G . N. Papatheodorou
492
etric Study of the Palladium( il I) ~ Chloride ~ Vapor~Complex ~
l
u
~
. Papatheodorou Ths James Franc/( institute. University of Chicago, Chicago, ///iflois 60637 (Received May 8 . 19721
The reaction of solid cu-PdCl2 with gaseous AlzCl6 to form a deep red gaseous complex has been studied spectrophotometrically. Thermodynamic considerations suggested the stoichiometry for the reaction PdAl&ls(g): ANR = 7 2 kcal/mol, A& = 9.45 eu. The visible and uv electronic PdC1p(s) 4- &Cls(g) absorption spectrum of the PdAl&ls complex was interpreted in terms of a square-planar PdC14 sharing edges with two tetrachloroaluminates. The data for the above reaction were compared wlth the data available in the literature for the corresponding reaction of NiClz. The electronic absorption spectrum of NlAl&lg(g) i s reported and interpreted as a NiCls octahedron sharing faces with two tetrachloroaluminates
-
1. ~
n
t
~
~
~
~
~
t
~
In recent years the existence of vapor complexes of metal chlorides, with highly volatile chlorides of aluminum and iron(Ilil), has attracted the interest of different investigators. These gaseous complexes are widely distributed throughout the periodic table, including metal monochlorides to metal pentachlorides. The first complexes to be investigated were the NaAlC141 and NaFeC14.2-4 Later the alkaline earth chlorides (Mg and Ca),5,,6divalent 3d metal (Mn, Fe, Co, Ni, and Z n ) j chlorides, and the chlorides of Cd5 and Pb5 with either AlzCls or E'ezGle have been studied by vapor pressure equilibrium methods. Representative chlorides from the lanthanides (NdC13) and from the actinides (UG14 and UC15) have been investigated spectroscopically by Gruen arid c o ~ o r k e r s Zvarova . ~ ~ ~ and Zvarag have shown the formation an.d separation, by means of gas chromatography, of gaseous coniplexes of the lanthanide (Ce, Pr, Pm, Cd, Tb, Dy, Tm, Yb, and Lu) chlorides with AlzCl6. The general formula for th.e gaseous complexes with aluminum chlorides is (MClk),m-(&C16),. In all cases studied, the value of m is 1, while the value of n varies between 0.5 and 2. In the present report the existence of a dark red vapor complex of palladium chloride with aluminum chloride is established. Fi.irt*herrnore,the equilibrium
has been studied b> measuring spectrophotometrically the absorbance of the gaseous complexes over a-PdClz(s). The thermodynamic quantities for the above equilibrium and e of the Pd-AI-CI gaseous comthe spcctrosco ~ i properties plexes are discussed in terms of the stoichiometry and structure of the gaseous molecules.
Chernzccils m d E'quiprnent The anhydrous aluminum chloride was prepared from high-purity aluminum metal and gaseous HCl lo The y-palladium(ll) chloride was purchased from Matthey &shop, h ~ . The~salt - was dried at 150" for several hours under VSCUUI I of less than 1 g~ The anhydrous maThe Journal of Physical Cnernfstry, Voi. 77, No. 4, 1973
