J . Phys. Chem. 1991, 95, 868-871
868
Crystal Structure of [C O ~ ~ ~ * Na+]4Si,,AI,2018*2Br2, B~~-]~[ a Bromine Sorption Complex of Dehydrated Partially Cobalt( I 1)-Exchanged Zeolite A Yang Kim,* Suk Hee Lee, Chemistry Department, Pusan National University, Pusan 609- 735, Korea
Duk So0 Kim, Chemistry Department, Cheju National University, Cheju 690- 756, Korea
and Karl Seff* Chemistry Department, University of Hawaii, Honolulu, Hawaii 96822-2275 (Received: May 18, 1990)
The crystal structure of a bromine sorption complex of dehydrated partially Co( 11)-exchanged zeolite A (Co4Na4-A) has been determined by single-crystal X-ray diffraction techniques. Whether treated with bromine vapor at 24 "C for 1.5 h or at 80 "C for 24 h, the structure is essentially the same. Both data sets were solved and refined in the cubic space group Pm3m at 21 ( I ) "C (a = 12.118 ( I ) A, R I = 0.066 and R2 = 0.064 with 185 reflections, and a = 12.1 11 (2) A, RI = 0.072 and R2 = 0.074 with 241 reflections, respectively, for which I > 3 o ( I ) ) . A redox reaction has apparently occurred between Co(l1) and Br2 to yield Co(II1) and Br,- ions. The 4 Na+ and 4 Co3+ions per unit cell occup 6-ring sites on 3-fold axes. Each Na+ ion is recessed ca. 0.33 8, into the sodalite unit, whereas each Co3+ion is ca. 0.54 on the large-cavity side of the plane of its 3 coordinating oxygens. Four quite asymmetric Br3- ions each bridge between a Co3+ion and an 8-ring framework oxygen (Co-Br-Br = 116", (Br-Br-Br)- = 138", and Br-Br-O = 177"; the corresponding Co-Br-Br-Br-O distances are 2.59 ( I ) , 2.28 ( I ) , 2.64 (3), and 3.40 (2) A, respectively). The near-linear Br-Br-O angle is indicative of a charge-transfer interaction. Two other bromine molecules similarly form charge-transfer complexes with 8-ring oxygens. Only with Br2, and neither with CI, nor 12, is Co2+oxidized and X< formed.
x
in a bent mannere8 The chlorine molecule is essentially basic with Introduction respect to the hard acid Co(II), and the dichlorine bond is Dihalogens are readily absorbed by zeolites to complex by a lengthened by a large amount, approximately 0.5 A, upon comvariety of mechanisms. In the earliest work, in the crystal structure pIexationa8 of a bromine sorption complex of zeolite 4A (NalZ-A),Iv2 six This work was undertaken to investigate halogen sorption by dibromine molecules were sorbed per unit cell, and they appeared zeolite A further and to determine the positions of sorbed bromine to interact neither with the anionic framework nor with Na+ ions. molecules. The resulting structures show X3- and Co(II1) ions, When Br2 gas was sorbed onto a single crystal of vacuum-dehyboth novel findings, indicating that the mechanism of sorption drated Ag,,-A, 6 bromine molecules are sorbed per unit cell; 3.6 of Brz by CoNa-A is quite different both from that of C12 (coBr2 molecules coordinate to draw 3.6 of the 8 6-ring Ag+ ions into ordination to Coz+) and from that of I2 (charge transfer from a large cavity, and 2.4 Br2 molecules form charge-transfer complexes framework oxygen). with framework oxygens (0-Br-Br = 174 (4)0).3 In the crystal structure of an iodine sorption complex of Experimental Section Ca4Na4-A, the 5.65 diiodine molecules sor'oed per unit cell formed Crystals of synthetic zeolite 4A were prepared by a modification charge-transfer complexes with framework 8-ring oxygen^.