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the action of light, Co(III) nitropentammoniate oxalate crystals decompose. ... described earlier for the preparation of lead [19, 20] and silver [21] oxalate single ...

Journal of Structural Chemistry, Iiol 39, No. 3, 1998






P. B. Novikov, D. Yu. Naumov, E. V. Boldyreva, and N. V. Podberezskaya

UDC 541.17 + 541.49 + 548.735 + 548.78

The structure of [Co(NH3)sN02]C20 4 is solved and refined (space group I m m m , a = 7.428(2), b = 9.790(3), c = 6.568(1) ~, V = 47Z 6( 2) A 3, Z = 2; R 1 = 0.0177, wR 2 = O.0279 for F 2 > 4cr(F 2); R1 = 0.1177, wR 2 = 0.0643 for all data; residual electron density from 0.125 to -0.140 e/~4 3). Specific contacts in the structure are analyzed. Crystal chemical interpretation is suggested to explain the occurrence o f photodecomposition rather than photochemical bond isomerization under the action of light in cobalt(Ill) nitropentammoniate oxalate crystals, in contrast to all previously investigated cobalt(HI) nitropentammoniates.

INTRODUCTION Co(III) nitropentammoniates are interesting in the action of visible (blue region) and UV radiation on the crystals of their anion salts (chloride, bromide, iodide, nitrate, chloride nitrate), leading to a reversible intramolecular nitro-nitrito isomerization [1-9]. In this reaction, the nitro group is coordinated to cobalt via oxygen (nitrito coordination) but not via nitrogen (nitro coordination). In solutions, photodecomposition of the complex occurs along with photoisomerization [6, 10, 11]. In crystals, isomerization is a homogeneous process, forming a continuous series of solid solutions of the two isomer forms [3, 4, 12]. Although this is an inner-spheric reaction, its rate may be affected by the closest crystalline surroundings [13]. Earlier, a comparative kinetic study was performed for solid state isomerization in the series chloride-bromide-iodide-nitrate-chloride nitrate; it was shown that the reaction rate depends on the anion [2, 14, 15]. The dependence of the rate of the solid state nitrito--nitro isomerization on the inner-spheric anion was also examined in [7]. It was shown that the isomerization rate differs between various polymorphous modifications [14]. Unlike other nitropentammoniates studied previously, which undergo nitro-nitrito bond isomerization under the action of light, Co(III) nitropentammoniate oxalate crystals decompose. This is evident from changes in the IR spectra of the nitro isomer after UV irradiation of the sample (Fig. 1). In principle, the nitrito isomer of cobalt(III) pentammoniate oxalate may be obtained by direct precipitation from a cool weakly acid solution containing cobalt(III) carbonatopentammoniate nitrate by adding sodium nitrite, as is usually done to prepare all other known nitrito isomers of cobalt(III) pentammoniates [16]. The question naturally arises as to why light-induced nitro--nitrito bond isomerization occurs in the previously studied crystals and solutions but not in the cobalt(III) nitropentammoniate crystals, whereas photodecomposition takes place in the latter crystals and former solutions but not in the former crystals. As stated in the literature, the possibility of photochemical reactions in molecular crystals often depends not only on the properties (such as absorption spectrum) of individual molecules but also on the environment of the molecule in a crystal. Examples were given of two chemically identical but crystallographically independent molecules

Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences. Novosibirsk State University. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Struktumoi Khimii, Vol. 39, No. 3, pp. 412-423, May-June, 1998. Original article submitted July 25, 1997. 0022-4766/98/3903-0333520.00 ©1998 Plenum Publishing Corporation



J vAs(NO2)Ia


-, "



0.8J b >, 0.4]~VAs(ONO) ~,.,.,VAs(ONO) 0.0 -- 0.8 0.4 0.0 ' !



d 0.8J 0.4 0,01 . . . . ~





V t C113 - 1

Fig. 1. IR absorption spectra of cobalt(III) nitropentammoniate samples: a) bromide before UV irradiation; b) bromide after UV irradiation; c) oxalate before UV irradiation; d) oxalate after UV irradiation.

