932
J. Phys. Chem. 1993,97, 932-936
Location of Pd(1) Species and Adsorbate Interactions in PdH-SAPO-5 Molecular Sieve Determined by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopies Thieny Saint-Pierre, Xinhua Chen, and Lany Kevan' Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: September 8, 1992; In Final Form: November 9, 1992
Electron spin resonance (ESR) and electron spin echo modulation (ESEM) have been used to determine the location of Pd+ and its interaction with water, methanol, and hydrogen in PdHSAPO-5. After activation a t 600 ' C two different Pd+ species are observed: A (811 = 2.96) and B (gll = 2.68) with a common gL = 2.13. The adsorption of water on activated samples results in the decomposition of water with formation of 02-and the simultaneous disappearance of the B signal. The ESR results and the 31Pmodulations observed in ESEM are consistent with a location of species A in a site I. However, after adsorption of methanol, the A signal is absent which is most likely due to a migration of these species toward another site where they are more accessible to methanol molecules. ESEM results show that two methanol molecules coordinate to Pd+ located in a site II+, with another one interacting at a larger distance from a neighboring channel. When hydrogen is adsorbed, a third species C (811 = 2.40) with gl = 2.12 is observed.
Introduction The aluminophosphate (AIPO4-X) molecular sieves are microporous materials with channels and cages.'** These molecular sieves have some structures that are homologous with certain aluminosilicateor zeolite structures such as AlP04-37 which has the faujasiteor zeolite X structure. AlP04's also have some unique structures not found in zeolites such as AlP04-5 and AlP04-11. In order to create anion-exchange capacity,silicon can be partially substituted for phosphorus to yield silicoaluminophosphate (SAPO-X) molecular sieves3s4which have the same structure as the analogous AlPOis. The SAPO-5 molecular sieve is a substituted AlP04-5 composed of 4-ring, 6-ring, and 12-ring straight channels which are interconnectedby 6-ring windows (Figure 1). This channel-type structure does not have an analogous zeolite structure. The difference in the number of A102- and POZ+tetrahedra creates a negative charge of the framework, balanted by H+in H-SAPO-5 which is the calcined form of SAPO-5. The SAPO materials show interesting propertiessuch as shape selectivitySuseful for catalysis. They can be loaded with transitionmetal ions such as Pd+ by ion exchange of H+ by the metal and be used for non acid catalysis. Pd-loaded catalysts are widely used for hydrocarbon ~ y n t h e s i s ~and - ~ ethylene dimerization.8-9 Hence, it is important to determine the location, oxidation state, and adsorbate geometry of Pd in SAPO materials to develop a basis for the application of these ions in catalytic processes. Pd(1) and Pd(II1) species have been characterized and located in X and Y zeolites by electron spin resonance (ESR), electron spin echo modulation (ESEM), X-ray diffraction, and X-ray photoelectron ~ p e c t r o s c o p y . l ~ESR ~ ~ and ESEM give information about the local geometry of the transition-metal ion and the number and distance of adsorbates around such a paramagnetic center. In this paper ESR and ESEM results are used to determine probable locationsof Pd(1) in solid-state ion-exchanged HSAPO5 . It has been found that in activated samples Pd(1) occupies two positions. When methanol is adsorbed, a Pd species migrates from site I at the center of a double 6-ring toward site 11, inside a 12-ring channel near a 6-ring window where it coordinates to adsorbates: Experimental Section Synthesis and Exchange hocedure. The SAPO-5 molecular sieve was synthesized according to a Union Carbide Patent 0022-3654/58/2097-0932S04.00/0
Figure 1. SAPO4 structure. Cation sites are designated by analogy with X zeolites.
