The role of H2O in controlling bond topology:I. The[6]Mg(SO4)(H2O)n(n= 0–11) structures

Abstract

AbstractHere, we examine the role of (H2O) in controlling polymerization of structural units in simple hydrated oxysalt compounds of the form Mg(SO4)(H2O)n. As the number of (H2O) groups in the structure increases, the number ofM—M(M= Mg) linkages decreases in a 1 : 1 ratio with the (increasing) number of (H2O) groups until noM—Mlinkages are left, and then the number ofM—T(T= S) linkages decreases with the (increasing) number of (H2O) groups until noM—Tlinkages are left (atn= 6). The change in bond topology is monotonic as a function of (H2O) content except forn= 5 where, instead of replacing anM—Tlinkage, the fifth (H2O) group is held in the structure by hydrogen bonds only. The valence-sum rule of bond-valence theory constrains the anion-coordination numbers to [2], [3] and [4] in these structures. The handshaking lemma of graph theory allows us to derive an equation relating the coordination numbers of the cations (Mg = [6], S6+= [4], H = [2]) to the coordination numbers of the anions. There are four integer solutions to this equation, and the Mg(SO4)(H2O)nstructures correspond to two of these solutions. Structures are known forn= 0–2.5, 4–7, 11; considering the variations in connectivity and coordination number as a function ofn, a possible structural arrangement is proposed for Mg(SO4)(H2O)3.