[NiL(3)]X(2) . H2O (X is Cl-, Br- and ClO4-; L is N-propylethane-1,2-diamine), [NiL(2)(H2O)(2)](ClO4)(2), [NiL(2)X(2)](X is Cl-, Br-, 0.5SO(4)(2-) and 0.5SeO(4)(2-)), [NiL(2)’(H2O)(2)](ClO4)(2) . 2H(2)O (L’ is N-isopropylethane-1,2-diamine), [NiL(2)’X(2)] (X is Cl-, Br-, NCS- and 0.5SeO(4)(2-)) and [NiL(2)’SO4]. 3H(2)O have been synthesised from solution and their thermal studies have been carried out in the solid phase. [NiL(2)X(2)] (X is Cl- and Br-), NiLCl(2) and NiL’SO4 have been prepared pyrolytically from their respective parent species in the solid state. [NiL(2)](ClO4)(2) and [NiL(2)’](ClO4)(2) were prepared pyrolytically from their parents species, and also by storing [NiL(2)(H2O)(2)](ClO4)(2) and [NiL(2)’(H2O)(2)](ClO4)(2) . 2H(2)O in a CaCl2 desiccator, respectively; these transformations are accompanied by a pink to yellow colour change due to octahedral to square-planar configurational changes. All other bis(diamine) species have octahedral geometry. Among them, [NiL(2)X(2)] (X is Cl- and Br-) and [NiL(2)’X(2)] (X is Cl-, Br- and NCS-) possess trans-octahedral configuration, whereas [NiL(2)X] (X is SO42- and SeO42-), [NiL(2)’SO4]. 3H(2)O and [NiL(2)’SeO4] have cis-configuration. On heating, trans-[NiL(2)’Br-2] and trans-[NiL(2)’(NCS)(2)] undergo reversible endothermic phase transitions (86-99 degrees C, Delta H = 0.52 kJ mol(-1) for heating, and 92-81 degrees C, Delta H = -0.53 kJ mol(-1) for cooling for the former, and 162-224 degrees C, Delta H = 0.27 kJ mol(-) for heating and 208-150 degrees C, Delta H = -0.25 kJ mol(-1) for cooling for the latter), most likely due to conformational changes of the diamine chelate rings.