Four new representatives of the natrochalcite structure type s.s. – A+Me22+(X6+O4)2[(H2O)(OH)], monoclinic, space group C2/m, Z = 2 – with Me2+ = Co or Ni were synthesised under low-hydrothermal conditions and their crystal structures were refined from single-crystal X-ray diffraction data up to sin 𝛉/λ = 0.904, measured at room temperature and for two compounds also at 105 K. Room temperature data are: NaNi2(SO4)2[(H2O)(OH)]: a = 8.605(1), b = 6.185(1), c = 7.336(1) Å, β = 114.78(1)°, V = 354.5 Å3, R1 = 0.021; NaCo2(SO4)2[(H2O)(OH)]: a = 8.677(1), b = 6.317(1), c = 7.396(1) Å, β = 115.40(1)°, V = 366.2 Å3, R1 = 0.022; NaCo2(SeO4)2[(H2O)(OH)]: a = 8.939(2), b = 6.382(2), c = 7.554(2) Å, β = 115.09(1)°, V = 390.2 Å3, R1 = 0.023; KCo2(SeO4)2[(H2O)(OH)]: a = 9.078(1), b = 6.471(1), c = 7.687(1) Å, β = 113.99(1)°, V = 412.6 Å3, R1 = 0.019.

The structure type is built up from chains of edge-sharing MeO6 octahedra, which are connected by XO4 tetrahedra and very strong hydrogen bonds to {Me2(XO4)2[(OH)(H2O)]} sheets. The linkage between these sheets is accomplished by the larger A cations and intermediate to weak hydrogen bonds.

Natrochalcite-type compounds are of particular interest due to the occurrence of formal H3O2 units forming very strong intramolecular hydrogen bonds, e.g. O···O = 2.44 Å in natrochalcite itself, NaCu2(SO4)2[(H2O)(OH)]. The replacement of Jahn-Teller distorted Cu2+ cations by Co2+ or Ni2+ in the title compounds leads to substantial changes of the MeO6 coordination polyhedra and, as further consequences, to increased distortions of the XO4 and AO8 polyhedra, to the occurrence of short interpolyhedral O···O contacts, and to a further shortening of the strong intramolecular hydrogen bond to O···O = 2.429(2) Å in NaCo2(SO4)2[(H2O)(OH)] and 2.420(2) Å in NaNi2(SO4)2[(H2O)(OH)]. These seem to be the shortest hydrogen bond lengths hitherto reported for minerals or mineral-type compounds with fully occupied oxygen donor-acceptor positions. Despite of this fact, the X-ray data corroborate a non-centric, i.e. split position of the hydrogen atom along these short donor-acceptor lines.

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