^ In of ~ exchange with aqueous 0.1 M Cothe crystal structures of iodine sorption complexes of C O ~ . ~ N ~ ~ - A , ~Charnell's m e t h ~ d . Ion was done by the static method.I0 This yielded material 2.5 iodine molecules per unit cell were sorbed at 70 "C within whose approximate stoichiometry was Co4Na4Si12Al1 2 0 4 8 - ~ H 2 0 30 min and 5 iodine molecules per unit cell a t 80 "C after 24 h. per unit cell, subsequently to be referred to as Co4Na4-A,, exAgain, each iodine molecule makes a close nearly linear approach clusive of water molecules. Two of the largest single crystals from (1-1-0 = 175"), indicative of charge-transfer complexation, to this experiment, each about 0.08 mm along an edge, were selected an 8-ring oxygen. for X-ray diffraction study. Each crystal was placed in a finely I n the crystal structures of chlorine sorption complexes of drawn Pyrex capillary, attached to a vacuum system, and cauEu( I I)-exchanged zeolite A6 and Ag+-exchanged zeolite A,7 tiously dehydrated by gradually incrementing its temperature (ca. chlorine gas is reported to have oxidized Eu(l1) to E u ( I V ) ~and 25 OC/h) to 360 "C at a constant pressure of 2 X IOd Torr. hexasilver to AgCIe7 In the latter structure, an additional 6 Finally, the system was maintained at this state for 48 h. To dichlorine molecules are sorbed per unit cell; these form prepare the bromine complex, the first dehydrated Co4Na4-A charge-transfer complexes with framework oxygens (O-CI-CI = crystal was treated with zeolitically dried Brz vapor (Br, vapor 166 (2)O). When C12 gas was sorbed onto a single crystal of pressure ca. 160 Torr) at 24 "C for 1.5 h, and the second crystal vacuum-dehydrated Co4Na4-A, dichlorine coordinated to Co( 11) at 80 OC (Br, vapor pressure ca. 1400 Torr)'' for 24 h. Each deep blue crystal became black after exposure to Br2 vapor. Finally, each crystal, still in its bromine atmosphere, was sealed in its ( I ) Meier, W. H.; Shoemaker, D. P. Z . Kristallogr., Kristallgeom., Kristallphys.. Kristallchem. 1966, 123, 375-364. capillary by torch. (2) The nomenclature refers to the contents of the unit cell. For example, Na12-A represents Na12SiI2A112048. (3) Kim, Y.; Seff, K. J . fhys. Chem. 1978, 82, 925-929. (4) Seff, K.; Shoemaker, D. P. Acta Crysrallogr. 1967, 22, 162-170. (5) Kim, Y.;Lee, S. H.; Seff, K. Bull. Korean Chem. SOC.1989, IO, 426-430. (6) Firor, R. L.;Seff, K. J . Am. Chem. Soc. 1978, 100, 976-978. (7) Kim, Y.;Seff, K. J . Am. Chem. SOC.1978, 100, 3801-3805.
(8) Subramanian, V.;Seff, K.; Ottersen, T. J . Am. Chem. Soc. 1978, 100, 291 1-1913. (9) Charnell, J. F. J. Cryst. Growth 1971, 8, 291-294. (IO) Riley, P. E.; Seff, K. fnorg. Chem. 1974, 13, 1355-1360. ( I I ) Handbook of Chemistry and Physics, 67th ed.; Chemical Rubber Co.: Cleveland, OH, 1986/1987; p D-193.
0022-3654/9 1/2095-0868%02.50/0 0 199 1 American Chemical Society
Structure of a Bromine Complex of Cobalt Zeolite A
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 869
A1
02 SI 01
I
Y
Figure 1. Stereoview of the unit cell of [ C O " ' . B ~ , - ] ~ [ N ~ + ] ~ S ~ , ~Ellipsoids A ~ ~ ~ Oof~ 20% ~ . ~ probability B~~. are shown.
