Synthesis, characterization of triamine complexes of nickel(II)selenocyanate and their thermally induced dimerization: X-ray single crystal structures of [Ni(dpt)(SeCN)2(H2O)] and [Ni(aepn)(SeCN)2(H2O)] (dpt=bis(3-aminopropyl)amine and aepn=(2-aminoethyl)(3-aminopropyl)amine)