MgSO4·11H2O and MgCrO4·11H2O based on time-of-flight neutron single-crystal Laue data

Abstract

Hexaaquamagnesium(II) sulfate pentahydrate, [Mg(H2O)6]SO4·5H2O, and hexaaquamagnesium(II) chromate(II) pentahydrate, [Mg(H2O)6][CrO4]·5H2O, are isomorphous, being composed of hexaaquamagnesium(II) octahedra, [Mg(H2O)6]2+, and sulfate (chromate) tetrahedral oxyanions, SO42−(CrO42−), linked by hydrogen bonds. There are two symmetry-inequivalent centrosymmetric octahedra:M1 at (0, 0, 0) donates hydrogen bonds directly to the tetrahedral oxyanion,T1, at (0.405, 0.320, 0.201), whereas theM2 octahedron at (0, 0, {1 \over 2}) is linked to the oxyanionviafive interstitial water molecules. Substitution of CrVIfor SVIleads to a substantial expansion ofT1, since the Cr—O bond is approximately 12% longer than the S—O bond. This expansion is propagated through the hydrogen-bonded framework to produce a 3.3% increase in unit-cell volume; the greatest part of this chemically induced strain is manifested along theb* direction. The hydrogen bonds in the chromate compound mitigate ∼20% of the expected strain due to the larger oxyanion, becoming shorter (i.e.stronger) and more linear than in the sulfate analogue. The bifurcated hydrogen bond donated by one of the interstitial water molecules is significantly more symmetrical in the chromate analogue.