Figure 2. Stereoview of one corner of the large cavity showing the Br; ion coordinated to Co(lI1) and forming a charge-transfer complex with O(1). Ellipsoids of 20% probability are shown.
The space group Pm3m (no systematic absences) was used throughout this work for reasons discussed previ~usly.l~*'~ Preliminary crystallographic experiments and subsequent data collection were performed with an automated, four-circle Enraf Nonius CAD4 diffractometer equipped with a graphite monochromator and a PDP micro 11/73 computer. Molybdenum radiation was used for all experiments ( K q , X = 0.70930 A; Kaz, X = 0.713 59 A). The unit cell constants determined by leastsquares refinements of 25 intense reflections for which 19' I26 I24' are 12.1 18 ( I ) and 12.1 11 (2) A, respectively. For each crystal, reflections from two intensity-equivalent regions of reciprocal space (hkl, h Ik II and lkh, 1 I k Ih ) were examined by using the w - 26 scan technique. The data were collected by using variable scan speeds; most reflections were observed at the slowest scan speeds, ranging between 0.13' and 0.29' in w/min. The intensities of three reflections in diverse regions of reciprocal space were recorded every 3 h to monitor crystal and instrument stability. Only small, random fluctuations of these check reflections were noted during the course of data collection. For each region of reciprocal space, the intensities of all lattice points for which 26 < 70' were collected. The raw data for each crystal were corrected for Lorentz and polarization effects; the reduced intensities were merged and the resultant estimated standard deviations were assigned to each average reflection by the computer programs PAINTand WEIGHT.I4 A spherical absorption correction ( p R for each crystal ca. 0.40)15 was applied (the calculated transmission coefficients ranged from 0.556 to 0.565) which understandably had little effect on the final error indexes. Of the 846 pairs of reflections, only 185 pairs for the first crystal and 241 pairs for the second for which I > 3 4 0 , respectively, were used in subsequent structure determinations.
Structure Determination Full-matrix least-squares refinement was initiated using the atomic parameters of the framework atoms [@,AI), O(l), 0(2), and 0(3)] and a Co2+position from the iodine sorption complexes of partially Co(I1)-exchanged zeolite A.5 Anisotropic refinement of the framework atoms and isotropic refinement of Co2+at Co( 1) for crystals 1 and 2 converged to R , = C(F,- IF,I)/CF,, = 0.265 and R2 = ( C w ( F , - I F c 1 ) 2 / C w F ~ ) 1=/ 20.353 and R I = 0.252 (12) Seff, K.; Mellum, M. D. J . Phys. Chem. 1984.88, 3560-3563. (13) Seff, K. Acc. Chem. Res. 1976, 9, 121-128. (14) Frenz, B. A,; Okaya, Y . Sfructure DeferminafionPackage, Version I .2.0; Enraf-Nonius: Delft, Holland. 1984. ( I 5 ) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. 11, p 302.
