A new type of hydrogen bonded networks [M(H2pdc)2(H2O)2] . 2H2O [M = Mn (1), Fe (2), Co (3), Ni (4), Zn (5); H3pdc = 3,5-pyrazoledicarboxylic acid] have been synthesized via hydrothermal reactions and their structures have been characterized. Upon a cooling-heating cycle, these compounds undergo a reversible structural interconversion process via hydration-dehydration: [chemical equation: see text]. The process is associated with distinct color changes. The dehydrated [M(H2pdc)2] (M = Mn, Fe, Co, Ni, and Zn) are amorphous and highly reactive. Further chemical reactions of these reactive intermediates show that they may act as effective precursors towards assembly of new supramolecular compounds that may otherwise be inaccessible by other synthetic routes. An interesting structure containing an "open-box" molecule [Co4(Hpdc)4(py)12] . 4py . 2H2O . 2CH3OH (6) (py=pyridine) has been isolated by using dehydrated [Co(H2pdc)2] as the precursor, and its crystal structure has been analyzed. Crystal data for 1-6: monoclinic, space group P2(1/c) and Z = 2 with a = 10.186(2), b = 12.473(2), c = 6.831(1) A, beta = 108.80(3) degrees (1); a = 9.896(2), b = 12.402(2), c = 6.810(1) A, beta = 108.15(3) degrees (2); a = 9.981(2), b = 12.426(2), c = 6.807(1) A, beta = 108.23(3) degrees (3); a = 9.896(2), b = 12.402(2), c = 6.810(1) A, beta = 108.15(3) degrees (4); a = 10.001(2), b = 12.430(2), c = 6.834(1) A, beta = 108.32(3) degrees (5); a = 9.9617(1), b=18.5080(2), c = 28.4786(3) A, beta = 93.076(1) degrees (6).