terials ~ were ~ handled under vacuum in tight containers or in a drybox with water vapor level less than 2 ppm. The spectrophotometric measurements were performed on a Cary Model 14H spectrophotometer equipped with a high-temperature cell compartrnentl2 (1-cm maximum path length). The spectra were digitally recorded on paper tape, and the calculation and plotting of molar absorptivities were carried out by a digital computer. The quartz optical cells were the uv type rectangular 1-cm cells pur chased from Pyroceli. Method Each optical cell was connected with a 10 cm long, 3-mm i.d., side quartz tube. A calibrated D c m 3 burret and water were then used for measuring the volume of the cell. The side tube was Calibrated in cm3 of the total volume (cell plus tube). These procedures permitted the determination of the cell volume with an error of less than 0.5%. The height of each cell did not exceed 9 cm and the total volume varied between 8 and 24 cm3. In a typiral experiment amouiits of AlCls and PdC12, preweighed on a microbalance, were transferred into the dry and degassed cell. The cell was evacuated and sealed on a premarked point of the side tube. The amounts of AlClz were adjusted so that all the AlQ in the cell was in the vapor phase a t temperatures between 500 and 600°K. The ceil was then heated for 15-30 rnm at 700°K and finally transferred into the furnace of the s ~ e c ~ r o p h o ~ o m eter. The vertical and horizontal temperature gradients of the high-temperature compartment of the spectrophotometer was less than 2". hi most cases the spectra were re(1) E. \N.Dewing, J. Amer. Chem. SOC..77, 2639 (1955). (2) C. M. Cook and W. E. Dunn, J. Phys. Chern.. 85,1505 (1961). (3) E. G. Korshunov, I. S. Morozov, V. I. lonov, and M. A. Zorrna, izv. Vyssh. Ucheb. Zaved.. Tsvel. Met.. 3 , 72 (1960). (4) R. R. Richards and N. W. Gregory, J. Phys. Chem.. (1964). (5) (a) E. W. Dewing, Mature i i o n d o n i . 214. 483 (1967); ( b ) Met. Trans.. 2, 2169 (1370). (6) K. N. Semenenko, T. N. Naumova, L. N. Gorokhov, and A. V. Bovoselova, Doki. Akad. Nauk. USSR. 154. 648 (1964). (7) H . A. Oye and D. M. Gruen, J , Arner. Chem. Soc.. e l , 2229 (1369). (8) D. M. Gruen and R. L. McBeth, Inorg. Cliern.. 8 , 2625 (1969). (9) T. S. Zvarova and I . Zvara, J. Chrornaiogor.. 44, 604 (1569). (10) N. J. Bjerrum, C. R. Boston, and G P. Srni!h, inn/-g. Chem.. 6 , 2261 (1967). (11) Malvern, Pa. 19355. (12) C. R. Boston and G. P. Smith, J Sci. ;nstrum.. Ser. 2 . 2, 543 (1969).
Palladium Chloride-Aluminum Chloride Vapor Complex
473
TABLE I: Determinationuf Molar Absorptivity
500 550
485.5 488 490.5 493 496 499 502
600 650
700 750 800
174 189 194 20.1 21 1 21 6 223
600
650 700 750
217 227 236 244
575 600 650
700 750 800
206 212 218 227 235 241
550 600
:!05
650
:!12
700 750
218
197
226
mol. V = 9.95 (:rn3. X 10 alm; n l b d = 1.024 X X 1 0 - ’ T atni; n,,,, = 7 038 X 10 mol; V = 15.25 i m 3 . P., = 4.468 X 10 -3Talm.fl,,
bPo
= 4.65
= 4.326
. ~ ~frequency at which the maximum absorptivity occurs) values of u , ~ , (the were found to be the same for all four experiments.
