Chiral templating activity of tris(bipyridine)ruthenium(II) cation in the design of three-dimensional (3D) optically active oxalate-bridged {[Ru(bpy)3][Cu2xNi2(1-x)(C2O4)3]}n (0 ≤ x ≤ 1; bpy = 2,2'-bipyridine): Structural, optical, and magnetic studies