Vibrational behavior of SO42- guest ions included in K2Me(CrO4)2•2H2O (Me = Co, Ni) and crystal structures of K2Me(CrO4)2•2H2O (Me = Co, Ni)

Crystal structures of the isotypic compounds K2Co(CrO4)(2)center dot 2H(2)O and K2Ni(CrO4)(2)center dot 2H(2)O were refined from X-ray powder diffraction data (triclinic, space group P (1) over bar, Z = 1, a = 6.432(1)/6.425(1), b = 7.396(1)/7.355(1), c = 5.643(1)/5.619(1) angstrom, alpha = 96.61(1)/96.78(2), beta = 106.64(1)/106.60(1), gamma = 109.75(1)/ 109.84(1)degrees, V = 235.3/232.4 angstrom(3), R-wp = 8.22/9.27% for the Co/Ni-compound). The structures are composed of Me2+O4(H2O)(2) octahedra and CrO4 groups forming krohnkite-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 compounds containing krohnkite-type chains. Vibrational spectra (infrared and Raman) of K2Me(CrO4)(2)center dot 2H(2)O(Me = Co, Ni) as well as infrared spectra of mixed crystals K2Me(CrO4)(2-x)(SO4)(x)center dot 2H(2)O (where x are approximately 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 experiments reveal that v(1) of the CrO42- ions appear at higher frequencies thin two of the three components of v(3), i.e. v(1) > v(3b) (v(3a), v(3b) and v(3c) are site-group components of v(3), v(3c) being the lowest wavenumbered component). The SO42- guest ions matrix-isolated in both matrices exhibit three infrared bands corresponding to the three site-group components of the asymmetric stretching and bending modes in good agreement with the low site symmetry of the CrO42- host ions (C-1 site symmetry). The values of Delta v(3) (site-group splitting)are larger than those of Delta(v4) (84 and 10 cm(-1) in the cobalt matrix, and 88 and 8 cm(-1) in the nickel one, respectively), thus indicating a stronger distortion of the SO42- guest ions with respect to the S-O bond lengths than to the O-S-O bond angles. The influence of the SO42- guest ion concentrations on the infrared band positions is also commented. Matrix-isolated in K2Me(CrO4)(2)center dot 2H(2)O (Me = Co, Ni) sulfate ions exhibit a stronger degree of energetic distortion than that of the same ions included in K2Me(CrO4)(2)center dot 2H(2)O (Me= Mg, Zn, Cd) as deduced from the values of Delta v(3) and Delta v(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 Me2+ ions (for example, Delta v(3) and Delta v(max) of SO42- ions matrix-isolated in K2Zn(CrO4)(2)center dot 2H(2)O have values of 73 and 163 cm(-1), and 88 and 173 cm(-1) in K2Ni(CrO4)(2)center dot 2H(2)O, respectively). Metal ions with CFSE not equal 0 (crystal field stabilization energy, Co2+ and Ni2+) are more resistant to angular deformations, i.e. to bond angle changes in the respective 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 (Me2+ and Me+) on the values of Delta v(3) and Delta v(max) of matrix-isolated SO42- ions in different matrices with krohnkite-type octahedral-tetrahedral chains - chromates K2Me(CrO4)(2)center dot 2H(2)O (Me = Mg, Co, Ni, Zn, Cd) and selenates Na2Me(SeO4)(2)center dot 2H(2)O (Me = Co, Ni. Zn, Cd) is analyzed. SO42- guest ions included in chromate matrices K2Me(CrO4)(2)center dot 2H(2)O (Me = Co, Ni, Cd) are remarkably less distorted than in selenate ones Na2Me(SeO4)(2)center dot 2H(2)O (Me = Co, Ni, Cd) as deduced from the values of Delta v(3) and Delta v(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 Compounds, i.e. the frequencies of v(OD), (matrix-isolated HDO molecules) is discussed in terms of hydrogen bond distances O-w center dot center dot center dot O, proton acceptor capacity of the oxygen atoms deduced from their bond-valence sums and Me-OH2 interactions (synergetic effect). (C) 2008 Elsevier B.V. All rights reserved.