corded in the range between 600 and 800°K and in incervals of 50°K. The pressure 1’‘ of Alz&(g), without correction for consumption due to reaction 1, was calculated from the difference 1.’’ = P o - PI). Here POis the ‘%ea1 gas” pressure of A l z C l ~ ,and PI, is the correction7 duci to the dissociation’“ of the dimer A12C16. Detwmination of ,2/lolar A hsorptiuity. The cells were filled with small amounts of PdClz to ensure that all of the PdClz was in the gaseous complex state in the range of temperatures; studied. The “apparent” molar absorptiv) the gaseous complex(es) of wavelength X was ity ~ ( h for then determined from the relation
where V is the volun-le of the optical cell, h is the 1-cm optical path length. A is the absorbance at wavelength A, and npci the total number of Pd moles placed into the cell. Four different spectrophotometric experiments were carried out for t.his determination of c ( h ) . The charactcristics of each of these experiments are shown in Table I. The spectra were recorded from 4 to 30 kK. No bands were found in the 4 - 12 kK range. A strong absorp5on band close to 20 kK and a very weak band close to 15 kK were observed (Figure 1). From the listed values in Table 1: it appears that varies almost linearly with temperature. A plot of cmnx us. ?’ for all four experiments giws a set of straight lines with slopes very close to 0.15 14-1 c m - l deg -l. However, the absolute magnitude of tmax is subject to a noiisyste niatic variation from one experiment to another. Averaging all four experiments, we can express cIllilx as a function of 7
The constant term ( I 17.5) is calculated with a 5% ve,riation, which wc attribute to errors associated with the weighing of the small amounts of PdC12. On the other hand. the second t.erm of the equation (i.e., the relative change of absorptivity with temperature) is well established with an error of less than 1%. A comparison of the last experiment (12-109 in Table I) with the other three experiments indicate:, that, within experimental error, the absorptivity ( iR independent of the A12C16 pressure. This implies that either only one gaseous species is present or that two or more species with equal “atomic absorptivi-
Figure 1. Absorption spectra of t h e palladium chloride-aluminum chloride vapor complex as a function of temperature.
ties” are present. Thus in a mixture of 1 gaseous complexes (PdC12),(A12C16), ( m = m1,m2, . . . m, and n = n1,n2, . . . nl) the molar absorptivities t 1 , t z . . . ( 1 o f t h e absorbing complexes are related to the apparent molar absorptivity ( c, = m,r, i = 1,: ,...
I
(4)
The uv spectrum of the gaseous complexes was recorded in a somewhat different way. Due to the high absorptivity of the uv bands, the number of moles of PdClz to be complexed in the gaseous phase (in order t o give absorbances in the range permitted by the instrument) is very small and cannot be weighed accurately. However, we were able to determine the absorptivity in the uv region by preparing a very dilute, but unknown, concentration of the gas and by measuring the relative intensities of the uv and visible bands. For wavenumbers below 50 kK, there is only one uv band present at -41 kK. The absorbance of this band was measured with the 0 to 2 slidewire while for the same cell the absorbance of the visible band (at -20 kK) was measured with the 0.0 to 0.1 slidewire. It was found that at 600°K cmitx(41 kK)/crni:x(20 kK) I= 80. From this relation and the value of fmax(20kK) of 210 (at T = 600OK) we were able to approximate the absorptivity scale in the uv region. Solid PdCl2 in Equilibrium with A12(,’16 Gas. In order to determine the nature of the solid phase(s) in equilibrium with gaseous A12C16 and to exclude the possibility of a PdC12-A1&16 solid compound above 5Oo”K, we performed the following experiments. Excess quantities of y-PdCI2lrLand small quantities of A12C16 were sealed in an evacuated 10-cm long quartz tube whose volume was about 20 cm3. The quartz tube was then placed in a vertical windowed furnace and the temperature of the furnace was raised slowiy to 650°K. The sample was kept a t that temperature for 3 hr to ensure equilibrium. The upper end of the tube was then quenched in water, and all the aluminum chloride, with some gaseous complex, was solidified. After reaching room temperature, the tube was opened and the X-ray diffraction pattern of the solid in the lower end of the tube was (13) JANAF Thermochemical Data. The n o w Chemical Co Midland Mich (14) J R Soulen and W H Chappeli J Phys Cherrr 69, 3669 (1965)
The Journal of Physical Chemistry, Vol 77, No 4 . 1973
G . N. Papatheodorou
474
taken. The pattern showed that a-PdClz15J6 was the only solid phase present. No additional lines, indicating either the p17 and y forms or possible solid compound formation with AlZC16, were present. We have repeated the same type of experiment a t two different equilibration temperatures, 550 and 750°K. In all cases the only solid found (after breaking the tube) was the a-PdClz. Finally, in a fourth experiment, the quartz tube containing the cu-PdClz and AlzC16 was placed in a vertical windowed furnace in such a way that the excess a-PdCl2 was maintained in the bottom of the tube a t 550"K, while the other end of the sample tube was in a cooler zone a t about 520°K. The tube was maintained in this way for several days. Daily visual observations indicated the formation of small red single crystals in the middle of the sample tube. These crystals were identified by X-ray diffraction as a-PdCl2. This last experiment eliminated any possibility of solid complex compound formation and shows that the decomposition of the gaseous complex yields the a-PdClz form. Partial Pressures of the Gaseous C o m p l e d e s ) . The optical cells were filled with excess quantities of PdClZ. The apparent partial pressure of the gaseous complex(es) over the solid a-PdClz was determined from the relation
(5) where A,,, is the measured absorbance a t vmax and tmax is the apparent absorptivity of the complex(es) as given in eq 3. Measurements of Pc as a function of time and a t constant temperature showed that the equilibrium was reached very fast. An equilibration time of less than 10 min was sufficient before making spectral measurements. Seven separate experiments with different pressures (P') of AlzC16 were performed and the values of,,,A , P,, and R' are given in Table I%.