and R2 = 0.375, respectively. From the initial difference Fourier function, Br( l ) , Br(2), and Na( 1 ) were readily located and refined (see Table I). In zeolite A, the charges of the exchangeable cations should sum to 12 per unit cell. The ions at Co(l) and Na(1) are associated with 6-rings and lie on 3-fold axes (see Table I). The number of ions per unit cell at these positions cannot sum to more than 8; otherwise an unacceptably close interionic distance would occur. Nicely at this limit, full-matrix least-squares refinement converged to the occupancies Co( 1) = 4.0 ( 1 ) and Na( 1) = 3.9 (2). The alternative assignment of chemical identities led to ca. 10.0 Na+ and 1.6 cobalt ions on the 3-fold axes of unit cell. This sums to appreciably more than 8 and so is not acceptable. Furthermore, similar positions for Co(1) and Na(1) were found in previous s t r ~ c t u r e s . ~ J ~ The occupancy numbers at Br(1) and Br(2) refined to ca. 6.2 and 5.9 bromine atoms, respectively. These were reset and fixed at 6.0, the maximum number of atoms at each of these positions for packing reasons. It is furthermore reasonable that these two occupancies should be equal because the distance involved indicates that these positions represent dibromine molecules. A subsequent difference Fourier function revealed the bromine position Br( 3). The occupancies at Br(3) and Co(l), and their interatomic distance, indicated that they are associated with each other, so their occupancies were constrained to be equal (see Table I). The O-Br(l)-Br(2) angle is ca. 177', indicating that "Br, molecules" have formed charge-transfer complexes with framework oxygens. Actually, the 6 Br( 1)-Br(2) "molecules" cannot be equivalent because only 4 can associate further with Br species at Br(3). The remaining 2 molecules of dibromine must occupy positions that are at least somewhat different, but this could not be resolved crystallographically. The structures were refined to give the final error indexes R I = 0.066 and R2 = 0.064, and R , = 0.072 and R2 = 0.074, for the two crystals. The final difference Fourier function was featureless except for one peak at (O,O,O) with peak height 1.9 (1 4) e for the first crystal and one at (O,O,O)with peak height 3.5 (13) e A-3 for the second. Atomic scattering factor^'^.'^ for Bro (for Br(1) and Br(2)), Br- (for Br(3)), and (Si,A1)1,75+were used. The Co3+, Na+, 0-, function describing (Si,AI)',75+is the mean of the Sio, Si4+,AIo, and AI3+functions. All scattering factors were modified to account ~~~
~
~
~
(16) Doyle, P. A.; Turner, P. S. Acfa Crystallogr., Sect. A 1968, 24, 390-397. ( 1 7) International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV, pp 73-87.
810
The Journal of Physical Chemistry, Vol. 9.5, No. 2, 1991
TABLE I: Positionrl, Thermal, and 0ccup.ney Parameters' Wyckoff atom position X Y z Crystal 1 (Dehydrated (%AI) 24(k) 0 I816 (3) 3670(3) O(I) 0 1970 (IO) 5000 0 2978 (7) 2978 (7) O(2) 12(i) O(3) 24(m) 1133 (6) 1133 (6) 3244 (7) 8(g) 1690 (20) 1690 (20) 1690 (20) Na(l) 8(g) 2088 (3) 2088 (3) 2088 (3) Co(l) 24(/) 1530 (IO) 4300 (20) 5000 Br(1) 24(/) 2620 (20) 3760 (IO) 9000 Br(2) 8(g) 3329 (7) 3329 (7) 3329 (7) Br(3) Crystal 2 0 1819 (3) 0 I980 ( I 0) 0 2982 (7) 1 1 24 (5) 1124 (5) 1660 (20) 1660 (20) 2096 (3) 2096 (3) 1520 (IO) 4350 (20) 2630 (20) 3830 (IO) 3322 (7) 3322 (7)
(Dehydrated 3671 (3) 5000 2982 (7) 3233 (7) 1660 (20) 2096 (3) 5000 5000 3322 (7)
&Ib
Co4Na4-A 34(2) 80 (IO) 1 IO (IO) 58 (5) 250 (IO) 36 (2) 540 (20) 490 (50) 350 (IO)
Kim et al.
