Polyhedron Vol. 12, No. 10, pp. 1149-I 155, 1993 Printed in Great Britain

0277-5387/93 $6.00+.00 Pergamon Press Ltd

THE PHENOMENON OF CONGLOMERATE CRYSTALLIZATION-XXXII. THE CRYSTALLIZATION BEHAVIOUR OF [cis-Co(en),(NO,),]I (I). COUNTER-ION

EFFECTS ON CRYSTALLIZATION PATHWAY OF RACEMATE SOLUTIONS--III

IVAN BERNAL,* JOZEF MYRCZEKPS and JIWEN CAIt

Chemistry Department, University of Houston, Houston, TX 77204-5641, U.S.A.

(Received 10 November 1992 ; accepted 26 January 1993)

Abstract-[Cis-Co(en),(N02)2]I (I), ICo04N6C4H 16, crystallizes from water at 18°C as both a racemate and a conglomerate. The structure of the latter was determined in this study. Unlike its chloride and bromide brethren, which crystallize exclusively as conglomerates, in the space group P2,, the iodide crystallizes in the tetragonal space group P41 (No. 76). The absolute configuration was determined by comparison with that of the parent compound, ( -)S,g-[A(Gil)-cis-Co(en)2(N02)2]Br (III), from which it was made by metathesis. Unlike the chloride and parent bromide, whose chiroptical symbols are [cis-(-)58g-A(8~)- Co(en),(NO,),]Cl (II) and [cis-( -)589 -A(&)-Co(en),(NO,),JBr (III), in the solid state that of [cis-(-)5,g -Co(en),(NO,)9 (I) is A(nn). The cations of I form infinitely hydrogen- bonded, homochiral, helical strings about the four-fold screw axis of the crystals. In turn, adjacent helical strings are held together by hydrogen bonds between the iodides and -NH2 hydrogens of the amine ligands. Such an arrangement is also present in the isomorphous chloride and bromide derivatives ; however, in those compounds the infinite spirals are wound about the two-fold screw axis of the space group P21.

In previous studies l-9 we reported conglomerate crystallization’ O of Werner coordination com- pounds belonging to a class characterized by having chic (from the Latin word for paddle or tiller) ligands, such as -NOz, which can be anchored into specific dissymmetric arrangements by hydrogen bonds to adjacent amine ligands, all of which have a pronounced tendency to crystallize as con- glomerates-mechanical mixtures of pure homo- chiral crystals derived from a solution of a racemate. Examples of compounds of such class, crystallizing as conglomerates and whose structures and absolute configurations have been determined, are the series [cis-c+Co(trien),(N0,)2] * Hz0 (X = Cl,’ I’), the series [cis-Co(en)2(N02)2]X (X = Cl,’ Br3) and [cis-Rh(en)2(N0,)2]C1.4

* Author to whom correspondence should be addressed. t Fellow of the Robert A. Welch Foundation. $ On leave from the Technical University of Wroclaw,

1-5 Wrociaw, Poland.

We have recently explored’ in some detail the effect of the counter-ion on the crystallization path- way selected by a series of ionic species in which either the cation or the anion was retained while varying the nature of the counter-ion, a topic which had been mentioned since the beginning of our stud- ies. ’ The conclusions reached 5 were that (a) in sub- stituted cobalt amine series hydrogen bonding plays an important role in selecting the crystallization pathway, (b) in cis-dinitro cobalt amines strong intramolecular bonds are formed by the axial amine -NH2 hydrogens with the -NO2 oxygens and the presence of these bonds seems to be a necessary condition for conglomerate crystallization, and (c) that counter-anions capable of forming strong hydrogen bonds with the cation -NH, hydrogens can often compete for the formation of hydrogen bonds, and, thus, block the formation of intra- molecular ones. When this happens cations which crystallize as conglomerates with halide counter- ions now crystallize as racemates. However, anions

1149

1150 I. BERNAL et al.

capable of forming strong hydrogen bonds with the metal amine hydrogens can function in different ways depending on the nature of the cation: (1) in a positive manner (conglomerate crystallization results, i.e. in mi(en)JN0,$7), or (2) in a negative fashion (racemates result, e.g. [cis-Co(en)2 (NO&]NO,).* (d) It appea

f s that in the [c&metal

(en),(NO,),] series a sign, of the intramolecular hydrogen-bonded interactions is the fact that the two en rings have opposite ~ torsional chirality (e.g. one is 6, the other one A).~’ For more details the interested reader is dire&e

i’

to our arguments and examples in the published 1 terature.‘-g

With respect to compounds such as I the fol- lowing information was known prior to this study :

Space Compound group Ref.