111. Discussion Thermodynamic Treatment of t h e Data. According to reaction 1, the equilibrium constant of the ith gaseous complex ( n = a,, rn = m,) with partial pressure P, is
K , -- P J ( P '
-
1
CN,P,)"'
(6)
1-1
In the aasence of information ( e g., mass spectrometric studies) regarding the principal gaseous complexes present, a thermodynamic treatment of the data for all possible reactions leacling to different complex species is futile. However, the treatment can be significantly simplified from the experimental fact that the measured apparent pressure P, is Dropertional to P'.
P, = (11b)P'
(7)
The validity of this proportionality is illustrated in Figure 2 . Relation 7 cmplies18 that (a) each gaseous complex contains one AlzAl6 molecule per mole and that (b) there is only one predominant complex species present, The general formula of this complex is (PdCl~),,&Cl6 with m = 1,2,3, . . . . The, thus far, investigated g a s e o u ~ l -or ~ solidls complexes of LilzClfi with transition metal chlorides contain always one transition metal atom per mole of complex, while the number of Al atoms varies from 1 to 4. Thus, we consider it reasonable to make the assumption that in the The Journal of Physicai Chemistry, Voi. 77, No. 4 , 1973
TABLE I I : Partial Pressures of the Palladium Gaseous Complex Experiment no. and characteristics E-1 02 v = 12.2 cm3 Po = 3.31 x 1 0 - - 3 ~ E-103 V = 21.5 c m 3 Po = 1.39 x 1 0 - 3 ~
E-1 04 V = 9.5 61113 p0 = 5.21 x 10-37E-105' V = 8.5 cm3 p0 = 5.59 x 1 0 - 3 ~ E-1 06 V = 24.55 cm3 PO = 0.756 X 1 0 - 3 T
E-1 11 V = 24.3 cm3 Po = 1.073 X 10-3T
E-1 12 V = 11.6 crn3 Po = 3.688 X 1 0 - 3 T
7, "K
Amax
P,, atm
P',atm
602.5 648 702 549 601 650 699.5 747.5 801.0 601 624.2 651.5 699.5 651 676.7 701.5 601.5 651.5 700.0 749.5 800.5
1.895 2.500 3.280 0.5025 0.7425
0.450 0.619 0.848 0.113 0.176 0.248 0.330 0.427
1.980 2.107 2.137 0.761 0.826 0.878 0.914 0.928 0.905 3.110 3.220 3.340 3.530 3.587 3.700 3.801 0.447 0.472
550.5 600.5 650.0 699.5 749 799 600 649.5 699.5 750.5 800.0
1.000 1.280
1.600
0.501 0.71 7
1.815 3.02 3.60 4.225 5.125 4.545 5.165 5.885 0.455 0.590 0.735 0.875 0.980 0.385 0,575 0.795 1.055 1.24 1.397 2.040 2.830 3.760 4.775 5.70
0.873 1.049 1.31 1 1.128 1.310 1.521 0.108 0.147 0,190 0,234 0.271 0.087 0,736 0.197 0.272 0.331 0.386 0.484 0.702 0.962 12 7 8 1.570
0.485 0.482 0.455 0.588 0.635 0.674 0.701 0.702 0.676 2.196
10K 2.95 4.16 6.59 1.75 2.71 3.94 5.66 8.54 12.45 2.99 3.72 4.57 5.90 4.59 5.48 6.67 3.19 4.50 6.40 9.45 14.69 1.74 2.73 4.14 6.35 8.96
13.38 2.83
2,351
4.25
2.482 2.570 2.590
6.32 9.89 15.3
a A 0.5-cm path length cell was used but the absorbance is referred to 1-cm path length.
