Vibrational behavior of SO42 - guest ions included in K2 Me(CrO4)2×2H2O (Me =Co, Ni) and crystal structures of K2 Me(CrO4)2×2H2O (Me =Co, Ni)

Author(s): Christian Lengauer, Manfred Wildner, Mitko Georgiev, Donka Stoilova
Abstract: Crystal structures of the isotypic compds. K2Co(CrO4)2·2H2O and K2Ni(CrO4)2·2H2O were refined from X-ray powder diffraction data (triclinic, space group P 1 , Z = 1, a = 6.432(1)/6.425(1), b = 7.396(1)/7.355(1), c = 5.643(1)/5.619(1) Å, a = 96.61(1)/96.78(2), ß = 106.64(1)/106.60(1), ? = 109.75(1)/109.84(1)°, V = 235.3/232.4 Å3, R wp = 8.22/9.27% for the Co/Ni-compd.). The structures are composed of Me 2+O4(H2O)2 octahedra and CrO4 groups forming kroehnkite-type octahedral-tetrahedral infinite chains which are linked by the potassium ions to layers and further to a three-dimensional framework. The proposed hydrogen-bonding scheme provides further linkage within the chains as well as between neighboring layers. The structures belong to type A' of the large group of compds. contg. kroehnkite-type chains. Vibrational spectra (IR and Raman) of K2 Me(CrO4)2·2H2O (Me = Co, Ni) as well as IR spectra of mixed crystals K2 Me(CrO4)2- x(SO4)x·2H2O (where x are approx. 0.05, 0.1 and 0.2 for the cobalt matrix, and 0.02, 0.1 and 0.15 for the nickel one) are presented and discussed with respect to the normal vibrations of the tetrahedral ions and the hydrogen bond system. The spectroscopic expts. reveal that ? 1 of the CrO 4 2 - ions appear at higher frequencies than two of the three components of ? 3, i.e. ? 1 > ? 3b (? 3a, ? 3b and ? 3c are site-group components of ? 3, ? 3c being the lowest wavenumbered component). The SO 4 2 - guest ions matrix-isolated in both matrixes exhibit three IR bands corresponding to the three site-group components of the asym. stretching and bending modes in good agreement with the low site symmetry of the CrO 4 2 - host ions (C 1 site symmetry). The values of ?? 3 (site-group splitting) are larger than those of ?? 4 (84 and 10 cm-1 in the cobalt matrix, and 88 and 8 cm-1 in the nickel one, resp.), thus indicating a stronger distortion of the SO 4 2 - guest ions with respect to the S-O bond lengths than to the O-S-O bond angles. The influence of the SO 4 2 - guest ion concns. on the IR band positions is also commented. Matrix-isolated in K2 Me(CrO4)2·2H2O (Me = Co, Ni) sulfate ions exhibit a stronger degree of energetic distortion than that of the same ions included in K2 Me(CrO4)2·2H2O (Me = Mg, Zn, Cd) as deduced from the values of ?? 3 and ?? max (the difference between the highest and the lowest wavenumbered components of the stretching modes). These spectroscopic findings are owing to the electronic configuration of the Me 2+ ions (for example, ?? 3 and ?? max of SO 4 2 - ions matrix-isolated in K2Zn(CrO4)2·2H2O have values of 73 and 163 cm-1, and 88 and 173 cm-1 in K2Ni(CrO4)2·2H2O, resp.). Metal ions with CFSE ? 0 (crystal field stabilization energy, Co2+ and Ni2+) are more resistant to angular deformations, i.e. to bond angle changes in the resp. MeO6 octahedra, thus facilitating the distortion of the adjacent tetrahedra as compared to those with CFSE = 0 (Mg2+, Zn2+, Cd2+). The influence of the metal ion nature (Me 2+ and Me +) on the values of ?? 3 and ?? max of matrix-isolated SO 4 2 - ions in different matrixes with kroehnkite-type octahedral-tetrahedral chains - chromates K2 Me(CrO4)2·2H2O (Me = Mg, Co, Ni, Zn, Cd) and selenates Na2 Me(SeO4)2·2H2O (Me = Co, Ni, Zn, Cd) is analyzed. SO 4 2 - guest ions included in chromate matrixes K2 Me(CrO4)2·2H2O (Me = Co, Ni, Cd) are remarkably less distorted than in selenate ones Na2 Me(SeO4)2·2H2O (Me = Co, Ni, Cd) as deduced from the values of ?? 3 and ?? max owing to weaker static fields caused by the larger K+ ions as compared to that caused by the smaller Na+ ions. The strength of the hydrogen bonds in the title compds., i.e. the frequencies of ? OD (matrix-isolated HDO mols.) is discussed in terms of hydrogen bond distances Ow···O, proton acceptor capacity of the oxygen atoms deduced from their bond-valence sums and Me-OH2 interactions (synergetic effect).
Organisation(s): Department of Mineralogy and Crystallography
External organisation(s): Bulgarian Academy of Sciences (BAS)
Journal: Journal of Molecular Structure
Volume: 920
Pages: 289-296
No. of pages: 8
ISSN: 0022-2860
Publication date: 2009
Peer reviewed: Yes
Austrian Fields of Science 2012: 105113 Crystallography, 104021 Structural chemistry
Portal url: https://ucrisportal.univie.ac.at/en/publications/79a91542-08b5-411f-a296-8cddc1b7b777

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