[cis-Co(en),(NO,),]Cl (II) P2, 1 [cis-Co(en),(N02)2]Br (III)1 P2, 3

[cis-Co(en)z(NO&lN03 (IV) P2,/c 11 [cis-Co(en),(N02)2]N02(V) Cc 8

Thus, it seemed interesti g to document whether or not the pattern noted in the ethylenediamine derivatives could be ob erved with additional charge compensating ani

1

ns. This is particularly interesting for the iodide I) in view of the obser- vation, by Yamanari et al.’ ‘2(a) that II and III crys- tallize as conglomerates, hereas I crystallizes as both a racemate and a con

1

lomerate. It seems then that the iodide (I) provid s a golden opportunity for exploring the differences in intra- and inter- molecular interactions during the course of one or the other crystallization mode. Our results of such exploration are given beloi.

EXPERIMENTAL

Preparation of the compotinds and crystal growth

Rucemic [cis-Co(en),(N02),]I (I). [Cis-Co(en), (NOJ2]Br was prepare as described before. l3 Compound I was obtaine

1

by the addition of solid NaI to a stirred, warm ( O’C), saturated solution of [cis-Co(en),(NO,),]Br until precipitation. We already knew ‘33,12(a) that ) in the cis-dinitro series (both trien and en) the solubility order is Cl > Br > I. The solutio was cooled overnight in a refrigerator and filtered The dried solid was used to prepare a room-tempe

i

ature, saturated solution which was allowed to s owly crystallize at room temperature. Crystals ob ained in this fashion were completely useless for si gle-crystal X-ray diffrac- tion work. We tried for 5 years to obtain useful crystals but without success.

Eventually, we decided to use pure, chiral [cis- Co(en),(N02),]I derived from our previously prepared ( - ) 589-[h@A) -cis-Co(en),(N02)dBr and grow crystals in the dark, since we had observed that [Co(en),oxalato]I decomposes when in solu- tion and exposed to laboratory lights. ’ 3

( ->589-[Cis-Co(en)2(N02)2]I (I) was prepared by metathesis with NaI, as described above, but using ( -)58g-{A(i%)-Cis-CO(en)2(N02)2]Br. Crystals grown in the dark are of varying quality. The worst ones are as bad as those obtained from racemic solutions. However, after several crystallization batches were sampled, one produced a very good crystal, which was used in the current study.

Elemental analyses for I were unnecessary since this is a known compound I’@) prepared from another known compound.‘3’2(a)

Collection of X-ray dzjiiaction abta

All data collection and processing were per- formed with the Molecular Structure Corporation TEXRAY-230 Modification * 4 of the SDP-Plus’ ’ programs. The reader is referred to this manual for the details of the various routines mentioned and for the procedures recommended by them.

Nearly cubic, amber-brown crystals (I) were for- med on slow evaporation of a water solution (ca 18C) of the optically active compound, kept in the dark. The crystal was mounted on a translation head and onto an Enraf-Nonius CAD-4 diffrac- tometer. A set of 25 reflections were centred and used to define the orientation and Niggli16 matrices. The cell is primitive, tetragonal ; the only systematic absences are those for which 011 # 4n, which suggest the space groups P41, P43, P4,22 or P4322 since the compound was known to be a pure enantiomorph.

MO-K, radiation was used throughout. Data for I were collected over the range 4” G 20 < 60”. Two reflections were used as intensity standards in order to monitor crystal orientation and electronic sta- bility. Intensity standards were collected every 2 h and an absorption correction was applied to the structure factors. No decay or systematic variation were observed and details of data collection and processing are listed in Table 1. The scattering curves of Cromer and Waber ” were used through- out.

The phase problem was solved using a Patterson summation, from which the position of I was readily extracted. Heavy atoms were refined aniso- tropically and every five cycles of refinement new positions were calculated for the hydrogen atoms of the cation (C-H = N-H = 0.95 A), added to the list and the structure refined until convergence.