with different crystal surroundings, in which only one molecule undergoes tight-induced transformation while the other remains inert [17, 18]. A comparison of the optical absorption spectra of sofid [Co(NH3)5NO2]C204 with those of other nitropentzmmoniates (e.g., [Co(NH3)sNO2]Br2) fails to explain whyphotoisomerization does not take place in oxalates, at least by absorption of fight in the blue part of the spectrum, in the region of d - d transitions, where all cobalt(Ill) nitropentzmmoniates absorb similarly. Photodecomposition of solid cobalt nitrozmmoniate oxalate may be promoted by the special properties of the oxalate anion compared to halide ions or to the nitrate ion studied earlier. These properties alone, however, are insufficient to provide photodecomposition in crystals. An effective phototransfer of an electron from anion to cation requires a favorable mutual arrangement of the anion and cation in the structure. Thus the reason for the specific behavior of crystalline cobalt(III) nitropentammoniate oxalate under irradiation may be sought in peculiarities of the environment of the complex cation in a crystal structure. To verify this hypothesis, we grew single crystals of [Co(NH3)5NO2]C204, determined their crystal structure by the single crystal X-ray diffraction method, and carried out a crystal chemical analysis. EXPERIMENTAL Cobalt(HI) nitropentammoniate oxalate crystals have poor solubility in water. The crystals were grown using the diffusion method, described earlier for the preparation of lead [19, 20] and silver [21] oxalate single crystals. Solutions of cobalt(III) nitropentammoniate nitrate (0.1 M) and sodium oxalate (0.1 M) were slowly fed into a reactor 334

filled with water where cobalt(Ill) nitropentammoniate crystals were gown. The time of one diffusion synthesis was several weeks. Cobalt(III) nitropentammoniate nitrate was synthesized from cobalt(III) carbonatopentammoniate nitrate as described in [16]. Cobalt(III) carbonatopentammoniate nitrate was synthesized from cobalt(II) nitrate, ammonium carbonate, and ammonia as described in [22]. The composition of the samples synthesized was monitored by the IR spectra and powder diffractograms. [Co(NH3)5NO2]C204 resulted as thin needles yellowish brown in color and typically 0.210.01x0.01 mm in size. Despite the small size and unfavorable habit, we managed to perform single crystal X-ray analysis for one of the crystals. For a representativity test, we also recorded a powder diffractogram of the whole sample; this diffractogram was in fair agreement with the one calculated from single crystal data. Data were collected on an Euraf-Nonius CAD-4 four-circle automatic diffractometer at room temperature [MOKa, 2 = 0.71073 /~; 0 / 20 scan mode for 20 from 6.88 to 49.02*; - 8 -< h 4cr(F 2) and R 1 = 0.1177, wR2 = 0.0643 for all data collected. The weighting scheme was w = 1/o2(F2). Residual electron density Was from 0.125 to -0.140 e/~ 3. Table 1 lists the atomic coordinates and isotropic displacement parameters of nonhydrogen atoms; Table 2 gives anisotropic displacement parameters. Figure 2 shows the atomic numbering and connectivity scheme as well as selected bond lengths and angles for the complex cation and oxalate anion. The type of crystal lattice was unambiguously established from the intensity ratio of nonzero reflections, although few of these were measured because of the extremely small size of the single crystal and the difficulty of obtaining larger crystals of good quality. The absence of secondary extinctions, defining the glide planes in the/-lattice, did not allow us to unambiguously choose the space group from Immm, Imm2,1222. In space group Immm (Rf = 1.8%), the cation of the complex is disordered at the site of the NO 2 group due to the presence of a mirror plane m, which is perpendicular to the X axis through the NO 2 nitrogen atom and the cobalt atom. The fact that this is a mononitro rather than dinitro complex follows from the cation:anion ratio, which is 1:1. Further calculations with space groups I2mm and/222 gave considerably increased R factors (6 and 10%, respectively). Due to statistical disorientation of the cations and few measured Ihld, we failed to reliably locate the hydrogen atoms and to obtain more exact Co-NO 2 and Co-trans-NH 3 bond lengths and geometry of the NO 2 group. We tried to eliminate the disorder by using a model suggesting twinning, which is not detectable with a polarizing microscope. However, this considerably worsened the results of the refinement (gave increased R factors and errors of bond length and bond angle determination and less

TABLE 1. Coordinates and Isotropic Thermal Parameters (ilk2)< 103) of Independent Atoms in

ICo(NH3)5NO2]C204 (× 104) Atom Col NI* N2 N4* O1 C1 02

X 0 -2622(8) 0 2622(8) 3721(8) 3945(9) 3208(4)




0 0

42(1) 45(2) 46(1) 45(2) 69(3) 41(2) 67(1)

Y 0 0 1401(4)




1070(6) 0 1129(3)



TABLE 2. Anisotropic Thermal Parameters (/~,21103) of Atoms in [Co(NH3)5NO2]C204

*N1 (from trans-NH3) could not be separated from N4 (from the NO 2 ligand) because of disordering.