822
833
812
Co4Na4-A Treated with Br2 at 80 22 (2) 18 (2) 19 (2) 100 (IO) 40 (IO) 27 (9) 80 (IO) 32 (6) 32 (6) 61 (5) 57 (8) 61 (5) 199 (9) 199 (9) 199 (9) 37 (2) 37 (2) 37 (2) 660 (20) 1570 (50) 470 (30) 570 (40) 360 (30) 470 (30) 390 (IO) 390 (IO) 390 (IO)
823
occupancyC
6 (5) 0 20 (20) -40(10) 380 (40) 9 (6) 0 0 -150 (20)
24d 12 12 24 4 4 6 6 4
7 (5) 0 40 (20) -30 (IO) 360 (20) 10 (5) 0 0 --140 (20)
24d 12 12 24 4 4 6 6 4
813
Treated with Br2 at 24 OC for 1.5 h) I9 (2) 16(2) 0 0 50 (IO) 40 (IO) 0 0 29 (6) 29 (6) 0 0 58 (5) 61 (9) 70(10) -40(10) 250 (IO) 250 (IO) 380 (40) 380 (40) 36 (2) 36 (2) 9 (6) 9 (6) I010 (40) 470 (30) -1250 (40) 0 360 (30) 460 (30) -130 (60) 0 350 (IO) 350 (IO) -150 (20) -150 (20)
OC for 24 h) 0 0
0 0 0 -30 (IO) 360 (20) 10 (5)
0 70 (IO) 360 (20) 10 (5) -1780 (40) 0 -240 (50) 0 -140 (20) -140(20)
"Positional and anisotropic thermal parameters are given X104. Numbers in parentheses are the esd's in units of the least significant digit given for the corresponding parameter. bThe anisotropic temperature factor = exp[-(dl1h2 + bZ2k2+ b33!2 + 8&k + 8l3h/+ 823kl)l. COccupancy factors given as the number of atoms or ions per unit cell. dOccupancy for (Si) = 12; occupancy for (AI) = 12.
TABLE 11: Selected Interatomic Distances (A) and Angles (deg)" crystal 1 crystal 2 (Si.AI)-O( I ) 1.623 (4) 1.623 (4) (Si,AI)-0(2) 1.639 (8) 1.639 (8) (%,AI)-O( 3) 1.686 (7) 1.688 (7) Na( 1)-O( 3) 2.11 ( I ) 2.12 (2) CO(I )-O( 3) 2.155 (7) 2.164 (7) 2.605 (6) 2.574 (6) Co( I)-Br(3) Br( 3)-Br(2) 2.265 (9) 2.288 (9) Br(2)-Br( 1) 2.69 (3) 2.58 (3) Br( I )-O(I ) 3.38 (2) 3.41 (2) Br( 1)-0(2) 3.47 ( I ) 3.48 ( I )
O(1 )-(Si,Al)-O(2) O(1 )-(Si,Al)-O(3) 0(2)-(Si,Al)-O( 3)
O(3)-(Si,AI)-O( 3) (Si,AI)-O( I )-(%AI) (Si,Al)-O( 2)-(Si,AI) (Si,Al)-O( 3)-(Si,AI) 0(3)-CO( l)-0(3) 0(3)-Na( I )-0(3) 0(3)-Co( l)-Br(3) Co(l )-Br(3)-Br(2) O(1 )-Br( 1 )-Br(2) Br( 1 )-Br(2)-Br(3)
114.3 (6) 1 1 1 . 1 (4) 105.4 (3) 105.7 (3) 167.0 (20) 151.5 (5) 135.0 (5) 114.2 (2) 117.8 (8) 104.3 (2) 115.5 (5) 175.8 (8) 138.0 (8)
1 1 3.8 (5) 1 1 I .9 (4)
105.6 (2) 105.1 (3) 166.3 (9) 151.3 (4) 135.0 (5) 113.4 (2) 117.2 (6) 105.2 (3) 117.2 (6) 178.6 (8) 138.7 (9)
"Numbers in parentheses are estimated standard deviations in the units of the least significant digit given for the corresponding value.