PdCl2(s)-Al&l6(g) system the predominant gaseous complex is a mononuclear ( m = 1) palladium species and that the equilibria leading to complexes with m :Z 2 occur only to a slight extent. This view is also supported by the recent20 isolation of a diamagnetic palladium aluminate compound with an 1:1PdClZ:A1&16 stoichiometry. (15) H. Schafer,
U. Wiese, K. Rinke, and
K. Erendet, Angew. Chem..
79,244 (1966). (16) A, F. Wells, Z. Krisraliogr. Mineraiogr. Petrogr,. Abi. A , 100, 189 (1938). (17) K. Brodersen, G. Thiele, and H. G . Schnering, Z. Anory. Aiig. Chern.. 337,120 (1965). (18)From eq 4-7 it follows that 1
1
P , = x m , P , , P' = (P,/K,)"' 3I =1
1=1
,
=1
This last equation, based on the experimentally determined relation 8, is an identity; thus n = nj = n2 . . = 1 and P, = 0 (i ?' i ) . For the only nonvanishing partial pressure (e.g.. P I ) we have (l,'KL)4- 1 = b which determine the equilibrium constant of reaction 1. (a) J. A. Ibers, Acta Crystaiiogr.. 15, 967 (1962);(b) R, F. Belt and H. Scott, Inorg. Chern.. 3, 1785 (1964);(c) J. Brynestad, S. von Winbush, H. L. Yakel, and G . P. Smith, Inorg. Nuci. Chem. Lett.. 6, 889 (1970); (d) J. Brynestad, ti. L. Yakel, and G. P. Smith, Inorg. Chem.. 9, 686 (1970). G. N. Papatheodorou, to be submitted for publication.
.
Palladium Chloride-Alurriinum Chloride Vapor Complex
475
With the values rn = n = 1equilibrium 1becomes PdClz(~)-+- AI,Cl&)
---+
PdAlzCl&)
5 (8)
and the equilibrium constant is
K
=
P,/(P'
- P,)
The values of I< are listed in the last column of Table 11. A deduction of the enthalpy and entropy of this equilibrium, according to thc second law, is shown in Figure 3. Thirty-two experimentally determined values of R 111 K are plotted us X/T The least-squares treatment of the data shown yields a value of AHR = 7.2 f 0.16 kcal/mol for the enthalpy of reaction 8 over the range of temperature studied. For the same temperature range a value for the entropy of reaction ASR = 9.45 f 0.75 is obtained. A comparison between the angles and distances of the PdCi4 square 111 solid PdC1216 and of the A l a 4 in gaseous13 A12C16 shows that changes of bond lengths and bond angles by less 1,han 3% make the C1-C1 edge of the AIC14 tetrahedron equal with the Cl-C1 side of the squareplanair PdCla. TE view of this consideration we propose in Figure 4 a structural model for the PdAl2Cls gaseous complex. The angle 01 was introduced in order to determine the position of hl atom (and the AlC14 units) with respect to the PdCla plane. Using this model (w = 0) and the structural parameters of the A12C16 dimers13 we calculated a statistical21 third law translational and rotational entropy fer reaction 8: ASTR = 3.8 f 0.3, 500°K < '7' € 800°K. This value, and the experimentally determined entropy, suggest a small vibrational entropy contribution of -5 eu. We shall now compare the thermodynamic functions of reaction 8 with the available in the literaturesb corresponding functions for the NiC12-AlzC16 reaction.