Conglomerate crystallization-XXX11

Table 1. Summary of data collection and processing parameters for (-)ss&~-Co(en)~(NC%lI (I)

Space group Cell constants

a (A) b (A) c (A)

Cell volume (A’) Molecular formula Molecular weight (g mol- ‘) Density (talc; z = 4 mol/cell) (g cm-‘) Radiation employed Absorption coefficient, p (cm- ‘) Transmission coefficients Data collection range Scan width Total data collected Data used in refinement

K = CllFJ -I~cllPl~oI K, = Pw*(lFel - Ir;,l~‘/~lI;,121”2 Weights used

p4,

7.721(3) 7.721(3) 20.087(14) 1197.39 C4H ,6N604C~I 398.05 2.208 MO-K, (1 = 0.71073 A) 39.930 0X874-0.9993 4” < 20 < 60” A0 = 0.90+0.35 tan 0 2036 1574 0.0475 0.0561 w = [a(F,)]- 2

1151

Bond lengths, angles, torsional angles and hydrogen bonds for I are listed in Table 2.

The absolute configuration of (I) was determined by refinement of the coordinates of the two enantio- morphs. The original coordinates (space group P4,) gave the known absolute configuration of the cation of the parent bromide, namely A ; consequently, it was assumed to be the correct one inasmuch as there is no reason to expect inversion at the cobalt

centre ; moreover, the sign of the rotation (-) of the solution of I used to grow the crystals matches that of the parent bromide.

RESULTS

The absolute configuration of the cation is shown in Fig. 1, which displays the contents of the asymmetric unit. As expected from the prepara-

HI2 0

H6

Fig. 1. The contents of the asymmetric unit. Note that the configuration about cobalt is A conformations of N(l)--C(l)-C(2)-N(2) and N(3)--C(3)--C(4)-N(4) are 1 and 1. hydrogen bonds between 0( 1) and H(2) and between O(4) and H( 15) are evident from this

details of the bond lengths see Table 2(D).

while the Also the

view. For

1152 I. BERNAL et al.

Table 2. Bond distances (A) and angles (“) for I

(A) Distances”

Co-N( 1) 1.950 Co-N(2) 2.0141 Co-N(3) 1.981’ Co-N(4) 1.946 Co-N(5) 1.877 co-N(6) 1.930

(B) Angles’

WF-W) 84.32

N(l)-=-N(3) 90.70 N( l j-co-N(4) 173.51

N(l)--Co--N(5) 93.65 N(ljCo-N(6) 92.99 N(2)-&--N(3) 92.47 N(2jCo-N(4) 91.51 N(2)-&-N(5) 90.73 N(2 )---&--N(6) 176.11 N(3jCo-N(4) 84.50

(C) Torsional angles

N(2jCo-N( 1 jC( 1) N(4)-&--N( l)+-C( 1)

N(5)--Co-N(ljC(1) N(6jCo-N( 1 jC( 1)

N(l)--Co-N(2)cC(2) N(3>-Co-N(2)t-C(2) N(4jCo-N(2)+-C(2)

N(5jCo-N(2H-_C(2) N(6>--Co--N(2#--C(2) N(ljCo-N(3jC(3) N(2jCo-N(3jC(3)

N(4t_Co--N(3jC(3) N(5jCo-N(3)-C(3) N(6jCo-N(3)---C(3)

N(lt--Co-N(4jC(4) N(2jCo-N(4jC(4)

N(3jCo--N(4k-C(4) N(5 jCo--N(4jC(4) N(6)-&-N(4jC(4) N(ljCo-N(SjO(1) N(ljCo-N(5ijO(2) N(2jCo-N($-O(l)

(D) Selected list of hydrogen bonds

WjH(2) 2.298 0(4jH(l5) 2.301

0(1)-H(l) 2.267 0(4jH(l6) 2.291

0(3jH(l6) 2.333 0(2)--H(l) 2.364

WjN(5) 1.265 N(3jC(3) 1.505 0(2)-N(5) 1.229 N(4jC(4) 1.437 0(3)-N(6) 1.224 C(ljC(2) 1.540 0(4jN(6) 1.215 C(3jC(4) 1.466 N(l>--C(l) 1.480 N(2jC(2) 1.474

N(3)-&--N(5) 174.84 N(3jCo-N(6) 90.37 N(4jCe-N(5) 91.37 N(4)-&--N(6) 91.41 N(5)-&--N(6) 86.63

