Magnetic Susceptibility and EPR Spectra of (μ-Hydroxo)bis[pentaamminechromium(III)] Chloride Monohydrate Jørgen Glerup* ,1a and Høgni Weihe 1b Department of Inorganic Chemistry, H.C. Ørsted Institute, University of Copenhagen, Universitetsparken 5, DK-2100 København Ø, Denmark, and Institut fu ¨r anorganische und physikalische Chemie, Universita ¨t Bern, Freiestrasse 3, CH-3000 Bern 9, Switzerland ReceiVed January 10, 1997 X From magnetic susceptibility measurements the J-value of (µ-hydroxo)bis[pentaamminechromium(III)] chloride hydrate was found to be 32.3 cm -1 . The EPR spectra of the complex diluted in the analogous cobalt(III) complex have been recorded down to 4 K. The spectrum from the quintet state has been identified by the temperature variation of the intensity. Below 200 K the quintet spectra consist of two spectra originating from two different sites in the crystal. Both spectra have been computer simulated, and the following two parameter sets were obtained for the two different ions in the crystal: D e )-0.0512(1) cm -1 , E e )-0.0263(1) cm -1 ; D e )-0.0498 cm -1 , E e )-0.0263 cm -1 . Introduction The cation “acid rhodo” (µ-hydroxo)bis[pentaamminechro- mium(III)] (5+) was first prepared by Jørgensen 2 in 1882, and since that time, it has been subject to considerable interest. Two modifications of the chloride salts are known, mono- and dihydrates, both of which have been examined by X-ray crystallography. 3,4 In a recent paper Gu ¨del 5 has shown by single-crystal spectroscopy that the acid rhodo monohydrate [(NH 3 ) 5 CrOHCr- (NH 3 ) 5 ]Cl 5 ‚H 2 O contains two inequivalent sets of complexes in the unit cell which contradicts the crystal structure determi- nation. 3 In recent work 6-8 we have simulated EPR spectra of antiferromagnetically coupled dinuclear chromium(III) com- plexes, and with this technique at our disposal we will show that the EPR spectra of the acid rhodo mono hydrate diluted in the analagous cobalt(III) complex definitely confirm that the crystals contain two different sites. Furthermore we will show that the usual spin Hamiltonian operator (eqs 4-6; Vide supra) is not sufficient simultaniously to simulate the EPR spectra from the triplet, quintet, and septet states which emerge as a result of the antiferromagnetic coupling between the two chromium- (III) ions. Experimental Section Magnetic Measurement. The magnetic susceptibility was measured on a Faraday balance which has been described elsewhere. 9 EPR Spectra. The EPR spectra have been recorded with a Bruker ESP 300 spectrometer equipped with an Oxford ESR-900 continous- flow cryostat. The spectra were recorded at 9.38 GHz, 20 mW microwave power, and with modulation frequency 100 kHz and modulation amplitude of 5 G. Synthesis. [(NH 3)5CrOHCr(NH3)5]Cl5‚H2O was prepared according to Linhard et al., 10 and [(NH3)5CoOHCo(NH3)5]Cl5·H2O was prepared according to Siebert et al. 11 A mixture of the chromium complex (1%) and the cobalt complex was recrystallized 10 from 10 -2 M acetic acid. X-Ray Powder Diagrams. These were measured on a Guinier- Ha ¨gg camera using Cu KR radiation. The data from the Guinier film was fitted by a least-squares program to a tetragonal cell. For the cobalt and chromium complexes the following cell constants were obtained: Co, a ) 15.980(5) Å, c ) 7.330(3) Å, Cr, a ) 16.269(4) Å, c ) 7.393(3) Å. The X-ray data 3 for the chromium complex were solved in the centrosymmetric space group P4 2/mnm with the cell constants a ) 16.259(7) Å and c ) 7.411(7) Å. We conclude that the Cr- and Co-containing crystals are isomorphous. Computer Simulation. The simulation of the EPR spectrum was performed by generating the energy matrix for each orientation of the molecule relative to the magnetic field. The resonance condition for each transition was then found by successive diagonalizations and iterations of the energy matrix, and the relative intensities were calculated from the eigenvectors multiplied by the appropriate Boltz- mann factor at the actual temperature. Summation of all the transitions over the whole space, where each transition is represented by a differentiated Lorentzian curve, gives the simulated spectrum. A more detailed description of the numerical method will be given elsewhere. Results and Discussion Magnetic Susceptibility. The temperature dependence of the magnetic susceptibility of [(NH 3 ) 5 CrOHCr(NH 3 ) 5 ]Cl 5 ‚H 2 O was measured in the temperature range 4-300 K. As seen in Figure 1, the susceptibility curve has a maximum at ap- proximately 70 K and the effective magnetic moment declines from 5.0 µ B at room temperature to almost zero at 4 K. This is the expected behavior characteristic for an antiferromagneti- cally coupled dimeric chromium(III) complex. The susceptibil- ity data was analyzed by fitting to the following expression: Here E i are the energies of the 16 components of the ground- state manifold. The energies of the different components were X Abstract published in AdVance ACS Abstracts, June 1, 1997. (1) (a) University of Copenhagen. (b) Universita ¨t Bern. (2) Jørgensen, S. M. J. Prakt. Chem. 1882, 25, 321. (3) Veal, J. T.; Jeter, D. Y.; Hempel, J. C.; Eckberg, R. P.; Hatfield, W. E.; Hodgson, D. J. Inorg. Chem. 1973, 12, 2928. (4) Engel, P.; Gu ¨del, H. U. Inorg. Chem. 1977, 16, 1589. (5) Riesen, H.; Gu ¨del, H. U. Inorg. Chem. 1986, 25, 3566. (6) Bang, E.; Eriksen, J.; Glerup, J.; Mønsted, L.; Mønsted, O.; Weihe, H. Acta Chem. Scand. 1991, 45, 367. (7) Glerup, J.; Weihe, H. Acta Chem. Scand. 1991, 45, 444. (8) Sanzenbacher, R.; Bo ¨ttcher, A.; Elias, H.; Glerup, J.; Haase, W.; Hu ¨ber, M.; Jensen, T. B.; Neuberger, M.; Olsen, C. E.; Springborg, J.; Zehnder, M. Inorg. Chem. 1996, 35, 7493. (9) Josephsen, J.; Pedersen, E. Inorg. Chem. 1977, 16, 2534. (10) Linhard, M.; Weigel, M. Z. Anorg. Allg. Chem. 1959, 299, 15. (11) Siebert, H.; Feuerhake, H. Chem. Ber. 1969, 102, 2951. ′ mol )- N H ∑ i ∂E i ∂H exp(-E i /kT) ∑ i exp(-E i /kT) + K + C/T (1) 2816 Inorg. Chem. 1997, 36, 2816-2819 S0020-1669(97)00029-3 CCC: $14.00 © 1997 American Chemical Society