€ 3
c
Q
I
0
.5
I .Q
I .5
2.o
Pc ,Atm. Figure 2. Plots of P' vs. P, at different temperatures. T h e slopes b were determined from a least-squares treatment of t h e data.
diamagnetic complex. In Table 111 we give the absorption data of PdC1d2- in the solid state and in solution. The asd transitions are based on the data signments of the d of Day, et a l , 2 2 on K2PdCl4 and on the data and ligand field calculations o f Martin and coworkers23 on solid K2PtC14. The assignments of the uv band are based on MO calculation^.^^ These high-intensity absorptions are attributed to ligand-to-metal charge transfer transitions
-
(L M). A comparison of the visible spectrum of the solid K2PdC14 and of PdC142- in solution indicates that the two first spin-allowed bands of the K2PdClg are not separable in the solution, while the third spin ailowed band of the solid seems to disappear under the first charge transfer band in the solution. In the last column of Table I11 we have listed for comparison the absorption data for the gaseous PdAlzCls complex. It appears that in the visible region the same band(s) is(are) present for both the gaseous and solution spectrum. Their main differences are a substantial enhancement of intensity, a slight red shift, and some broadening in going from the aqueous HCI solution to the gas. All these differences are reasonably attributed to temperature differences. As Figure L shows, the broadening of the band and the red shift increases with increasing temperature. In fact, an extrapolation of the spectrum in Figure 1 to room temperature, where the PdC142- solution spectrum was taken, gives emax (300°K) -'I I60 and VmaX (300°K) e 21 kK. -+
NiCl,(s) -I- A12Cl&)
-+
NiA12C18(g)
(9)
AH = 7.5 kcal /mol AS = 9 eu It appears that the stabilities of the PdA12Cls and NiA12C18 gaseous complexes are very similar. In his work, Dewingsb concluded that the similarities of the stabilities of the MAlzCln (M := Ca, Mn, Co, and Ni) complex can be best interpreted i%S implying that the bonding of the divalent ions in the complex is very similar to that in the solid. All MClz solids have similar structures with the divalent metal in o8r:tahedral coordination while the PdClz solid is a completely different structure with Pd2-+ in square-planar coordination. These differences, however, do not alter the r.hermodynamic functions of reactions 8 and 91, which implies that the characteristic structure and coordination in the solid is also present in the gaseous complex. Thus \we should expect that local geometry of Nia+ in NiA12CI8 is very close to Oh while the geometry of Pd2-i-in PdAlzCXS is very close to Dzh. Electronic Spectra. In Figure 5 the absorption speccrum of the PdA12Clfb gaseous complex is shown in the region . The visible and near-ir spectrum is characterized by one high-intensity band at wave number close to 20 kK. b. low-intensity band can be also observed in the 14-15-kK region. The uv spectrum shows a very strong absorption band a t 40.2 kK. The only known all-chloride Pd(I1) complex is PdC142which is a typical example of a well-known square-planar
(21) A calculation of the moment of inertia of the gaseous complex, as a function of w , shows that the entropy contribution of the moment of inertia increases by only 0.7 eu for changes of w between 0 and 45".
