THESPECTRUM OF THE HEXACHLOROIRIDIUM(IV) ION
2207
The Spectrum of the Hexachloroiridium(1V) Ion by Thomas P. Sleight and Curtis R. Hare Department of Chemistry, State University of New York at Buffalo,Buffdo,New York 14214 (Received December 1 1 , 1967)
At 80°K the spectrum of Ir(1V) substituted into KzSnCls shows vibrational progressions in the 300-cm-1 symmetric stretching mode. Analysis of the progressions indicates the presence of two new transitions which until now, were unobserved. Ligand field and molecular orbital theories are compared and combined to make the assignment of the spectrum.
Introduction The interpretation and assignment of spectra of second- and third-row transition-metal complexes has not been as successful as in the case of the first row. The main reasons for this are that the observed bands are considerably more intense than the weak bands observed for the first row and that no simple theory has been successful in the interpretation of the available data in a quantitative fashion. Most of the transitions have been classed RS electron-transfer bands from ligand to metal orbitals and have been correlated by empirical considerations. Jprrgensen has supplied most of the spectroscopic data on second- and thirdrow halide complexes, and of these Ir(1V) has been the most comprehensively These works form the basis for the presently accepted assignments of the spectra. Two quantitative theories exist for the description of transition metal complex spectra: ligand field theory for transitions within the d multiplet, and molecular orbital theory for electron-transfer spectra. The former are usually well described by TanabeSugano6 matrices for first-row transition-metal complexes. However, the spin-orbit interaction must be included to interpret the fine structure observed in low-temperature spectra. Schroeder' has computed the spin-orbit matrix elements for d5 ions in cubic fields which can be applied to Ir(1V) spectra. Using the analogous d8 spin-orbit matrices, Dorain and Wheeler* have shown that the fine structure in the spectrum of R€!CIB~- can accurately be computed. Therefore, the visible spectrum of ReC162- has quantitatively been assigned to ligand field spectra. Cotton and Harrise have applied extended the Hiickel molecular orbital theory to several third-row complexes with a limited degree of success. Their results do not quantitatively agree with the spectra. Their extended Hiickel parameters may be used as a starting point for more extensive calculations in which the potential energy surface is reproduced.1° The extended Hiickel theory can be used to compute accurately all the spectral and bonding observables of transitionmetal molecules." The theory only requires the ap-
propriate parameters and approximation to the resonance integrals.12 Jgrgensen' has pointed out that the low-energy spectra of d6 Ir(1V) should be particularly simple, since the highest filled orbital has only one vacancy. Thus, the low-temperature crystal spectrum of IrCl62should display fine structure which may be interpreted with the available theories. For simplicity, it is desirable that the environment of the IrC162- chromophore should be of cubic symmetry. This has led to the study of Ir(1V) in the cubic environment of KzSnCla.
Experimental Section Large crystals (2-ml volume) of K&3nCls are easily grown from a 6 M HC1 solutioa of KCI and SnClc. 5Hz0. To avoid fogging (hydrolysis) of the crystal, it is necessary to maintain the HC1 concentration at about 6 M . The crystals are cubic and grow in a regular octahedral habit with the (111) face dominant. This crystal is isomorphous with K21rC&and has the K2PtCle structure. The MClB2- octahedral units occupy sites of complete octahedral symmetry, making KzSnClaan ideal matrix for the study of crystal spectra of octahedral complexes. The solubility of KzIrCle is much less than that of K2SnCla,and for this reason, it was difficult to incorporate the Ir(IV) into the Sn(1V) sites. After several attempts, a suitable orange-yellow crystal was obtained with Ir(1V) in (1) C. K.J@rgensen,Mol. Phya., 2 , 309 (1959). (2) C. K.JZrgensen, Acta Chem. Scand., 10, 518 (1956). (3) C.K.J@rgensen,Mol. Phya., 4, 235 (1961). (4) C. K.J@rgensen,Acta Chem. Scand., 17, 1034 (1963). (5) C. K. JGrgensen and W. Preets, Z . Naturforach., A, 22, 945 (1967). (6)Y.Tanabe and S. Sugano, J . Phys. SOC.Jap., 9, 753 (1954). (7) K.A. Schroeder, J . Chem. Phys., 37, 1587 (1962). (8) P.B.Dorain and R. G. Wheeler, ibid., 45, 1172 (1966). (9) F.A. Cotton and C. B. Harris, Inorg. Chem., 6 , 369,376 (1967). (10) T.P.Sleight, C. R. Hare, and G. A. Clarke, to be submitted for publication. (11) C. R. Hare, T. P. Sleight, W. Cooper, and G. A. Clarke, Abstracts, 154th National Meeting of the American Chemical Society, Chicago, Ill., Sept 1987. (12) L. C. Cusachs, J . Chem. Phys., 43, 5157 (1965). Volume 72, Number 6 June 1868
THOMAS P. SLEIGHT AND CURTIS R. HARE
2208
16
18
20
24
22
m-1 x 10-3 Figure 1. The room-temperature crystal spectrum of Ir(1V) in K&3nCla.
