174
1174
1293
1286
1290
1404 1444
1406 1462
1410
sh
389
304 367
303 366 418
835 948
1292
bs
1414
1455 1484
sh
1481
2062
2060
2078
2079
2798 2948 3018
2802 2956 3026
2914 3025
2916 3026
420 47 1 536 622 795
672 792 895
674 794 894 910
1017 1070 1123 1185 1301
1008
1007
1070 1120 1176 1290
bs
1383 1455
1390 1458
sh
1 I77 1319 1365 1404 1434 1487
2938 2978
2945 2980
297 386 470
673 79 1 896 1017 1067 1125 1 I93 I301 1382 1458
662 790 894 1010 1030 1070 1118 1182 1298 1354 1392 1450
1184 1310 1352 1402 1457
2946 2990
2947 2978
796 897 1006 1033
1078
2050 2108 2954 3022
bs 2977
bs 2978
"Uncertainty in values is 3 cm-'. bs = broad shoulder; sh = shoulder. TABLE 11: Electronic Transitions Observed by Tunneling SDectroscoDV
transition
t >
-
4Az
(Et4N) 2CoBr4
4T2
CoBr22600 2940' 3260
I-
H
co
I
Z W IZ H
/
I
Jr-
---
,A2-+
\
,TI
\ \
4450 4850 (sh) 5040 5500
\
CoCI>-
Co(NCS)>-
2950b
3950
4650 4900 5200 5520 5700 (sh) 5980 6230 (sh)
82OOb
"Positions in cm-'. bunresolved band; only the band maximum is given. Estimated
2000
4000 ENERGY (cm-1)
6000
Figure 7. Comparison of a tunneling difference spectrum (solid line) and an IR difference spectrum (broken line) in the near-IR region of the spectrum of (Et4N)2CoBr4.
t >
-I
H
ZnBra and the IR spectrum of (Et,N)Br, respectively. The subtraction factor was determined by minimizing the amount of CH stretch present in each difference spectrum. The tunneling spectra used in generating the IET spectrum shown in Figure 7 were the sum of 110 scans with 8-mV modulation set at 30 mV. The dotted curve segment is intended as a guide for the eye and does not represent data. Table I1 contains the peak positions of the structure seen in the tunneling spectrum that are not due to CH subtraction errors. Figure 8 contrasts tunneling (solid line) and IR diffuse reflectance (broken line) difference spectra of (EbN)2C~C14 obtained by subtraction of tunneling and IR reference spectra. The subtraction factor was determined by minimizing the amount of C H stretch present in each difference spectrum. The positions of electronic structure in the (Et4N),CoC14 tunneling spectrum are given in Table IT. Figure 9 presents the tunneling difference (solid line) and IR diffuse reflectance (broken line) spectra of (Me,N),Co(NCS), in the electronic transition region of the
co Z W IZ H
2500
4500 ENERGY (cm-1)
6500
Figure 8. Comparison of a tunneling difference spectrum (solid line) and an IR difference spectrum (broken line) in the near-IR region of the
spectrum of (Et,N),CoCl4. spectrum. The diffuse reflectance spectrum is not a difference spectrum. The structure seen in the 1R near 4000 cm-' is due
5222
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987
Hipps and Mazur
1
I
4000
6000
8000
ENERGY
(cm- 1)
\
l
A
10000
Figure 9. Comparison of a tunneling difference spectrum (solid line) and an IR spectrum (broken line) in the near-IR region of the spectrum of
(Me,N),Co(NCS),. to vibrational overtones. Band maxima for the tunneling bands are given in Table 11.
free ion
Tetrahedral field
+ Spin Orbit
Co(I1) Figure 10. Schematic electronic energy level diagram for tetrahedral cobalt(I1) in the weak field limit.
Discussion The tunneling, IR, and Raman spectra of (Et,N)CI, (Et,N)Br, Vibrational Spectra. The vibrational tunneling spectral band (Et,N),CoCl4, and (Et,N),CoBr4 are all very similar in the region positions obtained for (Me,N)NCS are in general agreement with 470-1500 cm-I. This is demonstrated in Figures 3-5 and in Table the IR and Raman results (Figure 1 and Table I). The major I. The tunneling difference spectrum [ (Et,N),CoCl4 - (condifference is in the C N stretching motion near 2100 cm-'. While stant)(Et4N)CI]in Figure 4 demonstrates this most clearly. Unlike both IR and Raman report a single band at 2060 cm-', two the (Me4N),Co(NCS)4 case wherein the ligand internal motions separate bands at 2050 and 2108 cm-' are observed in the tunsignificantly affect the difference spectrum, Figure 4B is very neling spectrum. Consideration of the tunneling spectrum obtained nearly the spectrum of a blank alumina junction. The exception from KSCN38clarifies the origin of this difference. Ionic comto this is the differential feature near 3000 cm-I. This feature plexes of NCS- that dissociate in solution have tunneling spectra is due to a small change in the C H stretching region of Et4N+ which reflect the two possible bonding modes of the NCSwhen the cobalt complex is present. The use of (Et4N)2ZnC14 ion-N-bonded near 2060 cm-' and S-bonded near 2100 cm-I. as the reference considerably diminishes the size of this feature Tunneling band intensities are very different, however, than in(see Figure 8). One curiosity is the small (zero?) intensity of the tensities observed by IR or Raman spectroscopy. These intensity Co-X ( X = C1- or Br-) stretching motions in the tunneling spectra. differences are completely consistent with the very different According to several author^,^-^ the v , and u3 stretches are nearly tunneling excitation process and empirical o b ~ e r v a t i o n . ~ ~ > ~ ~ -degenerate ~' and occur near 300 cm-' for CoCI4*- and 200 cm-' The positions of vibrational bands in the tunneling spectrum for (Et4N),CoBr,. As