Vibrational behavior of 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)2·2H2O 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, α = 96.61(1)/96.78(2), β = 106.64(1)/106.60(1), γ = 109.75(1)/109.84(1)°, V = 235.3/232.4 Å3, Rwp = 8.22/9.27% for the Co/Ni-compound). The structures are composed of Me2+O4(H2O)2 octahedra and CrO4 groups forming kröhnkite-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 kröhnkite-type chains. ional spectra (infrared and Raman) of K2Me(CrO4)2·2H2O (Me = Co, Ni) as well as infrared spectra of mixed crystals K2Me(CrO4)2−x(SO4)x·2H2O (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 ν1 of the CrO 4 2 - ions appear at higher frequencies than two of the three components of ν3, i.e. ν1 > ν3b (ν3a, ν3b and ν3c are site-group components of ν3, ν3c being the lowest wavenumbered component). The SO 4 2 - 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 CrO 4 2 - host ions (C1 site symmetry). The values of Δν3 (site-group splitting) are larger than those of Δν4 (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 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 concentrations on the infrared band positions is also commented. Matrix-isolated in K2Me(CrO4)2·2H2O (Me = Co, Ni) sulfate ions exhibit a stronger degree of energetic distortion than that of the same ions included in K2Me(CrO4)2·2H2O (Me = Mg, Zn, Cd) as deduced from the values of Δν3 and Δνmax (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, Δν3 and Δνmax 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, respectively). 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 respective MeO6 octahedra, thus facilitating the distortion of the adjacent tetrahedra as compared to those with CFSE = 0 (Mg2+, Zn2+, Cd2+). fluence of the metal ion nature (Me2+ and Me+) on the values of Δν3 and Δνmax of matrix-isolated SO 4 2 - ions in different matrices with kröhnkite-type octahedral–tetrahedral chains – chromates K2Me(CrO4)2·2H2O (Me = Mg, Co, Ni, Zn, Cd) and selenates Na2Me(SeO4)2·2H2O (Me = Co, Ni, Zn, Cd) is analyzed. SO 4 2 - guest ions included in chromate matrices K2Me(CrO4)2·2H2O (Me = Co, Ni, Cd) are remarkably less distorted than in selenate ones Na2Me(SeO4)2·2H2O (Me = Co, Ni, Cd) as deduced from the values of Δν3 and Δνmax 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 νOD (matrix-isolated HDO molecules) 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).