---WI--W) 109.65

(3-W9-4(2) 110.09 Co-N(3)-C(3) 108.71

Co--N(4)--c(4) 110.02

CO-N(~)--(W) 121.87

Co-N(5)--0(2) 122.13

o(ljN(5)--0(2) 115.95 Co-N(6)-O(3) 117.92 Co-N(6>-0(4) 121.98

0(3jN(6W(4) 119.93

N(lt_C(l)--C(2) 105.25

N(2jC(2F--W) 106.84

N(3)--+3)--C(4) 108.69

N(4)--C(4)--C(3) 107.33

-22.7 N2)--Co-N9---0(2) 121.0 27.5 N(3)-&--N(5j0(2) -7.3

-113.1 Wb-Q+-NW-W) - 153.1 160.1 NW-Co--NW--W) 29.4 -7.1 WC--Co--NVW(l) 115.6

-97.6 N(6 j-co-N(5 jO(2) -61.9 177.8 N(1 j-co-N(6 jO(3) 28.1 86.5 N(l j-co-N(6 jO(4) - 147.0 39.2 N(2jCo--N(6W(3) -18.0

179.4 N(2)--Co--N(6jW4) 166.8 -96.3 N(3 j-co-N(6 jO(3) 118.9

-5.0 N(3 j-co-N(6 jO(4) - 56.3 32.0 N(4)--Co--N(6j-0(3) - 156.6 86.4 N(4)--Co--N(6jW4) 28.2 20.5 N(5 j-co-N(6 jO(3) -65.3 70.4 N(5)--Co--N(6jW4) 119.5

-21.9 Co-N(1 jC(l j-c(2) 46.2 161.2 Co-N(2)--c(2)--c(l) 33.8

-112.2 Co--N(3)--c(3)--c(4) 30.4 22.8 Co-N(4 jC(4 >-C(3) 44.5

- 154.7 N(l>-C(l)--C(2jN(2) -51.6 -61.6 N(3jC(3)-+4jN(4) -48.6

N(ljH(2). . . O(1) 117.6 N(4 jH(15). . . O(4) 117.1 N(l)--H(l)...O(l) 145.5 N(1) at 1 -y, x, l/4+2 N(4)--H( 16) . . . O(4) 143.4 N(4) at y, -x, z-l/4 N(4jH(16). . . O(3) 141.4 N(4) at y, -x, z-1/4 N(ljH(1) . . . O(2) 138.6 N(1) at 1 -y, x, l/4+2

n Estimated errors in distances range from 0.005 to 0.01 A. ‘Estimated errors in an es range from 0.02 to 0.05”. No e.s.d.s are shown sin hydrogen atoms were not refined. Numbers in estimated standard deviations in the least significant digits.

Conglomerate crystallization-XXX11 1153

tion the cation has the configuration A ; however, the helical chiralities of the two en rings are ,l[<N(l)-C(l)-C(2)-N(2) = -51.67 and A[< N(3)-C(3)-C(4)-N(4) = -48.6”], which is an interesting solid state result since that of the bromide III, from which I was prepared, was h(U), implying that intra- or intermolecular inter- actions, or both, are responsible for a change which increases the conformational energy of the system. la

The packing of the molecules in the unit cell is shown in Fig. 2, which illustrates the fact that the cations form infinite spiral strings, propagating along the four-fold screw axis, and held together by hydrogen bonds between the -NO, oxygens and the --NH2 hydrogens, typical of which are those listed in Table 3(D). The spiral strings are made up of homochiral cations inasmuch as we used a resolved species.

In I the helical strings are, in turn, held together by hydrogen bonds between the iodide and hydro- gens of the --NH1 moieties. The strings have the same helical chirality and, in that regard, resemble the helical strings of a polypeptide derived from homochiral amino acids.

Fig. 2. A portion of the unit cell. Note the infinite spirals of hydrogen-bonded cations into pockets of which fit the

iodide anions which link adjacent spiral strings.

Table 3. Torsional angles for [cis-Co(en),(NOJ jC1, bromide and iodide (I)

Chiroptical symbol Chloride Bromide Iodide”

A(dl) WA) WA)

N(l)--Co-N(5FW) - 137.3 - 135.0 - 154.7 N( l)-Co-N(5)-0(2) 39.9 44.9 22.8 N( 1 )-Co-N(6)--O(3) - 165.5 - 169.2 - 147.0

N( 1 FQ--N(6)--O(4) 15.86 12.6 28.1

N(4Fo-N(5)--0(1) 41.63 42.8 29.4

N(4)_-Co---N(5FW2) - 141.2 - 137.3 - 153.1 N(4)--Co-N(6)-O(3) 16.41 13.4 28.2

N(4Fo-N(6)_0(3) - 162.3 - 164.8 - 156.6

“Angle labels were rearranged to correspond to those of the lower halides, since in the labelling for the -NO, oxygens 0( 1) and O(2) and O(3) and O(4) for the iodide were reversed compared with the labels for the other two.

