[M(H<sub>2</sub>O)<sub>2</sub>(15-crown-5)](NO<sub>3</sub>)<sub>2</sub>:  A System Rich in Polymorphic and Modulated Phases<sup>†</sup>

The series of structurally related compounds [M(H<sub>2</sub>O)<sub>2</sub>(15-crown-5)](NO<sub>3</sub>)<sub>2</sub>, M = Cu, Mg, Zn, and Co (four phases) has been extended to include three new phases for M = Mn and two new phases for M = Fe. Two new structure types have been discovered, so nine phases of two basic types (two- or three-dimensional H-bonding pattern) are now known. For the 2-D H-bonded (or layered) structure type, four variants having different modulation patterns have been observed. All phases have more than one crystallographically independent formula unit (<i>Z</i><i>‘</i> > 1) The cation conformations of all the independent units in all the structures are essentially identical. Reversible phase transitions during which crystallinity is preserved link the two layered structures of the Mn compound and of the Fe compound. A reason for the existence of so many related phases has been identified.