AbstractThe temperature dependence of the second moment (M2) of 1H NMR absorption of the partially deuterated (deuterated amine protons) complexes [M (daco‐d2)2] X2 · n D2O (M = Cu(II) and Ni(II); daco = 1,5‐diazacyclooctane; X = ClO4 or NO3; n = 0 for X = NO3 and n = 2 for X = ClO4) has been measured. Among them, [Cu(daco‐d2)2](NO3)2 showed a distinct drop in M2 at the same temperature region (ca. 363 K) where the complex is reported to undergo the thermochromic phase transition. The complexes [Cu(daco‐d2)2](ClO4)2·2D2O and [Ni(daco‐d2)2](ClO4)2·2D2O showed a gradual decrease of M2 around 220 and 280 k, respectively, without any colour changes and phase transitions, whilst M2 of [Ni(daco‐d2)2](NO3)2 shows no marked decrease with increase of temperature up to 400 K. M2 of rigid and motional lattices of [Cu(daco‐d2)2](NO3)2 is theoretically calculated using the reported X‐ray crystal structure data. The molecular motions of [Cu(daco‐d2)2](NO3)2, [Cu(daco‐d2)2](ClO4)2·2D2O and [Ni(daco‐d2)2](ClO4)2·2D2O are explained in terms of dynamic disorder of metal diamine chelate rings.