Col NI* N2 N4* O1 C1 02

U11 U22 72(1) 24(1) 51(4) :56(4)


18(1) 18(4) 85(3) 18(2) 24(2) 51(4) 156(4) 180) 46(4) i 28(4) 134(7) 46(5) i 33(4) 44(2) 3 5 ( 2 )

45(5) 121(3)



0 0 0(2)

0 0


0 0 0 0

0 0 0


U12 0 0 0 0 -8(5) 0


*N1 (from trans-NH3) could not be separated from N4 (from the NO 2 ligand) because of disordering.


01a ~,i04.1(7]/01 y ""1.328(7)


L-,gO - ""

! ~'~ N2, 1.960(4) 1~1.948(61







Fig. 2. Atomic numbering and connectivity in the complex cation [Co(NH3)5NO2] 2+ and oxalate anion C2042-. Bond lengths are in angstr6ms, bond angles in degrees.

realistic values of these parameters). Should a larger single crystal be grown, the accuracy of bond length and bond angle determination might be improved. Then one would expect to obtain more reliable information on disordering of complex cations in the structure and more exact geometrical parameters. The relative arrangement of the oxalate ions and cations of the complex would not change even if the space group and atomic coordinates were re-refined. DISCUSSION OF THE STRUCTURE The crystal structure of [Co(NH3)5NO2]C204 is closest cubic packing of complex cations in the unit cell obtained from the initial cell via the matrix (1, 0, 1; 0, 1, 0; -1, 0, 1). The oxalate anions occupy all octahedral voids. Thus the structure is of NaC1 type. It is slightly closer-packed than the structures of other cobalt(IH) nitropentammoniates studied earlier. The packing coefficient (calculated with the KPACK program [25]) is 0.74. For comparison, the packing coefficients of the iodide (orthorhombic modification [26]), chloride nitrate [27], chloride [28], and bromide [28] are 0.67, 0.68, 0.70, and 0.72 [29], respectively. The environment of the complex cation in the structure of [Co(NH3)sNO2]C204 is depicted in Fig. 3. Projections of the structure on the plane of the NO 2 group and on the perpendicular plane are shown. Designations of atoms are given in accordance with the symmetry codes of Table 3. Selected distances defining the contacts of the complex cation with the environment in the structure are listed in Table 4. For comparison, Fig. 4 shows the surroundings of the complex cation in isostructural nitropentammoniates: chloride (a), bromide (b), chloride nitrate (c), and iodide in the orthorhombic modification (d) [29]. Table 5 lists, also for comparison, selected contacts of the complex cation with the surroundings in the crystal structures of [Co(NH3)sNO2]XY [XY = C12, Br2, I2, CI(NO3) , (NO3)2] [29]. In all structures whose fragments are shown in Fig. 4, the halogen atoms lie just above and below the nitro nitrogen, the nitro oxygens form long hydrogen bonds with the c/s-NH3 ligands of the neighboring cations, t r a n s - N H 3 form hydrogen bonds with the anions, and c i s - N H 3 are hydrogen-bonded with the oxygen atoms of the nitro groups of the adjacent cations. The environment of the complex cation in [Co(NH3)sNO2]C204 is denser than that in the other salts of the series; the contacts with the neighboring atoms, in particular, with atoms involved in hydrogen bonds, are shorter. The arrangement of the oxalate anions relative to the nitro group is such that there is far less "empty space" near the nitro 336




~ O2g/3.284 "W~~'~ -CLc/3.284 /q~O2b"3-284 ~ Try1 ~t O2e/'3-284~91~ N 2 c / 1 . 8 8 3 ~ C1b/3.284 ~ Cla/-3.284 _ t-~'~N2d/1.883 N2f/.l.SS~ -'° 0 1 ~ ~ 1 0 °'~2g/.1.883 --



Clg/0 ~

O2a/.3.284",--v/~-~ v--/O2/-3.284 Cl/-3.284/ "~N4/0 N2c/"1.40 . ~ i ~ o10' ~ ~ N'N~2/'1"401~ ff~l







O2j/O(,~>/ O2n/-3.284~ O2r/3.284~ C1k/3.284 O2m/-3.284t7~



h~O2h/0 2 o l - 3 . 2 8 4

02s/3.284 Cli/-3.284

l .