(example 13).3 H3P04 (85% Mallinckrodt), A1203.H20(Vista Chemical), tripropylamine (Aldrich Chemical), fumed silica (Sigma Chemical), and deionized water were used to prepare a homogeneous gel with the following molar composition: 2.0Pr3N:
0.3Si02:A1~O3:P2O~:4OH20. The gel was then placed in a stainless steel pressurevessel lined with Teflon and heated at 150 'C for 48 hat autogenouspressure. The autoclavewas then quenched, and the reaction material was washed three times with deionized water before filtration. The structure of the synthesized material was examined by powder X-ray diffraction (XRD) using a Philips PW 1850diffractometer. The XRD patterns of the as-synthesized material agree with that of SAPO-5 in the l i t e r a t ~ r e . ~The J ~ as-synthesized SAPO-5 was heated in flowing oxygen for 48 h at 600 'C to remove the templating agent and form HSAPO-5 in which the framework negative charge is balanced by H+. Pd was exchanged into HSAPO-5 using a solid-state reaction method. A 0.02-g quantity of Pd(NH3)4CbH20 (Morton Thiokol) was ground with 1 g of HSAPO-5 in a mortar until the powder looked homogeneous. The mixture was pressed in a stainless steel die with a force of 2 tons to prepare a pellet of 12 mm in diameter and 3 mm in thickness. The pellet was then broken into small chunks, placed in a porcelain boat, and heated in a furnace in air at 600 'C for 6 h. The reaction product is cooled slowly to room temperature and ground to a fine powder. After this treatment PdHSAPO-5 is brown. Activation Treatment. PdHSAPO-5 was loaded in 3-mm0.d. by 2-mm4.d. Suprasilquartz tubes andevacuated toa residual pressure of 1 X Torr overnight at 600 OC to give sample type Q 1993 American Chemical Society
7--7/eAp0-5
Adsorbate Interactions in PdHSAPO-5
The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 933 g1 =2.04
PdH-SAPO-5lD,O
200 G
200 G
*
I
t
xi0
gblBL=2.13
Figure 2. ESR spectrum at 77 K of activated PdHSAPO-5.
TABLE I: ESR Parametem for PdH-SAPO-5 sample treatment species: SAPO-5 site ma gLa 2.964 2.135 activatedb Pd+: I 2.685 2.135 Pd+: I1 2.962 2.114 +D20 Pd+: I Pd2+-021.988 2.043 +MeOH Pd+(Oz)3(CH30H)2:II* 2.684 2.1 15 +H2 Pd+: I 2.942 2.125 Pd+: I1 2.665 2.401 Pd+: I1 or II* Estimated uncertainty is *0.004. Sample dehydrated at 600 O C overnight exposed to 02 for 8 h at 600 O C and evacuated at the same temperature for 3 h. A. Type A samples were heated at 600 OC in static dry oxygen at 100 Torr for 8 h to give sample type B. Type B samples were evacuated at 600 OC for 3 h to give “activated” samples. The activated samples were exposed to adsorbates at their room temperature vapor pressure for 20 min prior to ESR and ESEM measurements. The adsorbates used were DzO (Aldrich Chemical), CHSOD, and CD3OH (Stohler Isotope Chemicals). Activated samples were also exposed to 400 Torr of H2 overnight at room temperature. The sampletubes were sealed after exposure to adsorbates and were stored in liquid nitrogen prior to ESR and ESEM measurements. Spectroscopic Measurements. ESR spectra were measured at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northern TN-1710 signal averager. Each spectrum was obtained after multiple scans to achieve a satisfactory signalto-noise ratio. Each acquired spectrum was transferred from the signal averager to an IBM PC/XT compatible computer for analysis and plotting. The magnetic field was calibrated with a Varian E 4 0 0 gaussmeter. The microwave frequency was measured by a Hewlett-Packard H P 5342A frequency counter. ESEM spectra were recorded with a Bruker YSP 380 pulsed ESR spectrometer. Three-pulse echoes were measured by using a 90°-~-900-T-900 pulse sequence with the echo measured as a function of T. The ESEM spectra were recorded at several T values: 0.256 ps to detect 31Pmodulation and 0.28 ps to detect 2D modulation.