the K2SnC16 lattice. The crystal was polished by grinding with SnOz, and the spectrum was determined normal to the (111) face. The spectrum was recorded a t ambient and liquid nitrogen temperatures using a Cary Model 14 spectrophotometer. This instrument was operated at a minimum slit width in order to have the highest spectral resolution possible. The crystal was glued to a sheet of copper which had a suitable aperture. This was then mounted in an attachment to the spectrophotometer which allowed the crystal to be “cooled” to liquid nitrogen temperatures indirectly while protected by vacuum. I n this way temperatures within a few degrees (80°K) of the boiling point of liquid nitrogen could be achieved. The reagents used for the crystal preparations were Fisher reagent grade KC1 and SnClr.5H20. The KzIrC&,was from Engelhard Industries and was used without further refinement. The Na2IrCl6* 6H20, which was used for solution spectra, was obtained from Alfa Inorganics.
Results The room temperature spectrum of KzSn(Ir)C16 is given in Figure 1. The spectrum is similar to that determined in HC1 solutions,’ except that the broad band at 23,750 cm-1 is single peaked in the crystal. This band is double peaked in CH30H, CHICOOH, and HC1 solutions but single peaked in CH3CN, DMF, DMSO, CH3SO2C1, CH3CH0, and (CH3CO)zO solutions. The spectrum a t liquid nitrogen temperatures is given in Figure 2. This spectrum is distinguished by The Journal of Physical Chemistry
the resolution of a series of fairly well-defined shoulders and sharp peaks; the energies of these transitions are given in Table I. The interval between sharp peaks of the intense bands is 312 f 5 cm-1, while the interval between the shoulders in the 21,000 to 22,000 cm-l region is approximately 150 cm-I. The spectrum of this latter region is given in Figure 3 on an expanded scale and is structure rich. The transitions in this region are obscured in the solution and roomtemperature crystal spectra by the adjacent bands.
Table I: Observed Spectrum of IrC162- at 80°K Peak no.
1 2 3 4 5 6
7 8 9
10 11
Observed energy, cm-1
Peak no.
17,840 18 ,080 18,710 19 ,600 19,820 20,010 20 ,320 20 ,640 20 ,960 21,090 21 ,270
12 13 14 15 16 17 18 19 20 21 22
Observed Energy, cm-1
21 ,390 21,586 21 ,740 21 ,907 22 ,030 22,230 22,957 23,237 23 ,545 23 ,861 24 ,160
I n order to separate the electronic states and their associated progressions, a plot was made of the energy of each shoulder or peak vs. its number in the sequence. That is, the first shoulder is given the number 1, the second 2, etc. This plot is given in Figure 4; the
THESPECTRUM OF THE HEXACHLOROIRIDIUM(IV) ION
20
Figure 2.
