Complex formation between N,N,N',N'-tetrakis(2-aminoethyl)ethane-1,2-diamine (penten) and the metal ions Mn 2 , Co 2 , Cu 2 , Zn 2 , Cd 2 , Hg 2 , Ag , Pb 2 , and Tl 3 (in 1.00m NaNO 3 and 258) was investigated by potentiometry and spectrophotometry. These are the first reported values of the stability constants for this ligand with Ag , Pb 2 , and Tl 3 . The X-ray crystal structure of [Tl(NO 3 )(penten)](NO 3 ) 2 was determined. In this structure, Tl 3 shows a coordination number of seven made up of the six N-donors and one O-atom of NO À 3 .