ESR Results Activation Treatment. After solid-state ion-exchangereaction the samples are brown and are ESR silent. The type A and type B samples are respectively gray and brown and do not show any ESR signal. However, the activated sample which is light gray produces an ESR signal. The activation procedure of heating in vacuum after high-temperature exposure to oxygen produces paramagnetic Pd species, but the mechanism is unclear. The ESR spectrum of an “activated” sample at 77 K shows an anisotropicsignal characteristic of speciesin an axially symmetric environment. Two signals designated as A (41= 2.96) and B (gil = 2.68) withacommongL valueat 2.13 (Figure2) wereobserved. The two signals are assigned to Pd+ in two different environments.15 Thegvaluesof thesespecies aresimilar to those observed in various ion-exchanged zeolites. Ben Taarit et al. reported a
PdH-SAPO-5ICH3OH
4r gEl1=2.68
%PPH
200 G
w
I
Pd+ species with gl = 2.97 and gL = 2.10 in CaPd-Y zeolite.13 So if one considers the large anisotropy, the values of the g tensors, and the reducing conditions under evacuation at high temperature in the last step of the activation process, the observed signals can be attributed to Pd+ d9 species. Adsorption of Water. After water adsorption on activated PdH-SAPO-5 a new ESR spectrum was observed. A new strong narrow signal with reversed gvalues g, = 2.04 and gll= 1.98 was observed immediately after exposure to water. At the same time signal B disappeared, but signal A was still weakly observable (Figure 3). The gl position seems slightly shifted to gL = 2.1 1, but this is likely due to the overlap with the strong g, component of the new species. After reaction with water the spin concentration of the spectral region corresponding to Pd+ species A and B is reduced to one-half of its value in an activated sample, but the total spin concentration including the new sharp signal is nearly the same as in an activated sample. The sharp signal with gl = 1.98 and g, = 2.04 can be assigned to Pd2+-02-. A similar ESR spectrum was obtained in Pd-mordenite after adsorption of 1 Torr of oxygeni3and in Pd-Ca-X zeolitesI5after adsorption of water on a sample activated in the same way as done here. The ESR spectrum of the same sample recorded 1 month later does not show significant differences in shape or intensity. Adsorption of Methanol. Samples with adsorbed methanol showed a Pd+ signal in an axially symmetric environment (Figure 4). The A signal disappears after methanol adsorption, and one observes a slight shift of thegl component togl = 2.1 1. However, the total spin concentration remains about the same as before adsorption. The Pd+ signal characterized by gl = 2.68 and gL = 2.1 1 slowly decreased in intensity over several days. After 1 day, the integrated intensity of the signal is reduced by 25%. This might be attributed to a reaction of methanol with the Pd species. The mechanism is unclear and will not be discussed further here. Adsorptionof Hydrogen. Hydrogen was adsorbed on activated samples overnight at room temperature at a pressure of 400 Torr. It was pumped off before ESR measurements. Signals A and B are still observed with a slight shift to higher field: 2.94
Saint-Pierre et al.
934 The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 1.OrA
-EXPT .- - -
gA,,=2.94
a
-cn
*
200 G
L
Figure% ESRspectrumat 77 KofactivatedPdHSAPO-5 withadsorbed H2. PdH-SAPO-SQO 31P modulation
,
z
g 0.23
s
5
2 = 0.28 Ps
a
'.''._..'-............,,,,,_,,_b
N=6 R = 0.47 nm
6 0.41
PdH-SAPO-5 I CH30D
Y
.............. .....'.......__, ......._..
9 0.6
II
5 0.4, I-
t gA,B*C,=2.12
0.8
l
a
o
r
n
ni
PdH-SAPO-5 I CD3OH
3 0.8
Q 0.21
!
i I
0
CALC
Shell 1
1
2
3
4
2
5
11 1 1 N R, nm A, MHt 6 0.43 0 . 7 3
0.32
T, PS
-'I
p 0.2 0
II1
0
3
2
1
4
5
T, PS withadeorbedDzOshowing3lPmodulation( ~ ' 0 . 2 5 6 ~ ) .(b)Simulated
Figure 8. Three-pulse experimental and simulated ESEM spectra at 4.2 K of P d H S A P O - 5 with adsorbed CD3OH.
ESEM spectrum. (c) Fourier transform spectrum of the ESEM signal in (a).
TABLE Ik ESEM Simulation Parameters for PdH-SAPO-5
Figure 6. (a) Thrce-pulse ESEM spectrum at 4.2 K of P d H S A P O - 5
with Various Adsorbates
and gel, = 2.67. But a more interesting result is that a third species, gCll = 2.40, is observed as shown in Figure 5. Although the g, component is broader, a common gl = 2.12 is tentatively assigned to all three Pd+ species. The total spin concentration remains about the same as before hydrogen adsorption.