2209
I
I
22
24
The crystal spectrum of Ir(1V) in KzSnCle a t 80°K.
viously stated, the exact interval between sharp peaks is 312 f 5 cm-'. This interval is in agreement with the symmetric stretching frequency of the Ir-Cl bond which would be 346 cm-'. It should be noted that the latter frequency is the symmetric mode for the ground electronic state, while this experiment gives the symmetric mode for the excited electronic states. If all of the observed progressions are of this symmetric mode, then the third line must be indicative of two closely lying electronic states with overlapping progressions. The separation of these two electronic n(300) cm-' (n = 0, 1, 2, 3); states would be 150 n cannot be discerned from this experiment. The origin of each of the vibrational progressions can be determined by extrapolation techniques. The origin of each of these progressions is assigned to v = 0, that is, the zeroth vibrational state of the appropriate electronic state. This assignment unfortunately is arbitrary and cannot be substantiated from these data alone, but the choice is quite reasonable and the error would only be one vibrational quantum in any event. On the basis of this assignment, the electronic transitions reach their maximum intensity a t v = 2. Any deviation of a point from the straight lines in Figure 4 (e-g., the third point of the first line) indicates either a spurious result, an error, or misassignment of the shoulder. In this case, the third shoulder of the first band might better be assigned to v = 3. The extrapolated origins of three of the observed electronic states (not corrected for the zero-point
+
22,000
21,000
a-1
Figure 3. The fine structure in the 21,00O-~rn-~ region for KzSn(1r)Cle a t 80°K.
shoulders are indicated with circles and the peaks are given by points. The best straight lines were drawn between the appropriate points. This aids the assignment of vibrational progressions associated with the electronic states. I n Figure 4 there are three lines of the same slope and one with one-half that slope. The interval between the shoulders or peaks in each of the three common slope lines is 300 cm-I, while, as pre-
Volume 72,Number 6 June 1068
THOMAS P. SLEIGHT AND CURTISR. HARE
2210
14
22
20
Energy
in
en-* x 10'
18
I
I
10
20
Peak Number
Figure 4. The vibrational analysis of K&h(Ir)Cle at 80°K:
0, shoulder;
energy of 150 cm-l, l/z hu) are 17,870, 19,600, and 22,957 cm-l. The lowest vibrations and the most likely origins of the other two transitions are 20,960 and 21,090 cm-'. IrC16-2 visible spectra were obtained in a variety of solvents already mentioned. The extinction coefficients are of the same order of magnitude (3 X lo3) as in HC1 solutions. Only in hydrogen-bonding solvents (Le., CH30H, CH3COOH, and HCI) is the double peak observed near 23,000 cm-l. The extinction coefficients determined here in the solid differ by a factor of 10 from those observed in solution. I n solution, the extinction coefficient is near 3400 at 20,000 cm-1, while the average of four different crystal determinations is 500. This discrepancy may be indicative of either a difference in the molar volumes of the solid and solution or a distortion of the IrC16-2 chromophore in solution, or both. Ligand-Field Theory. Several intense bands in the spectrum of ReC162- have been assigned to d-d transitions by Dorain and Wheeler.s I n view of these assignments, an attempt to correlate the absorption spectrum of IrC162- with the d6 spin-orbit matrices of Schroeder' was undertaken. The spectrum can be accurately fitted with the parameters Dq = 1836 cm-', B = 500 cm-1, y = 4.5, l = 2290 cm-' or Dq = The Journal of Physical Chemistry
*,
peak.
40
30
P
d
>0 P
W
Z
w20
2
3 DQ[kKI
Figure 5. The ligand field energy-level diagram with B = 450 cm-1, y = 4.5, and f = 2500 cm-1. The solid line, rs states; broken line, the r7states; dotted line, the restates.
1778 cm-1, B = 450 cm-1, y = 4.5, l = 2500 cm-I. The vibrational fine structure may be accounted for
THESPECTRUM OF
THE
2211
HEXACHLOROIRIDIUM(IV) ION
3.5
-15 I
2'. 0
I
3-0, r i n A
Figure 6. The ground-state and excited-states potential energy curves for IrCle-2 calculated using the Cusachs approximation for compared with the ground-state Morse curve.
