Five structures of co-crystals grown from aqueous solutions equimolar in 15-crown-5 (or 15C5) and [M(H(2)O)(6)](NO(3))(n), M = Al(3+), Cr(3+) and Pd(2+), are reported. The hydrogen-bonding patterns in all are similar: metal complexes including the fragment trans-H(2)O-M-OH(2) alternate with 15C5 molecules, to which they are hydrogen bonded, to form stacks. A literature survey shows that this hydrogen-bonding pattern is very common. In each of the two polymorphs of the compound [Al(H(2)O)(6)](NO(3))(3).15C5.4H(2)O there are two independent cations; one forms hydrogen bonds directly to the 15C5 molecules adjacent in the stack, while the other cation is hydrogen-bonded to two water molecules that act as spacers in the stack. These stacks are then crosslinked by hydrogen bonds formed by the three nitrate counterions and the three lattice water molecules. The hydrogen-bonded stacks in [Cr(H(2)O)(5)(NO(3))](NO(3))(2).1.5(15C5).H(2)O are discrete rather than infinite; each unit contains two Cr(3+) complex cations and three 15C5 molecules. These units are again crosslinked by the uncoordinated nitrate ions and a lattice water molecule. In [Pd(H(2)O)(2)(NO(3))(2)].15C5 the infinite stacks are electrically neutral and are not crosslinked. In [Pd(H(2)O)(2)(NO(3))(2)].2(15C5).2H(2)O.2HNO(3) a discrete, uncharged unit containing one Pd complex and two 15C5 molecules is ;capped off' at either end by a lattice water molecule and an included nitric acid molecule. In all five structures the infinite stacks or discrete units form an array that is at least approximately hexagonal.