Dehydration of the hybrid compound [Ni-3(OH)(2)(tp)(2)(H2O)(4)] (1) upon heating led to the sequential removal of coordinated water molecules to give [Ni-3(OH)(2)(tp)(2)(H2O)(2)] (2) at T-1 = 433 K and thereafter anhydrous [Ni-2(OH)(2)(tp)] (3) at T-2 = 483 K. These two successive structural transformations were thoroughly characterized by powder X-ray diffraction assisted by density functional theory calculations. The crystal structures of the two new compounds 2. and 3 were determined. It was shown that at T-1 (433 K) the infinite nickel oxide chains built of the repeating structural unit [Ni-3(mu(3)-OH)(2)](4+) in 1 collapse and lead to infinite porous layers, forming compound 2. The second transformation at T-2 (483 K) gave the expected anhydrous compound 3, which is isostructural with Co-2(OH)(2)(tp). These irreversible transitions directly affect the magnetic behavior of each phase. Hence, 1 was found to be antiferromagnetic at T-N = 4.11 K, with metamagnetic behavior with a threshold field H-c of ca. 0.6 T. Compound 2 exhibits canted antiferromagnetism below T-N = 3.19 K, and 3 is ferromagnetic below T-C = 4.5 K.