01cl~,~ ~.,AOIbl 0 O2q/3.284~/'~ ~ ~7102p/3.284 Cij/3.284 44a/0


b Nla o


Clbf~ O2d~ O2cv

O2c 02b t Cla



Ol N 42 l








~} N4a



Fig. 3. Environment of the complex cation in the structure of [Co(NH3)5NO2]C20 4. Projections on the plane of the NO 2 group (a) and on the plane perpendicular to it (b). Only one of the two possible orientations of the complex cation is shown. Symmetry operations used to determine the atoms are given in Table 3. group than in the structures of the other ammoniates studied previously. The oxalate anions "block" the nitro group, hindering its rotation and preventing it from changing its coordination to cobalt. One can assume that another hindrance is the greater "rigidity" of the structure relative to the structures of the other zmmoniates, preventing a structure distortion that could provide rotation of the nitro group, which takes place in the other nitroammoniates of the series. 337

TABLE 3. Symmetry Operations Used to Obtain the Atoms Surrounding the Cation in [Co(NH3)sNO2]C20 4 and Distances from the Atoms to the Plane through the Nitro Group (O1, O l a , N4 atoms) Atom

Symmetry operation


Symmetry operation

Col N1 Nla N2 N2a N2b N2c N2d N2e N2f N2g N4 N4a 01 Ola Olb Olc C1 Cla Clb Clc Cld Cle Clf Clg

x, y, z x, y, z x + l , -y, z x, y, z -x, y, -z x, -y, - z -x, -y, z 0.5 - x , 0.5 - y , z - 0.5 x + 0.5, y - 0.5, z - 0.5 0.5 - x , y - 0.5, 0.5 - z x+0.5, 0.5-y, 0 . 5 - z x, y, z x - 1, y, z x, y, z x, -y, -z x - 1, y, z x - 1, -y, -z x, y, z

Clh Cli

x - 1, y, z -x, -y, z x - 1, y, z - 1 -x, -y, z - 1 x, y, z x, -y, 1 - z


-y, z

x, y, z - 1 1 - x , -y, z - 1 x - 0.5, y + 0.5, z 0.5 - x , 0.5 - y , z x - 0.5, y - 0.5, z 0 . 5 - x , --0.5-y, z -

clj Clk 02 02a 02b 02c 02d 02e O2f O2g O2h O2i

o2j O2k O21 O2m O2n 020

O2p 0.5 0.5 0.5 0.5

O2q O2r O2s

TABLE 4. Selected Contacts* of the Complex Cation with Crystal Surroundings in


1 - x, y, - z

X-0.5, 0.5-y, 0.5-z 0.5 - x , 0.5 - y , Z - 0.5 0.5 - x , y - 0.5, 0.5 - z 0.5 - x, y - 0.5, 0.5 - Z x - 1, y, z x - 1, -y, 1 - z -x, -y, z -x, y, 1 - z x - 1, y, z - 1 x - 1, -y, - z -x, -y, z - 1 -x, y, -z


Surroundings of NO 2 group Contact




O (NO2)-N (trans-NH3) O1-Nla (Ola-Nla)


O (NO2)-N (cis-NH3) O1-N2d (O1-N2g)


O (NO2)-O (oxalate ion) O1-O2i O 1 - O 2 (O1-O2d) O 1 - O 2 a (O1-O2e)

3.095(7) 3.307(1) 3.945(4)

Hydrogen bonds of ammonia Contact


N (cis-NH3)-O (NO 2 ligand) N2d-O1 (N2g-O1)


N (trans-NH3)-O (NO 2 ligand) Nla-O1 (Nla-Ola)

N (cis-NH3)-O (oxalate N 2 - O 2 (N2-O2o) N2a---O2d (N2a-O2s) N2--O2i (N2-O2h) N2a-O2i (N2a-O2h) 338

-y, z

1 - x , y, 1 - z x, y, z - 1 x, -y, - z 1 - x , -y, Z - 1

2.912(8) ion) 3.049(3) 3.049(3) 3.096(4) 3.096(4)

TABLE 4 (Continued) 1


N (trans-NH3)-O (oxalate ion)

N1-O2s (N1-O2r) N1-O2o (N1-O2n) N1-O2j (N1-O2h)

3.492(1) 3.492(1) 3.840(4)

*Designations of atoms as in Fig. 3 and Table 3.