ESEM Results A strong ESE signal without deuterium modulation was observed for the samples adsorbed with DzO. This agrees with what was reported in activated PdCa-X ~eolites.1~ The lack of deuterium modulation supports the decomposition of adsorbed water and the assignment of the ESR spectrum to PdZ+-02-. The 3lP modulation was next investigated in the same sample. The field was set at 3310 G to study the Pd+ species. Clear modulation is observed (Figure 6a) which is confirmed by the Fourier transform spectrum of this signal (Figure 6c). The peak at 5.80 MHz correspondsto the Larmor frequency of 31Pat this field. The signal was simulated with six 31Patoms located 0.47 nmaway fromPd+(Figure6b). Incontrast, thesamplesadsorbed with methanol show little or no 31Pmodulation at the same field position. The samples with adsorbed CH30D and CD30H show 2D modulations (Figures 7 and 8). For the simulations of these spectra it was necessary to use a two-shell model. The ESEM simulation for CH3OD shows that two 2D nuclei are located at 0.32 nm from the Pd+ ion and one 2D nucleus is at 0.45 nm from
adsorbate
shell
N"
Rb
Aimc
CHjOD
1 2 1
2
0.32 0.45 0.43 0.32 0.47
0.21 0.02 0.07 0.1
CDJOH D2@
1 6
2
3
1
6
0.0
Number of nuclei. b Distance from the Pd+ ion to the nuclei, nm; estimated uncertainty is &0.01 nm. Isotropic hyperfine coupling, MHz; estimated uncertainty &lo%. ESEM recorded at 3310 0 (g 2.1 1) with T = 0.256 ps for 3'P modulation; with optimum T = 0.28 M for 2D, *D modulation is not observable. 0
-
Pd+. This indicates the direct coordination of two methanol moleculesand indirect coordination of anothermethanol molecule. The simulation of the ESEM signal from CD30Hgave six nuclei at a distance of 0.43 nm and three nuclei at a distance of 0.32 nm. Thus, there is good agreement between these two sets of parameters that will be used to discuss a possible location of Pd+ in the SAPO-5 structure. The ESEM parameters are summarized in Table 11.
Discussioll The H-SAPO-5 molecular sieve is composed of 4-ring, 6-ring, and 12-ring straight channels interconnectedby 6-ring windows. Byanalogywiththecation sitesinXzeolites,Cheneta1.21proprsed possible locations of the cation sites in SAPO-5(Figure 1). Site I is at the center of a double 6-ring that forms a 6-ring channel.
Adsorbate Interactions in PdHSAPO-5
The Journal of Physical Chemistry, Vol. 97, No. 4, 1993 935
This site is comparable to an SI site in the middle of a hexagonal prism in X zeolites. Two other sites can be identified in a 12-ring channel: site I1 in the center of a 6-ring window and site 11, displaced from the center of a 6-ring window toward the center of a 12-ring channel. The Pd3+species recorded in XlsJ6and Y17J9 Pd-exchanged zeolites was not observed in the HSAPO-5 molecular sieve after exposure to static oxygen at 600 OC (sample type B). The low framework charge of HSAPO-5 may account for this behavior. It is probable that the Pd3+ species cannot be stabilized in these conditions or that the content of Pd*+ exchanged into the sites is not sufficient to promote the mechanism (1) proposed by Naccache et al.1° 2Pd(II)
+ H 2 0 + (1 / 2 ) 0 2
-
ZPd(II1)
+ 20H-
(1)
When water is adsorbed on the activated samples, a splitting of the water molecules is observed. This decomposition of water was reported in the case of ~ e o l i t e s , l and ~ * ~a ~sharp , ~ ~anisotropic signal g, = 2.05,gIl = 1.96 was observed which is similar to that observed in PdHSAPO-5 (Figure 2). A Pd2+-02-complex with g, > gl has been assigned for this spectrum. Another alternative is a Pd2+-023- complex isoelectronic with F2- or C12- with an axially symmetric ESR signal with gl > gll.Is After water adsorption the only Pd+ species still present after reaction is species A. This is an indication that A is located in a site less accessible to water molecules than is species B. So water first reacts with the most accessiblespecies B, and eventually some of the unreacted water reacts with species A. The analysis of the 3lP modulation shows that species A is