Syntheses and crystal structures of dinuclear, trinuclear [2x1+1x1] and tetranuclear [2x1+1x2] copper(II)-d(10) complexes (d(10)double right arrow Zn-II, Cd-II, Hg-II and Ag-I) derived from N,N '-ethylenebis(3-ethoxysalicylaldimine)