Twenty-seven tris(diamine)nickel(II) complexes ([Ni(diamine)3]X2) were prepared and their thermal reactions were investigated in the solid phase, where the diamine is ethylenediamine(en), N-methylethylenediamine (N-men), N-ethylethylenediamine (N-een), 1,2-propanediamine (1,2-pn), 1,3-propanediamine (1,3-pn), 1,2-butanediamine (1,2-bn), D,L-2,3-butanediamine (D,L-bn), iso-butanediamine (i-bn), or D,L-1,2-cyclohexanediamine (D,L-chxn); and X is Cl-, Br-, or NO3-. Of these, twelve tris(diamine) complexes lost their one mole of coordinated diamine upon heating, resulting in the stable, uniform bis(diamine) complexes, [NiX2(diamine)2]. The configurations of the bis complexes were determined by means of electronic spectroscopy. Almost all the bis complexes have trans configurations; the formation of a cis-dianiono complex was observed only in the thermal reactions of [Ni(en)3]Cl2, [Ni(en)3]Br2, and [Ni(i-bn)3]Cl2.