X-ray analysis of Ni(II) complexes of the self-assembling system LH2, [pyda.H2][pydc] (pydc.H2 = 2,6-pyridinedicarboxylic acid and pyda = 2,6-pyridinediamine), indicated that two six-coordinated anionic [pyda.H]2[Ni(pydc)2].H2O (I) and neutral [Ni(pydc.H)2].3H2O (II) complexes are formed at different temperature conditions in water; in agreement with the results obtained from potentiometric pH titrations in the pH range 3.5–5.5.