surrounded by six atoms at a distance of 0.47 nm whereas the contribution of j l P is negligible for species B. Both the ESR and ESEM results are consistent with a location in site I. It is difficult however to determine the geometry of coordination of the Pd species. The high temperature used for the activation in vacuum seems to allow the migration of Pd2+ toward site I in the center of a double 6-ring where it is reduced to Pd+. This agrees with the work of Sachtler and co-workers on PdY ~ e o l i t e s ,which ~ ~ , ~shows ~ that at high temperature Pd2+ migrates toward the sodalitecages and hexagonal windows where it can be reduced in the presence of hydrogen. However, in our case the reduction mechanism is unclear. When methanol is adsorbed on an activated sample, signal A disappears. The total spin concentration is only reduced slightly so the disappearance of signal Acannot be explained by a reaction with methanol since no organic radical species is observed. Moreover, the Pd+ ion located in site I should not be accessible to the bulky methanol molecule because it is not accessible to water molecules. So it is suggested that species A migrates to the same location as species B to coordinate to methanol molecules in a more accessible site. The ESEM pattern of the CH3OD adsorbed samples reveals that twodeuteriumsare located0.32nm from thePd+,andanother deuterium is located 0.45 nm from Pd+. This suggests that two methanol molecules directly coordinate to Pd+ and another one interacts indirectly at a further distance. In support of this, the ESEM parameters of the samples with adsorbed CDjOH show that six deuteriums are located 0.43 nm from Pd+ and three others are 0.32 nm from the Pd+ species. This is good agreement between these two sets of parameters. So it is probable, as suggested by Chen et al.21 in the case of CuHSAPO-5, that a methanol molecule located in an adjacent channel is oriented in such a way that its methyl group is directed toward the Pd+ ion. Sites I1 and II* are the only possible Pd+ locations because site I does not allow direct coordination of the bulky methanol molecules with Pd+. Usually Pd+ is coordinated in a square planar geometry,26 but near a window site there are only three equidistant oxygens. Location of Pd+ in site I1 in the center of a 6-ring seems unlikely
A
Figure 9. Schematic diagram of a Pd+ ion at site 11. in SAPO-5 coordinateddirectlytotwomethanolmolecultsinthesame 12-ringchannel and indirectly coordinated to one methanol molecule in a neighboring channel.
because it would form a trigonal bipyramidal structure when Pd+ coordinates with two methanol molecules giving rise to reversed gvalues (81< gl) which are not observed. So a site II* location is proposed for Pd+. Then, when methanol is adsorbed, two methanol molecules interact with the Pd+ ion and pull it further away from the center of a 6-ring window toward the 12-ring to minimize the electrostatic repulsion between the oxygens of the hydroxyl group of methanol and the oxygens of the lattice. This coordination geometry is shown in Figure 9. It is interesting that the Pd+ signal looks axially symmetric and does not exhibit a high field minimum in the gl region as found for Cu2+ in SAPO-5.21 The apparent axial symmetry does not seem very diagnostic of possible coordination geometries for Pd+ in SAPO materials. After hydrogen adsorption a third signal originating from a Pd+ species is observed. The gvalues are comparable to the ones reported for Pd+ species with adsorbed CO or H2 in synthetic mordenitel3 (811 = 2.41,al = 2.35). On the basis of the structure of SAPO-5 and its possible anionic sites, it is rather unlikely that Pd+ is stabilized in a site other than the sites already occupied by Pd+ in activated samples. The ESR signal can reasonably be explained by the presence of a hydroxyl ligand formed during the adsorption of H2. Pd+ is then coordinated to three lattice oxygens and bound to a ligand in an axial position which decreases gl = 2.68 (signal B) to 811 = 2.40 (signal C).