DISCUSSION

Racemic [cis-Co(en),(N0,)2]I (I) crystallizes either as a racemate’2’a) or in the enantiomorphic space group P4 1, whose contents (z = 4) are homo- chiral, as shown above. Thus far, we have not been able to isolate X-ray quality crystals of the ra- cemate, and, as stated above, the enantiomorphic ones were obtained from pre-resolved material. However, we know that they should be the same as those precipitating during conglomerate cry- stallization since Aoki et al. I9 studied the structure of resolved ( -)5,g-[cis-[A(U)-Co(en),oxalato] Br * HZ0 while wescb) determined its structure using crystals from a conglomerate crystallization. The results are identical. The interested reader is referred to our previous paper on this subject.S(b)

It is noteworthy that the iodide (I) is not iso- morphous with its chloride (II) or bromide (III) analogues, which is in line with the observation that it is capable of crystallizing as both a racemate and a conglomerate ‘2(a) from the same racemic solution, while II and III crystallize as conglomerates. The same observations apply to the oxalato series, [cis- Co(en),Ox]X * nHzO (X = Cl, n = 4 ; X = Br, n = 1; X = I, n = 0). in which the chloride and bromide crystallize as conglomerates, while the iodide crystallizes as both.5(b),‘2(b),(c) In that series

one could argue that space group changes would not be surprising inasmuch as the degree of hydra- tion varies ; however, such is not the case with the current series. Finally, in the oxalato series similar difficulties arise from the quality of the crystals of the iodide salt, as we have demonstrated earlier.‘cb)

A pertinent fact is that while I winds its cations on a four-fold spiral screw, II and III crystallize in

1154 I. BERNAL et al.

the space group F2, and their cations pack in helical strings also, but the order of the screw axis is two. However, they all wrap themselves about a screw axis, in a helical arrangement internally held by -NOZ..- H,N- hydrogen bonds. In turn, strings of the same helical chirality are held together in a three-dimensional array ~ by hydrogen bonds between the counter-anion and the -NH2 hydro- gens of the cations. This

1

ature is shared by the three homochiral crystallin forms of I, II and III.

The following observati ns concerning the cry- stallization characteristics of the three halides of the [cis-[Co(en)2(N02)2]f cation are useful to under- stand the reasons for their crystallization behav- iour :

(1) The ionic radii of and I- are, respectively,2 d”

‘ix-coordinate Cl-, Br- 1.67, 1.82 and 2.06 A.

Therefore, their respective /volumes are 19.41,25.25 and 36.62 8L3. The differ ces between I and III, between I and II and betw en III and II are, respec-

$ tively, 17.11, 11.37 and 5. 4 A’. The experimental volume per molecule of I is 299.35 A’. The chloride (II) is characterized by molecular volume of 288.17 A3, while the mol ular volume of the bro- mide (III) is 292.67 A’. he experimental volume differences between I and III and between I and II are, respectively, 6.68 an 11.18 A’ ; the difference in volume between II a d III is 4.50 A’. These differences are too small to be due to the radius differences of the hali

i

counter-anion, which means expansion of the la tices is less than expected from a simple, linear, add tive model. Here we note that (see Fig. 2) the anio s lie in cages formed by the cationic strings, whit it helps stitch together. The same feature has bee

.;

observed5’b) in the pack- ing of II and III, despi the difference in space groups. Apparently the c loride fits well in the cage, which needs to expand only slightly to accom- modate the larger bro ‘de anion. This is not a trivial observation since i as in the series [Co(en), XIX, with X = Cl, Br and I, the radius of the anio

!

the current series, as well

plays a crucial role in the crystallization pathway. himura et al. have shown, from phase diagram is-dinitro”@) and the oxalato chlorides and des crystallize as con- glomerates, while the i can crystallize as both a conglomerate and a emate.‘2’b)~(c) In separate papers of ours,3,13 we ve shown that the reason for this behaviour is the cations form infinite hydrogen-bonded h 1 strings which extend throughout the length the crystal. Between these strings are cavities in ich the anions and the waters of crystallizati ; in turn, they link the strings by hydrogen s. The iodide anion is apparently large enou to force the cations apart

and disrupts, or otherwise modifies, the spiral formation. Furthermore, given the fact12 that the iodide derivative crystallizes as a racemate as well as a conglomerate, it appears that this anion loses the ability to differentiate between joining homochiral strings vs joining heterochiral strings, which is prob- ably due to (a) the formation of much weaker hydrogen bonds than the chloride and bromide anions due to its inherently lower basicity and (b) its larger radius, which mechanically disrupts the cages formed by the cationic spiral strings present in the lower halides, and the result is apathy to the form of packing of the spiral strings. (c) Finally, when forced to form an enantiomorphic lattice (as happened here) the pitch of the screw axis is changed from 2, to 4, in order to better accom- modate the bulkier iodide anion.