Hij
by adding multiples of 300 cm-l to the calculated states. The important states generated by the first set of parameters are illustrated in Figure 5 as a plot of energy vs. Dq. These assignments are not satisfactory because of the low value of Dq used and the fact that symmetric progressions suggest that these transitions are parity allowed rather than Laporte forbidden transitions which require odd vibrational modes. Molecular Orbital Theory. Cotton and Harrisg have recently carried out extended Huckel calculations on the IrCle2- ion with a limited degree of success. These results form a basis for more extensive calculations carried out in this laboratory which will be reported in more detail in a future communication.1° The results of these computations are pertinent, however, for the assignment of the IrCle2- spectrum. Figure 6 shows the potential energy curves of the ground and excited states of IrCla2- computed using the extended Huckel method with the Cusachs'2 approximation for the off-diagonal elements, H t j . Similar but not identical results were obtained with the Wolfsberg-Helmholz approximation. la The Morse curve for the ground state is also given in Figure 6 for comparison. The excited-state potential energy curves were calculated by populating the appropriate molecular orbitals. The similarity of the shapes of the Morse curve and the potential energy curves for the ground and excited states indicates a similarity in the vibrational frequencies. The ground-state alg mode is 346 cm-l l 4 and the progressions in the excited states were observed in this work to be 312 cm-1. Further, the excited-state potential minima are slightly displaced from the ground-state internuclear distance
(Figure 6) and agree with the observed maxima at Y = 2 in the spectrum. The excited states are also observed to be bonding in agreement with the previous suggestion of Jplrgensen. The assignment and the calculated energies of the transitions based on molecular orbital calculations are given in Table 11. The energies have been corTable I1 : Comparison of Observed and Calculated Electron-Transfer States for IrCla2- in Wave Numbers
State (t1g6)
'Tlu (tlu') 'Tzu (tzu') 'TI" (tlu') %* (tzB6) (e2) %, ( a d )
---CslodaWolfabergHelmholz
Cusaoha
Obad
18,720 19,900 23 ,870 34,140 35 ,770 36,350 42 ,880
20 490 20,630 22 ,680 30,830 33, 260 36 ,060 38 010
17,870 19, 600 22 ,957
a For the energy of the transitions to the eg orbital add 10 Dq (39,030 and 35,930 cm-' in the Wolfsberg-Helmholz and Cusachs approximations, respectively).
rected for the spin-orbit splitting of the ground state which stabilizes the ground state by c. The (2500 em-1) and to1 value of was derived from (580 cm-l) and the eigenvalues of the tzg orbital. The
r
(13) M.Wolfsberg and L. Helmholz, J . Chem. Phys., 20, 837 (1962). (14)L. A. Woodward and M. J. Ware, Spectrochim. Acta, 20, 711 (1964); D. M. Adams, J. Chatt, J. M. Davidson, and J. Gerratt, J . Chem. Soc., 2189 (1963); D. M. Adams, Proc. Chem. Soc., 336 (1961). Volume 72,Number 6 June 1988
THOMAS P. SLEIGHT AND CURTISR. HARE
2212 intense transitions at 19,600 and 23,000 cm-l may be assigned to transitions from the filled tl, and tz, orbitals to the partially filled tzgorbital. The WolfsbergHelmholz calculation suggests that the 17,800-cm-' band may be due to the transition from the tl, to the tzg molecular orbitals. Neither calculation predicts bands in the 21,000-cm-1 region, even when a spinorbit splitting of the ligand-like orbitals is included. The value of Dq computed, here, by the Cusachs method is 3590 cm-l, and the Wolfsberg-Helmholz value is 3900 cm-l. These values of Dq have been corrected for the spin-orbit stabilization of the ground state. The results are in agreement with Dorain and Wheeler's* value of 3035 cm-l for ReC16'- but not in agreement with our previous ligand field assignments. Molecular orbital theory does not account for all the observed transitions in the visible region. The agreement with other observables, such as the vibrational frequency, internuclear separation, Dq, ground-state-metal character, and charge on the metal ion, is excellent.1° The inability of molecular orbital theory to account for all the observed spectral transitions while other properties are well