Crystal Structure and Vibrational Spectra of [Ru(NH 3 ) 4 (OH)(NO)] ZnCl 4

The crystal structure of [Ru(NH 3 ) 4 (OH)(NO)] ZnCl 4 was determined by X-ray diffraction methods. Such study conﬁrmed the already reported data for the complex cation and allowed a description of the main features of the lattice structure. An infrared and Raman study of that compound was also performed in order to revise the existing assignment of the vibrational bands. This last study, backed by quantum chemical calculations, lead to a reinterpretation of the spectra. Data were also collected on the structural and vibrational characteristics of the ZnCl 42 - anion.


Introduction
As a consequence of preparative explorations related to the chemistry of nitrosyl complexes of ruthenium, the title compound was obtained and characterized.The [Ru(NH 3 ) 4 (OH)(NO)] 2? complex cation was the object of vibrational [1] and crystallographic [2] studies, being particularly interesting the investigation of a metastable isomeric state generated by irradiation with 450-500 nm light at low temperatures [3].The corresponding conjugated acid [Ru(NH 3 ) 4 (H 2 O)(NO)] 3? was also prepared and described [4].
In the present study the crystal structure of [Ru(NH 3 ) 4 (OH)(NO)]ZnCl 4 was determined by means of X-ray diffraction techniques and the vibrational properties were studied using infrared and Raman spectroscopy.The obtained vibrational spectra were interpreted with the aid of quantum chemical calculations.

Experimental
Preparative [Ru(NH 3 ) 5 Cl]Cl 2 was prepared according to Farquhar et al. [5].A solution of 146 mg (0.499 mmol) of that chloro complex in 15 ml of H 2 SO 4 0.1 M was treated with amalgamated zinc for 15 min with constant Ar bubbling.Thereafter, nitric oxide was bubbled in the solution for 2 h and liquid and gas were left in contact overnight.The nitric oxide in excess was then eliminated by bubbling with Ar, the solution filtered and left in a refrigerator allowing spontaneous evaporation of the solvent.This procedure resulted in the formation of small yellow crystals.
The nature of the crystals was proved by the structural determination, which showed the presence of zinc as ZnCl 4 2-in the lattice.That finding was confirmed by SEM/ EDAX spectra of the crystals.

X-Ray Crystallographic Study
Single-crystal X-ray diffraction data were taken at room temperature (293(2) K) on an automatic four-circle Enraf-Nonius CAD-4 diffractometer equipped with a rotating anode generator, using graphite-monochromated Cu Ka (k = 1.54184A ˚) radiation.Crystals belong to the acentric orthorhombic C m c 21 space group with Z = 4. Unit cell parameters and the orientation matrix for data collection were obtained from a least-squares refinement using the setting angles of 25 reflections in the h range 9°-59°.
The data collection, reduction, and absorption correction were performed with the CAD-4 [6] XCAD-4 [7], and PSISCAN [8] software, respectively.Crystal data, additional details of data collection and structure refinement are given in Table 1.
The structure was solved by direct methods with SHELXS-97 [9] and the model was refined by full-matrix least squares on F 2 with SHELXL-97 [9].The hydrogen atoms were stereochemically positioned and refined with the riding model, adopting isotropic thermal parameters 50% greater than the equivalent isotropic displacement parameter of the corresponding ammonia N or hydroxyl O-atom to which they are bonded.

Vibrational Spectra
The infrared spectrum of the substance diluted in a KBr pellet was run in a Bruker Equinox 55 FTIR instrument.The Raman spectrum of the pure substance was obtained in a Bruker IFS 66 FTIR instrument provided with the FRA 106 Raman accessory, in which 1064 nm light was used for excitation.

Calculations
The cation and anion optimized geometries were obtained before the calculation of the harmonic frequencies of the normal modes of vibration.For that purpose, density functional theory (DFT) techniques were used, with the B3LYP functional [14,15] and the Lanl2DZ basis set.The calculations were performed with the Gaussian.03suite of programs [16].The atomic displacement vectors corresponding to each molecular vibration were visualized by means of the Moldraw program [17], allowing a description of the different vibrational modes.