for anomalous dispersion.'*J9 Final positional, thermal, and occupancy parameters are presented in Table I; bond lengths and angles are given in Table 11. Discussion of the Structure In the bromine sorption complex of dehydrated CopNa4-A, 4 Na+ ions at Na( 1) and 4 Co3+ions at Co( 1) occupy 6-ring sites on the 3-fold axes of the unit cell (see Figure I ) . Each of 4 Br3ions bridges between a Co3+ ion and a framework oxygen at O(1) (see Figure 2). Each Na+ ion is recessed ca. 0.33 A into the sodalite unit, whereas each Co3+ ion is ca. 0.54 A on the largecavity side of the (1 1 I ) plane of 3 O(3)'s; this greater displacement of Co3+ indicates a movement toward tetrahedral coordination (see Table 11). Two additional bromine molecules interact with O(1 ) framework oxygens (see Figure I ) . (The mean geometry (18) Cromer, D. T. Acfa Crysfallogr. 1969, 18, 17-23. ( 1 9) Reference 17, pp 149-1 50.
of the two structures is used in this discussion.) The close approaches of the bromine species at Br(3) are to a cobalt ion at Co(1) and to a dibromine molecule at Br(2)-Br(l). This indicates the presence of Br3- ions coordinated to cobalt ions. Considering the occupancies, stoichiometry indicates that all 4 Co2+ ions per unit cell have been oxidized by two dibromine molecules as follows: 4c02+ + 2Br2 4C03+ 4Br-
-
followed by 4Br-
+ 4Br2
+
-
4Br3-
These Br3- ions are asymmetric in bond length (Br(l)-Br(2) = 2.64 A and Br(2)-Br(3) = 2.28 A) and bent (Br(l)-Br(2)-Br(3) = 138'). Br; ions have previously been found to be sometimes symmetric and sometimes asymmetric.20*2' Also the overall length of Isions, which have been studied in most detail, is approximately 0.5 A more than twice the single-bond length.20 In the crystal structure of cesium tribromide, the following geometry is re rted: Br(1)-Br(2) = 2.698 (6) A, Br(2)-Br(3) = 2.440 (6) and Br( l)-Br(2)-Br(3) = 177.5°.22 For comparison, the Br-Br distance in free dibromine is 2.29 A.23 In this work, bromine atoms have been found at two different 24-fold positions. These are interpreted to give two 24-fold molecular positions which are occupied statistically by two different kinds of dibromine molecules: 4 are combined with Br- to give Br3- and 2 remain as Br,. However, no more than six Br2/Br3- molecules can be accommodated at those positions. Otherwise unreasonably short inter-bromine distances would result. The closest approach of the Br( 1) position to the framework is to O(I ) . The Br3- ions and Br2 molecules act as Lewis acids with respect to the lone electron pairs of these O(1) framework oxygens. In support of this interpretation is the near linear, O(l)-Br(l)-Br(2) angle (ca. 177'). The bonding can be understood in terms of weak charge-transfer complexation: the electronegative oxygen at O(1 ) donates an electron pair axially to the vacant 4p CT* antibonding molecular orbital of the acidic terminal bromine atome4 The Br( 1 ) to O(I ) distances (ca. 3.39 (20) Wells, A. F. Sfrucfural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1986; p 395. (21) Cotton, F. A.; Wilkinson, G. Aduanced Inorganic Chemistry, 5th cd.; John Wiley: New York, 1988; p 579. ( 2 2 ) Breneman, G.L.; Willett, R. D. Acfa Crystallogr., Serf. B 1967, 23, 467-47 I . (23) Reference 1 1 , p F-159.
Structure of a Bromine Complex of Cobalt Zeolite A TABLE 111: Deviationspof Atoms (A) from the (111) Plane at O(3) atom crystal I crystal 2 O(2) 0.31 ( 5 ) 0.33 ( 5 ) CO( 1 ) 0.53 (2) 0.56 (2) M I ) -0.31 (17) -0.35 (13) W3) 3.13 ( 5 ) 3.14 (5) " A negative deviation indicates that the atom lies on the same side
of the plane as the origin, that is, in the sodalite cavity.