can be seen in Figure 6, the tunneling of (Me,N)2Co(NCS)4 (Figure 2 and Table I) are in excellent spectra of the chloride and bromide complexes in this region are agreement with those observed by IR. This is the expected result nearly identical. and indicates that the NCS- ion is coordinated to cobalt and is Electronic Spectra. Figure 10 shows a schematic of the evonot bonded to the alumina surface. This is most easily seen by lution in energy of states associated with the lowest three terms observing that the C N stretch in (Me,N),Co(NCS), occurs as of a cobalt(I1) ion in a tetrahedral field in the weak field limit. a single band at 2078 cm-' in the tunneling spectrum and at 2079 The ordering of levels is based on, and in agreement with, the work cm-I in the IR. Given the peak uncertainties, these positions are of several a ~ t h o r s . ~ ~Note * ~ ~that ~ - 'the ~ r7and rslevels arising identical. Figure 2b emphasizes the difference in the from the 4T2term are not actually degenerate, but are closely (Me,N)*Co(NCS), and (Me,N)NCS tunneling spectra. The spaced. Even when one considers the small reduction in symmetry strongly positive C N stretch and strongly negative features at 947 associated with the crystal structure of cobalt tetrahalide salts,' and 1291 cm-' are partially due to the 2 to 1 ratio of NCS- to the form of the far right hand side of the diagram does not tetraalkylammonium ion in the complex relative to the salt. It significantly change. Rather, the splitting is somewhat augmented. is interesting to note that these negative features (947 and 1291 For example, the calculated zero field splitting of the rsground ~ is 7.5 cm-' for the chloride complex and 3.4 cm-' for the cm-') are due to CH rocking and C-N stretching, r e s p e ~ t i v e l y . ~ ~ , ~level The differential feature near 470 cm-' is due to a shift in the NCSbromide., The r7and rs levels arising from the ,T, term are bend with complexation and is another indicator that NCS- is separated by about 60 and 40 cm-' for the bromide and chloride complexed to cobalt. The oscillatory structure seen in the difcomplexes, respectively.8 According to the first-order perturbation ference spectrum (Figure 2b) near 3000 cm-l is a direct consetheory, the overall width of the manifold of levels arising from quence of significant changes in the CH stretching vibrations on the ,TI term is about 6h and that of the ,T2 manifold is about complexation. This variation is seen both in tunneling and IR 2L4, For all of the complexes studied here, a h value of 150 f (see Table I). 20 cm-' is an excellent estimate.*' Consideration of small reductions in symmetry and of higher order interactions do not greatly change the values of the calculated manifold ~ i d t h s . ~ - ~ . ' ~ - ' * (38) Mazur, U.; Hipps, K. W. J . Phys. Chem. 1979,83, 2773. (39) H ~ p p s ,K. W.; Mazur, U. J . Phys. Chem. 1980, 84, 3162. We will discuss the significance of the tunneling data presented (40) Hipps, K. W.; Williams, S. D.; Mazur, U. Inorg. Chem. 1984, 23, in Figures 7-9 and in Table I1 in stages. We will be first concerned 3500.
(41) Hipps, K. W.; Aplin, A. T. J . Phys. Chem. 1985, 89, 5459. (42) Ebsworth, E. A,; Sheppard, N. Spectrochim. Acta 1959, 13, 261. (43) Bottger, G . L.; Geddes, A . L. Spectrochim. Acta 1965. 21, 1701.
(44) Weakliem, H. A . J . Chem. Phis. 1962. 36, 2117.
The Journal of Physical Chemistry, Vol. 91, No. 20, 1987 5223
Electronic Transitions in Cobalt(I1) with the gross features, that is, with the term assignments and the value of A for each complex. The most obvious beginning is with the assignment of the electric dipole allowed 4A2 4Tl transition. As may be seen from Figures 7-9, or from previously published optical ~ p e c t r a , ' ~ , ' the ~ - ~highest ' energy tunneling transition observed is clearly the 4Az 4T1 transition. The enhanced structuring and slight blue-shifting of the tunneling band relative to the optical band are due to the difference in measurement temperature (293 K for IR and 4.2 K for IET spectra). While it is gratifying that tunneling does such a good job of reproducing the optical spectrum, it is not particularly significant to the inorganic spectroscopist who already knew the position from conventional spectroscopy. Of much more interest to the inorganic spectroscopist is the lower energy band in each of the Figures 7-9. If one takes the average of the highest and lowest features in each of the structured bands (see Table 11) as the band center, one arrives at values of 4970, 5440, and 8200 for the centers of the 4A2 4Tl transitions in the bromide, chloride, and thiocyanate complexes, respectively. In a similar fashion, the band centers of the 4A2 4T2transitions are at 2930, 2950, and 3950. As discussed previously and shown in Figure IO, the ratio [E(4A,4Tl)/E(4A2-4T2)] is given by crystal field theory as 1.8. Taking the above band centers and forming this ratio yields 1.7, 1.8, and 2.1 for the bromide, chloride, and thiocyanate complexes, respectively. Thus we assign the lower energy of the two broad bands in Figures 7-9 as the 4A, 4T2transition. We might also note that the magnitude of A is expected to increase in the order Br< CI-