Structural Results
[Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 crystallize with one [Ru (NH 3 ) 4 (NO)(OH)] 2? cation and one ZnCl 4 2-anion located on a mirror plane.The complex ions with the atom-numbering scheme are shown in Fig. 1.Selected bond lengths and angles are listed in Table 2. Cations consist of Ru atoms octahedrally coordinated by four equatorial ammine N atoms, one axial nitrosyl N atom and one axial hydroxyl O atom.Similarly to the complex cation in [Ru(NH 3 ) 4 (NO)(OH)]Cl 2 [2] they have Cs symmetry with the Ru atom and the NO and OH axial groups lying on the crystallographic mirror plane and the equatorial NH 3 groups symmetry related by that plane, thus showing some deviation from the idealized C 4v symmetry.The Ru-NH 3 and Ru-NO bond distances, and the N-Ru-X angles (X = N or O), which are close to the ideal octahedral value of 908 and 1808, do not deviate significantly from the values observed in [Ru(NH 3 ) 5 (NO)]Cl 3 ÁH 2 O and [Ru(NH 3 ) 4 (NO)(OH)]Cl 2 [2] and they also are near to the value found for other nitrosylammineruthenium complexes [18][19][20][21][22].The metalnitrosyl conformation, Ru-NO = 1.726(9)A ˚, N-O = 1.17(1)A ˚, and Ru-N-O = 174.5(9)8, is close to that found , from which it might be inferred the presence of the hydroxyl group.Although from X-ray results the hydroxyl hydrogen atom could not be reliably located, the OH stretching vibration appear displaced to lower frequencies in comparison with that reported for the free OH -anion, 3556 cm -1 [24], or the calculated value (see Table 4), in agreement with the fact that this group might be involved in an H-bond interaction.In addition, the distortion from the ideal tetrahedral geometry of the ZnCl 4 2-anion (see Table 2) suggests that tetrachlorozincate chlorine atoms are also involved in H-bond interactions [cf.25].Thus, from these observations and taking the hydroxyl oxygen and tetrachlorozincate chlorine positions into consideration, the hydrogen atom has been located to maximize their interaction (see Table 3 and Fig. 2).
The [Ru(NH 3 ) 4 (NO)(OH)] 2? cations are held together by two interionic N-HÁÁÁO hydrogen bonds formed by the interactions of two ammine N-H groups, symmetric related by the mirror plane, and the hydroxyl oxygen atom of the neighbor cation.These interionic N-HÁÁÁO hydrogen bonds contribute to the development of infinite zigzag chains of [Ru(NH 3 ) 4 (NO)(OH)] 2? cations arranged along the c-axis.
Thus the [Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 crystal structure can be described as infinite zigzag chains of cations which favor the formation of large channels, parallel to the c-axis, where tetrachloride anions are located (see Fig. 2).The cations are interlinked along the chains through N-HÁÁÁO(H) hydrogen bonds formed by the equatorial ammine hydrogens and the hydroxyl oxygen atoms as acceptors, and linked to the anions through O-HÁÁÁCl and N-HÁÁÁCl bonds (see Table 3).

Vibrational Results
Representative infrared and Raman spectra of the studied compound are shown in Fig. 3, whereas the calculated and observed frequencies of the normal modes of vibration are presented in Table 4.
The calculated frequencies were of great aid to discriminate between the cation and anion bands; the last ones appear remarked in Table 4.The cation bands were assigned to the different vibrational modes taking into account the calculated atomic displacement vectors for each mode and also the assignments given by Mercer et al. [1].The ZnCl 4 2-bands assignments were also based in the calculations and in previous infrared and Raman spectroscopic studies [31,32].
The modes of vibration involving hydrogen atoms are strongly anharmonic and for that reason there is a notable lack of agreement between the corresponding calculated and observed frequencies.However, the calculated numbers served to assign the blocks of bands related to the vibrations of the NH 3 ligands.The 1007 cm -1 infrared band is assigned tentatively to the Ru-O-H bending mode, although it is something far from the calculated value of 700 cm -1 (Table 4); in fact, in nitrosyl ruthenium complexes having OH as ligand the corresponding band appears at 914-995 cm -1 [1].
The 563 cm -1 band was assigned to the Ru-NO stretching because of its rather large intensity in both infrared and Raman spectra, as predicted by the calculations.Such frequency seems to be more in line with the relation between the N=O and Ru-NO stretchings mentioned by Mercer et al. [1].The nearby 590 cm -1 band, weak in infrared and strong in Raman, is assigned to one of the Ru-N-O bending bands.
The ideally tetrahedral ZnCl 4 2-anion is strongly deformed in the crystal lattice, as the anion is on the crystallographic mirror plane (Table 2).As a consequence there is a splitting of the vibrational modes of E and F 2 symmetries, although only the most intense component bands could be observed because of severe overlapping with the low frequency bands of the cation (Fig. 3 and Table 4) in the corresponding spectral regions.

Conclusions
A new structural (X-ray diffraction) and vibrational (infrared, Raman) study was performed on the [Ru(NH 3 ) 4 (NO)(OH)] 2? cation in the crystal lattice of [Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 .Whereas the previously reported structural data were basically confirmed, the assignment of some vibrational bands was corrected on the basis of new infrared and Raman spectra (the last one was not reported before) and its interpretation with the aid of quantum chemical calculations.

Supplementary Material
Further details of the crystal structure investigation may be obtained in writing from the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number CSD-421506.

Fig. 1
Fig. 1 Molecular plot of the [Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 complex showing the labeling of the non-H atoms and their displacement ellipsoids at the 50% probability level

Fig. 2 Fig. 3
Fig. 2 Packing diagram of [Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 viewed down the a axis, showing in dashed lines the infinite zigzag N-HÁÁÁO hydrogen bond chains along the c axis

Table 1
Crystal data, structure determination and refinement summary for [Ru(NH 3 ) 4 (NO)(OH)]ZnCl 4 -bond from the NO nitrogen lone pair and two p-bonds involving the filled d xz and d yz Ru orbitals and the antibonding *p x and *p y NO orbital promoting the back-donor effect.The Ru-O bond length, 1.961(7)A ˚, has the same value as that observed in [Ru(NH 3 ) 4 (NO) (OH)]Cl 2 but is shorter than the Ru-OH 2 distance, 2.035(5) A ˚, in [Ru(NH 3 ) 4 NO(H 2 O)]Cl 3 ÁH 2 O [4] [23]haracter of the nitrosyl ligand; the NO stretching frequency is in agreement with such chemical character.Furthermore the value of the Ru-NO bond length, smaller than the sum of covalent radii 2.00 A ˚, also suggests, according to Enemark et al.[23], a multiple Ru-NO bond involving one r