Two ONNO-type Schiff bases, bis-N,N'-(salicylidene)-1,3-diaminopropane and bis-N,N'-(2-hydroxyacetophenylidene)-1,3-propanediamine, were reduced using NaBH4 and converted to two phenol-amine-type tetradentate ligands, bis-N,N'-(2-hydroxybenzyl)-1,3-diaminopropane ((LH2)-H-H) and bis-N,N'-[1(2-hydroxyphenyl)ethyl]-1,3-diaminopropane (LAC(H)H(2)). These ligands were used to prepare two Ni-II-Cd-II heterodinuclear complexes, namely [DMF center dot NiLH center dot CdI2 center dot DMF] and [DMF center dot NiLAC(H)center dot CdBr2 center dot DMF] in DMF medium. The molecular structure and unit cells of these complexes have been elucidated by the use of X-ray diffraction data. The thermogravimetric analysis of the compounds revealed that as the temperature is increased, the first coordinative DMF molecule was removed from the structure followed by a second coordinative DMF molecule with the complete decomposition of the complex. The activation energies and Arrhenius pre-exponential factor of these thermal reactions were determined by the use of isothermal Coats-Redfern, nonisothermal Ozawa-Flynn-Wall and Kissinger-Akahira-Sunose methods. The results obtained for the first thermal reaction were similar since the structure of both complexes remained intact during this process. Also, the theoretical calculations of the bond lengths, bond angles and natural bond orbital analysis of both complexes were carried out using the algorithms embedded in Gaussian 09 software.