[NiL2]X-2. nH(2)O (where L is bis(2-aminoethyl)methylamine, and n = 0 when X is Cl-, Br-, SCN- and CF3COO-, n = 3 when X is 0.5SO(4)(2-) and 0.5SeO(4)(2-)) and NiLX2. nH(2)O (n = 0 when X is Cl-, Br-, NO3-, n = 1 when X is CF3COO(-) and n = 3 when X is 0.5SO(4)(2-) and 0.5SeO(4)(2-)) have been synthesized and investigated thermally in the solid state. All the bis, as well as monotriamine complexes possess octahedral geometry. [NiI2](SCN)(2) upon heating undergoes irreversible phase transition (155-180 degrees C; Delta H = 5.6 kJ mol(-1)) yielding an isomeric species which on further heating shows reversible phase transition (182-205 degrees C for heating, Delta H = 2.8 kJ mol(-1); 170-200 degrees C for cooling, Delta H = -2.8 kJ mol(-1)) and [NiL2]SO3. 3H(2)O undergoes an endothermic phase transition after deaquation 172-200 degrees C; Delta H = 6.8 kJ mol(-1)). Nil(NO3)(2) also shows a phase transition (195-220 degrees C; Delta H = 8.9 kJ mol(-1)). The phase transitions are proposed to be due to the conformational change in the chelate rings of the triamine. NiLAO(4). 3H(2)O (A = S or Se) exhibit thermochromism, blue-to-greenish-blue for elimination of two molecules of water and greenish-blue-to-light-green for elimination of the residual water molecule. The effect of methyl substitution in N-2-position of bis(2-aminoethyl)amine is not noticeable with respect to thermal stability of the complexes, but is pronounced in the case of thermally induced solid-state isomerization.