Crystal structure and magnetic properties of catena-.mu.-sulfato-[N,N'-bis(2-hydroxyethyl)dithiooxamido(2-)-N,O,S:N',O',S']bis[aquacopper(II)]: a chain of copper(II) dinuclear units with a 594 cm-1 singlet-triplet separation and a 5.61 .ANG. copper-copper distance