COnClUSiOM The Pd3+species is not observed after oxidation in the SAPO-5 molecular sieve which may be due to a low negative framework charge as compared with zeolites. But after high-temperature activation in vacuum, ESR and ESEM reveal that Pd+ is stabilized and is located in twodifferent sites which are most likely exchange sites I and 11. The adsorption of water results in its decomposition with the formation of 02-. When methanol is adsorbed, one of the Pd+ signals disappears which can tentatively be explained by the migration of Pd+ from site I toward site I1 where it is more accessible to methanol molecules. An axially symmetric Pd+ signal is observed, and the ESEM results show that two methanol molecules are directly coordinated with Pd+ and that another one interacts at a further distance probably in a neighboring channel. A coordination geometry is proposed where Pd+ located at site 11* is coordinated to three lattice oxygens and two methanol molecules. The adsorption of hydrogen on activated sample leads to the formation of a new species which is suggested to be Pd+ with a hydroxyl ligand on the axis of a pseudopyramidal complex. Acknowledgment. This research was supported by the National Science Foundation and the Robert A. Welch Foundation.
References and Notes (1) Wilson, S.T.; Lok, B. M.; Flanigen, E. M. US.Patent 4 310 440, 1982. (2) Wilson, S.T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J . Am. Chem. Sm. 1982. 104. 1146. (3) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan. T. R.; Flanigen, E. M. US.Patent 4 440 871, 1984. (4) Lok, B. M.; Messina, C. A.; Patton, R. L.; Gajek, R. T.; Cannan, T. R.; Flanigen, E. M. J . Am. Chem. Soc. 1984, 106, 6092. (5) Kaiser, S. W. US. Patent 4 499 327, 1985.
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The Journal of Physical Chemistry, Vol. 97, No. 4, 1993
(6) Thomson, R.;Montes, C.; Davis, M. E.; Wolf, E. E.; J . Coral. 1990, 124, 401. (7) Chung, S. K.; Butt, J. B. Appl. Caral. 1990, 64, 173. (8) Lapidus, A.L.;Mal’tsev,V. V.;Shpiro, E.S.;Antoshin,G. V.;Garanin, G. V.;Minachev, Kh. M. Izv. Akad. Nauk. SSSRR, Ser. Khim. 1977,2454. (9) Ghosh, A. K.; Kevan, L. J . Am. Chem. Soc. 1988, 110, 8044. (IO) Naccache, C.; Primet, M.; Mathieu, M. V. Ado. Chem. Ser. 1973, No. 121, 266. ( I I ) Narayana, M.; Michalik, J.; Contarini, S.; Kevan, L. J. Phys. Chem. 1985,89, 3895. (12) Che, M.; Dutel, J. F.; Gallezot, P.; Primet, M. J . Phys. Chem. 1976, 80, 2371. (13) Ben Taarit, Y.; Vedrine, J. C.; Dutel, J. F.; Naccache, C. J . Magn. Reson. 1978, 31, 251. (14) Gallezot, P.; Imelik, B. Adv. Chem. Ser. 1973, No. 121, 66. (15) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1985, 89, 4553.
Saint-Pierre et al. (16) Michalik. J.: Heminn. M.: Kevan. L. J. Phvs. Chem. 1986.90.2132. . . (17) Ghosh, A. K.; Kevan, L. J . Phyi. Chem.-1990, 94, 1953. (18) Flanigen, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S.T. In New Developments in Zeolite Science and Technology; Murakami, Y.,Iijima, A.;
Ward, J. W.,Eds.; Proceedings of the 7th International Zeolite Conference; Elsevier: Amsterdam, 1986; pp 103-1 12. (19) Naccache, C.; Dutel, J. F.;Che, M. J . Coral. 1973, 29, 179. (20) Tressaud. A.; Khairoun, S.;Dance, J. M.; Hagenmuller, P. Z . Anorg. Allg. Chem. 1984, 517,43. (21) Chen, X.;Kevan, L. J . Am. Chem. Soc. 1991, 113, 2861. (22) Kasai, P. H.; Bishop, R. J. J . Phys. Chem. 1977,81, 1527. (23) Kuznicki, S. M.; Eyring, E. M. J . Am. Chem. Soc. 1978,100,6790. (24) Homeyer, S. T.; Sachtler, W. M. H. J. Caral. 1989. 117, 91. (25) Homeyer, S.T.; Sachtler, W. M. H. J . Caral. 1989, 118, 226. (26) Cotton, F.A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; pp 917-937.