8,)are close to the sum of the van der Waals radii of Br and 0 (3.35 A).*, Four Co3+ ions at Co( 1) lie near the centers of 6-rings on the 3-fold axes of the unit cell. Each Co3+ ion is bound to the 3 equivalent O(3) framework oxygens of its 6-ring at a distance of 2.16 8,. Each Co3+ ion is also coordinated by Br(3) at a distance of 2.59 A, which is the sum of the ionic radii of Br- and The geometry about Co3+ is tetrahedral, distorted somewhat toward trigonal pyramidal. Each Co3+ ion lies about 0.54 8, on the large-cavity side of the plane of the 3 O(3) atoms to which it is bound (see Table 111). Four Na+ ions at Na( 1 ) lie near the centers of 6-rings on 3-fold axes and are recessed 0.33 8,into the sodalite cavity from the (1 11) plane at O(3). Each Na( I ) ion is trigonally coordinated to 3 O(3) framework oxygens at 2.1 1 A. The 6-rings containing Co3+ cannot actually be equivalent to those containing Na+ as herein reported. The smaller, more highly charged Co3+ion is certain to draw its coordinating oxygens much closer. Yet the data, as usual, were inadequate reliably to resolve O(3) (and other positions that are also likely to suffer to a lesser degree from this distortion) into two components. Accordingly, Co3+ and Na+ are herein presented as bound to 3 averaged O(3) oxygens. The result, then, that Na-0(3) is found to be too short by 0.18 8, (2.1 1 8, compared to sum of radii = 2.29 ,&25) and that Co3+-0(3) is too long by 0.21 8, (2.16 8, as compared to sum of radii = 1.95 A25)is likely to be a consequence of this failure to resolve O(3). The actual distances are likely to be nearly the same as the sum of the corresponding ionic radii. That the virtual discrepancies are opposite, in the sense that they are, and nearly equal, supports this interpretation. Furthermore, they indicate (24) Reference 1 I , p D-188. ( 2 5 ) Reference 1 I , p F-157.
The Journal of Physical Chemistry, Vol. 95, NO. 2, 1991 871 that the radius of the cobalt ion is nearly that of Co3+and farther from that of
Dihalogen Sorption by Co*+-ContainingZeolite A When Iz was sorbed by Co3,,NaS-A,' diiodine molecules formed charge-transfer complexes with O(1) oxygens; Iz did not coordinate to Co(lI), Co(l1) was not oxidized, and I,- did not form. When C12 was sorbed by CO,N~,-A,~dichlorine molecules coordinated to Co2+; Clz did not form charge-transfer complexes with framework oxygens, Co(I1) was not oxidized, and C13- did not form. Here then are two different sorption mechanisms for these two dihalogens. Dibromine displays a third sorption mechanism which includes features of the other two, together with redox reaction. Dichlorine forms the weakest charge-transfer complexes of the three dihalogens studied, and is the hardest Lewis base, so understandably it coordinates to Co(I1) and does not form a charge-transfer complex with a framework oxygen; perhaps the relative instability of Cl,- contributes also to the failure of C12 to oxidize Coz+. Diiodine forms the strongest charge-transfer complexes, has the least electrochemical potential for redox reaction with Co2+, and is a soft Lewis base, not suited for coordination to the hard Co(I1) cation, so it is understandable that it simply forms a charge-transfer complex with a framework oxygen. Dibromine, although it does not have as great an electrochemical potential for redox reaction with Co2+as CI2, does so, perhaps because the product of that reaction, Br-, is readily accepted by Br2 to form Bry, which is readily stabilized further both by coordination to Co(II1) and by charge-transfer interaction with a framework oxygen. Acknowledgment. This work was supported by the Korean Trades Scholarship Foundation, by the Korean Science and Engineering Foundation, and by the US.National Science Foundation (NSF-INT-8711252). Registry No. [CO"'~B~~]~[N~+]~S~,,AI,,O,,~~B~~, 130727-1 1-6. Supplementary Material Available: A listing of observed and calculated structure factors with esd's for the two data sets gathered for [Co"'.Br
