The complexes, [NiL 2 (CF 3 CO 2 ) 2 ] [L = N-methylethane-1,2-diamine (meen) , N-ethylethane-1,2-diamine (eten) , N-propylethane-1,2-diamine (pren) and N-isopropylethane-1,2-diamine (ipren) ] have been prepared and X-ray single crystal structures of [Ni (pren) 2 (CF 3 CO 2 ) 2 ] 3 and [Ni (ipren) 2 (CF 3 CO 2 ) 2 ] 4 are determined. Both 3 and 4 have slightly distorted trans-octahedral geometry with N 4 O 2 -chromophore around nickel (II) . The crystals are stabilized by H-bonds and this interaction is higher in 4 due to close proximity of methyl H-atoms of isopropyl group with oxygen atoms of trifluoroacetate groups. The complexes 3 and 4 exhibit blue to light-violet, irreversible discontinuous thermochromism. [Ni (meen) 2 (CF 3 CO 2 ) 2 ] 1 also shows thermochromism, blue-violet to blue but reverts on keeping in humid atmosphere. On the other hand, [Ni (eten) 2 (CF 3 CO 2 ) 2 ] 2 undergoes decomposition without showing any phase transition. However, thermochromism here is considered to be due to a decreased in-plane ligand field strength resulting from the conformational changes in diamine chelate rings.