Gravar-mail: Crystal structures of three copper(II)–2,2′-bi­pyridine (bpy) compounds, [Cu(bpy)(2)(H(2)O)][SiF(6)]·4H(2)O, [Cu(bpy)(2)(TaF(6))(2)] and [Cu(bpy)(3)][TaF(6)](2) and a related coordination polymer, [Cu(bpy)(H(2)O)(2)SnF(6)](n)