described suggests that a combination of ligand field and molecular orbital theories would be more appropriate for the interpretation of the spectrum. Combination of Molecular Orbital and Ligand Field Theory. A combinatian of the previously discussed ligand field and molecular orbital assignments would account for all spectral and physical properties of IrC162-. However, the values of Dq used in ligand field theory are considerably different from the values obtained in molecular orbital theory. When the mOlecular orbital values of Dq are used in ligand field calculations with appropriate parameters ( B , y, and p ) , only quartets are predicted in the range of 17,000 and 22,000 em-'. A satisfactory but not unique solution is found with Dq = 2960 em-', B = 340 em-', y = 9.0, and p = 2700 em-'. The energy levels of this compvtation are similar to those depicted in Figure 5. With these parameters the transitions at 17,840, 20,960, and 21,090 cm-' are assigned to spin-orbit components of the 4T1g state: r8, I's, and re, respectively. The separation between the ??6 (4T1g) and the second I's (*T1,), is 144 em-', which is in agreement with the low-temperature crystal spectrum. Using this combination assignment, the intense transitions are quantitatively accounted for by molecular orbital theory and the less intense transitions by ligand field theory (Table 111). The enhanced intensity of the quartets may be accounted for by a vibronic coupling scheme suggested by Fenske.15 The above ligand fieId parameters predict other spin-forbidden transitions which should fall within the envelope of the intense electron-transfer transitions, and, therefore, they are not observed. The lowest The Journal of Physical Chemistry
Table I11 : Combination Assignment for the Spectrum of IrCls*-
Aasignment
---_ Obsd
Energy, cm-l-----. Calcd
17,840 19,600 20,960 21 ,090 22 ,957 27,800 sh 32,700 pk
17 ,790" 20, 630b 20,960a 21,110' 22, 680b 28 ,6 3 " 30,830b 31 ,10033, gooa
Ligand field states calculated using Dq = 2960 cm-', y = 9.0 and = 2700 cm-l. Electrontransfer states calculated by the Cusachs approximation, as in
B = 340 crn-l,
r
Table 11.
transition to a doublet is predicted to be at 28,700 cm-l. Jgrgensen' has observed a shoulder at 27,800 cm-l which may be assigned to this transition. The shoulder at 32,700 em-', observed in solution spectra,' is assigned on the basis of this scheme to either a tl, + tzg electron-transfer transition or to several closely lying ligand field doublets (2Tlg,2Tz,,and 2E,). The very intense transition at 43,100 cm-l may be assigned to transitions from filled ligand orbitals to the metal e, orbital. This combination assignment supports the previous assignment of Jgrgensen.
Conclusions The combination of molecular orbital theory and ligand field theory is necessary to quantitatively calculate and, thereby, to assign the spectrum of IrCla2-. Molecular orbital theory is capable of predicting transitions from the T orbitals of the ligand to the partially occupied metal orbitals. Ligand field theory is necessary to describe the less intense transitions which are within the d multiplet of the metal. The combination, thereby, incorporates the descriptive character of molecular orbital theory with the accuracy of ligand field theory and accounts for the number and energy of the observed bands. Therefore, this combination method best describes the over-all properties of IrCle2-. This approach has been suggested previously by J@rgensen'in his treatment of this system. The accuracy of this approach in fitting the spectrum of IrC162- is largely due to the ideality of the system. The ground state of IrCls2- has a tzg6configuration, and the low-lying transitions from the filled ligand T orbitals can only produce doublet states. Thus configuration interaction is unimportant and a oneelectron theory can be applied. This argument is (la) R, F. Fenske, J . Amer. Chem. Soc., 89, 262 (1967).
THIAZOLIUM IONS AND RELATED HETEROAROMATIC SYSTEMS not valid for the transitions to the unoccupied orbitals. The neglect of configuration interaction may account for the failure to predict accurately the 43,100-
2213
cm-I transition. This will be a very important consideration for the description of other transition-metal complexes.