(22) P. Day, A. F. Orchard, A. J. Thompson, and R . J. P, Williams, J. Chem. Phys., 42, 1973 (1965). (23) D. S. Martin, M. A. Tucker, and A . J. Kassmai?, inorg. Chem.. 4, 1682 (1965); as amended, 5,1298 (1966). (24) (a) A. J. McCaffery, P. N. Schaltz, and P. J. Stephens, d . Amer. Chem. SOC.. 90, 5730 (1968); (b) H. ito, J. Fujita, and K. Saito, Chem. Soc. Jap.. 40, 2584 (1967). The Journal of Physical Chemistry, Vol. 77, No. 4, 1973
G. N. Papatheodorou
476
+T,'K 850 750 650
6
WAVE NUMBER I k K )
Figure 5. Absorption spectrum of Pd(AIC14)2(g)from 4 to 50 kK. TABLE Ill: Assignments of the K2PdC14 Solid and PdCk2Solution Spectra K Z P ~ C I ~ ~ PdC14'solid, 300°K soh, 2 M HCI Assignment 'Ale ---*
1.1
-
1.3
1.5
l / T x IO3, Figure 3. Plot of R in
1.7
1.9
V,
kK
~max
17
7
(18)c (20)
(19)
22.6
128
23.0 (29.5)
80 (67)
V,
I
kK
K vs. l / l f o r reaction 8.
enlax
V,
kK
emax
1-15)
(16)
(67) 21
OK-'-
PdA12Cla gas, 300°K
___.--
20
220
40.2
15,800
152
(30.2)d (540) 35.8 10.400
I*
'E, (a)
(1.c
a From ref 22. transfer band.24b
Figure 4. MO~eCillat'model of the PdAlzCl8 gas.
Although the effect of temperature has not been measured for PdCi42- in solution, it has been measured in the case of KzPtCl4 crystalsz3 between 15 and 300"K, and in the case of PtCP42- centers in liquid alkali chloride solvents between 600 and 11QO0K.2YHere, also, an increase in temperature greatly enhances the intensity, shifts the peak to the red, and broadens the band. These effects are plausibly accounted for by vibronic interactions in which asymmetric vibrational states, which destroy the center of symmetry of MC142- anion, become increasingly populated with increasing temperature.28 Therefore, it is more likely that the broad band, occupying the visible spectrum of PdAlsCls, covers a t least two spin-allowed d --* d transitions of the palladium atom in a ligand field with LI,, local symmetry. approximately -_ 1x1 contrast with $he agreement observed in the visible region, a comparison of the uv spectra of P d C L - and IPdAlzC38 indicates delinite differences. The three charge The Journal of Phy.sica1 Chemistry, Vol. 77, No. 4, 1973
From ref 24.
Shoulder.
Qrbital forbidden charge
transfer bands in aqueous HC1 have been replaced in PdAlZC18 by one band of intermediate energy. Since the charge transfer transitions originate essentially from ligand-based molecular orbital levels, their energies should be very. sensitive to the electronic environment of the chloride ligand. Environments pulling away the electronic density of the ligand are expected to give rise to charge transfer transitions a t higher energy.26 In the case of the PdAlzCls molecule the bridged chloride i s strongly polarized by the aluminum atom, and the C1 electronic density is substantially different from that of Cl in PdC142M charge transfer is hin(aqueous HC1). Thus the L dered by the aluminum, and the energies of the charge transfer bands OCCUK a t higher energies, From these considerations, we suggest that the only uv band in the lA2, LE,(^) charge PdAlzCls spectrum is the lAl, transfer transition. A further support for this assignment is the similarity of the relative intensities of the uv and visible bands for the PdA12Cl8, with the relative intensities of the lA1, lAzlr lE,(r) and 'AI, IAz, bands in the PdC142- (in 2 M HC1) molecule. The association of the electronic absorption spectrum of the gaseous complex, with the spectrum of PdC1d2- in a
-
-
-
+
+
---t
N, Papatheodorou and p, Smith, Nut, ,norg Chem to be oubltshed (26) The substitution of Br in PdBr4z- by the less polarizable CI shifts the charge transfer band toward the blue The energy of the lAlg -+ 'AzU f l E u ( T ) band, for example, shifts to higher energies by - 6 kK as we go from Br to CI 24 (25)
Copper Phthalocyanine Polymorph
477
I
LL+.J--L&L
(
4
1
1
30
1
1
1
1
I
i
1
1
I
40
WAVE NUMBER ( X K I
1
!IC 7
Figure ti. Absorption spectrum of Ni(AIC14)2(g) from 4 to 50 kl
