Synthesis, structural characterization, electrochemical investigation, and near-infrared absorption of bis(dithiolene)nickel(II) complexes of type [Ni(L)2] [L = 1,2-di(4-chlorophenyl)ethene-1,2-dithiolate (1) and 1,2-di(4-bromophenyl)ethene-1,2-dithiolate (2)]