Thiazolium Ions and Related Heteroaromatic Systems. 11. The Acidity Constants of Thiazolium, Oxazolium, and Imidazolium Ions1
by Paul Haake2 and Larry P. Bausher Department of Chemistry, University of California, Loa Angeles, California 90094 (Received December 18, 1067)
The pK,’s of the protonated forms of several oxazoles, thiazoles, and imidazoles have been determined and are given in parentheses following the name of the compound : 4-methyloxazole (1.07), ethyl 4-methyloxaeole5-carboxylate (0.83),4-methyloxazole-5-carboxylic acid (1.09, 2.88), 4-methylthiaeole (3.07), ethyl 4-methylthiazole-5-carboxylate (1.69), 4-methylthiazole-5-carboxylic acid (pKHA= 3.51), 4-methylthiazole-2-carboxylic acid (1.20, 3.18), 4-methylimidazole-5-carboxylic acid (2.53, 7.02), and l-methylimidazole-2-carboxylicacid (1.53, 7.25). Therefore, oxazoles are -lOB less basic than imidazoles, and thiazoles are w104 less basic than imidazoles. The pK’s for azolium acids indicate that zwitterionic forms are favored for imidazole acids, but uncharged forms are favored for thiazole and oxazole acids.
Introduction There are relatively few data on the basicity of thiazoles and oxazoles. As part of structure and mechanism studies on azoles,a we have determined the pK’s of a series of thiazoles (l),oxazoles (2), and imidazoles (3.) Five of the compounds studied were carboxylic acids and the pK’s furnish some information on the equilibrium, azolecarboxylic acid e azoliumcarboxylate ~witterion.4,~The methods of Albert and Serjeant6 were used in the determinations, so the pK’s have been corrected for ionic-strength effects and false pK’s have been avoided. A computer program has been written for the determination of the pK’s of dibasic acids, including cases where pK1 is not widely different from pK2.
t-2 1,x-s
or in potassium bromide windows on a Perkin-Elmer 421 infrared spectrophotometer. Mass spectra were measured with an AEI MS-9 instrument with an ionizing voltage of 70 eV, heated inlet system. Allmelting points are corrected; all boiling points are uncorrected. pKa Determinations. Standard HC1 and carbonatefree, standard NaOH were prepared and titrated by standard methods. The pKa values were determined by potentiometric titration with a Beckman Model G pH meter, which was standardized with saturated potassium hydrogen tartrate at pH 3.56. The apparatus used was similar to the one described by Albert and Serjeant.6 A solution of the compound was placed in a water-jacketed beaker through which water was circulated from a constant-temperature bath maintained at 25.0 & 0.1”. During the titration, a slow stream of nitrogen was bubbled through the solution, which was stirred magnetically. The stirring and the stream of nitrogen bubbles was discontinued while
2,XmO 3, X N-R 5.
Experimental Section All nmr spectra were taken on a Varian A-60 spectrometer. The spectra were determined in carbon tetrachloride with tetramethylsilane added as an internal standard or in deuterium oxide with sodium 3-trimethylsilyln-propylsulfonate added as an internal standard. Infrared spectra were taken either in carbon tetrachloride
(1) Supported by Grant AM4870 from the U. S. Public Health Service and by an Alfred P. Sloan Research Fellowship to P. H. (2) Address inquiries to: Department of Chemistry, Wesleyan University, Middletown, Conn. 06457. (3) (a) P. Haake and W. B. Miller, J . Amer. Chem. Soc., 85, 4044 (1963); (b) L. Bausher, Ph.D. Thesis, University of California, Loa Angeles, Calif., 1967. (4) L. Ebert, 2. Phys. Chem., 121, 385 (1926). (5) A. Albert and E. P. Serjeant, “Ionization Constants of Acids and Baaes,” John Wiley and Sons, Inc., New York, N. Y., 1962. Volume 79, Number 6 June 1068