~N2c/-2.087 N3c/2.294~

2b/2"087 b/-2.294

ola/0C~' ' ~ C I O1/0 Ie/3.739 @ CiId/-0.067 CI1ff-3.739~ N3a/-1.511 (D Oled-3.805 o~.8o50

C11g/0.067~ v





Old/-3.317C) Cllb/[email protected]


2/1.230 1b/3.317



CI1a/-3.383~ O C I 1c/3.383 Nla~


y Ol.c/3.9130

Old/'3"425O Brlb/0.453~


N2a/-{.261{ N3a/-L496 Oo1e/-3.913

•i•• N1/0

~ ~D BrI~'3"46~OBr1c/3.460 Fig. 4. (Be~nnlng)

2/1-261 (~B1b/3"425 ri/-0.452



N2b/2.248& N2e/-2.248(~,YO 04100


N3c/.2.282~ N3b/2.282

o,,0c O3/1.078(~O3a/- 1.078 N4/0 0~a/-3.655 N2~-1.4o6 ~,., , , . , " ~ , . . . 0 alb,O / ",~--"" '-'uo2b,3.655 X,


/ flZ

/ ,, r


NI/0 ] 1~34b/-3.655 Olb/-3.6550 o ~ x r"~O4a/3655 Clla/O~ (-~ ~ " ('~O3d/-1.078 Ola/3 655"-' "k,-)O3e/1.078 " O3e/-2.573C) N5a/09 O3b/2.577(~ (~O4c/0 N2b/0.534


O~ rg[~ NSb/-1.441 N4/0

ll a/-1.453O X Olb/2.0000

O2/431~ ~Tll OIl0 I1/4. Ilbt-4.276 O O Ilc/1"734 N31-1.802 Ola/-1.453 880~ ~o1/0.107~ Y N5a/0. ~51-I.101 ~-N2/1.590



N1/0,208 0- x.~ilf,-3772 Oilg/0.402

O I1¢/3.226 Fig. 4. Environment of the complex cation in the structures of cobalt(III) nitropentammoniates: chloride (a), bromide (b), chloride nitrate (c), iodide (d). Projections on the plane of the NO 2 group. Atomic distances to the projection plane are given after a slash.

At the same time, the mutual arrangement of the oxalate anions and complex cations seems to be favorable for a photochemical redox reaction due to an electron transfer from anion to cation. Photochemical decomposition of the complex compounds of transition metals, including eobalt(III) compounds, in which the oxalate ion is an inner-spheric ligand, is well known from the literature [30, 31]. In respect of the effects preventing photoisomerization and providing photodecomposition in 340

TABLE 5. Selected Contacts of the Complex Cation with Surroundings in the Crystal Structures of [Co(NH3)sNO2]XY [XY = CIz, Br2, I2, CI(NO3) ] (A) [16]


[Co(NH3)sNO2IXY 12 (in Pnma) Br2

CI(NO 3)

Contacts of nitro group with Hal anions N-Hal,/~ O-Hal,/~

with cis-ligands O-N,

H-O, A N-H-O angles, deg

3.776 3.816 4.098

3.917 3.949 4.234

4.459 3.510 5.414

3.661 3.814 3.838

2.973 3.089 2.526 2.520 115.4 128.2

3.078 3.242 2.668 2.668 123.2 110.3

3.014 3.191 2.568 2.629 119.4 132.6

3.096 3.284 2.729 2.607 110.1 144.4

3.459 3.485

3542 3.604



Contacts of trans-ammonia N-Hal anion,/~


N-O (with NH 3 anion),/~,

3.016 3.348 3.713

[Co(NHa)5NO2]C204 crystals, it is necessary to distinguish between the essentially steric factors and the factors determined by the redox and spectral properties of the oxalate anion. For this purpose, it would be interesting to investigate the radiation-induced behavior of any cobalt(III) nitropentammoniate with an anion blocking the nitro group in a crystal structure, thus sterically hindering inner-spheric bond photoisomerization as this is done by the oxalate anion, but not taking part in the redox photodecomposition of the complex. CONCLUSIONS Determination and analysis of the structure of [Co(NH3)5NO2]C204ill comparison with the structures of the complexes containing other anions led to the following conclusions. The nonoccurrence of nitro-nitrito isomerization and the occurrence of photodecomposition under UV irradiation in crystals of this compound may be explained by the relative position of the oxalate anions and complex cations in the structure. The position of the oxalate ion prevents the NO 2 group from rotating and changing its coordination. The structure of the oxalate is denser (packing coefficient = 0.74) than that of isomerizing nitropentammoniates (packing coefficient = 0.67-0.72), whereas the hydrogen bonds are shorter, making the crystal structure more rigid and hindering rotation of the NO 2 group at least by structure distortion (as it occurs in isomerizing chloride, bromide, iodide, and chloride nitrate). This work was supported by the "Universities of Russia" Program, project 3H-34-94. We are grateful to A.V. Virovets for his interest in this work and assistance. REFERENCES 1.

2. 3. 4.

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