(2) The conformation and configuration of the cation in I is A(U) ; e.g. given the conformational symbol A both en rings have been coerced into the highest energy conformation (22) in order to form stronger intramolecular hydrogen bonds between the -NO2 oxygen and the -NH2 hydrogens now that in I there is a weaker hydrogen bonding net- work between cations and anions (see Figs 1 and 2). We have pointed out before**2’-23 that for a A(&?)-[cis-Co(en),(NO,)J+ cation, if through tor- sional motion about the Co-NO2 bond one -NO2 orients itself at the most advantageous position for hydrogen bonding with a 6 ring, it is impossible for the second -NO2 ligand to achieve an equally effective hydrogen bonding with another 6 ring while a proper interligand 0 * . * 0 distance (ca 3.1 A) is achieved. However, if the second en ring acquires the I conformation, a much more effec- tive intramolecular hydrogen bonding scheme is achieved by the second -NO2 ligand. This appears to be the reason for the occurrence of h(U), A(M), A(U) and A(66) [cis-Co(en)2(NOz)z]+ cations in the crystalline state. This phenomenon can best be dem- onstrated by comparison of the torsional angles defined by (axial nitrogen)-Co-(NO2 nitro- gen)--(NO, oxygen), as listed below (Table 3). For more details of these arguments, the reader is referred to the original papers.‘*2’-23

(3) The energy associated with en ring inversion has been recently measured24 to be 24.7&4.5 KJ mall ’ in the anion [Fe(CN)4(en)]-, implying that the intramolecular hydrogen bonds present in the cation must exceed that barrier in order to be effec- tive. Thus, we have an appropriate value of the strength of these hydrogen bonds.

(4) Finally, we should point out that the helical nature of the strings is not the result of packing forces in crystals belonging to space groups con- taining such symmetry elements. We know this

Conglomerate crystallization-XXX11 1155

is so because such strings are also found in com- pounds crystallizing in the space groups PI and Pi, where no such symmetry element is found. The point will be discussed in a separate paper” on the structure of K[Co(edda)(NO,)d and K[Co(edda) (NO,),] -Hz0 (edda = ethylenediamine-diacetic

acid), which crystallize, respectively, in space groups P2,2,2, and Pi. Such helical strings are found in both ; however, in the latter compound pairs of adjacent strings are of opposite helical chirality, while the anionic members of each string are homochiral.

5. (a) I. Bernal and J. Cetrulo, J. Coord. Chem. 1989,

CONCLUSIONS

We have previously’,21-23 emphasized that in order for cations of the two series [cis-Co(en), (NO3JX to crystallize as conglomerates the counter-anions (and/or waters of crystallization) must not compete for the hydrogen bonds of the -NH2 ligands. In the conglomerate halide series the anion is always hydrogen-bonded to the equa- torial -NH2 hydrogens and leaves the -NO2 oxy- gens free to form relatively strong hydrogen bonds with axial hydrogens and with the hydrogens of adjacent cations within that string, thus increasing the rigidity of the molecule and destroying the potential two-fold axis which these cations could have if the -NO2 ligands were properly oriented. We find the current results gratifying since the con- sistency in crystallization mode indicates our suggestions for the origin of conglomerate cry- stallization are reasonable and consistent with experimental facts obtained with a wide range of compounds of the cis-dinitro and oxalato series.

Supplementary material available. Compound I : anisotropic thermal parameters (1 page), structure factor tables (18 pages). The latter are available in PC diskette form from Ivan Bernal.

Acknowledgements-We thank the Robert A. Welch Foundation for a research grant (E-594 to IB) and for Fellowships to Jozef Myrczek and Jiwen Cai. The National Science Foundation provided the funds for purchasing the X-ray diffractometer.

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[Co(trien)(NO,)zJNO, and [Co(en),(NO,),JBr- Clearfield Symposium.

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