We report X-ray crystal structures of complexes of the membrane-permeable metal chelator N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) with Cu(II) and Zn(II), and solution studies of the Zn(II) and Cd(II) complexes. [Zn(II)(TPEN)](PF6)(2) exhibits a distorted octahedral geometry with hexadentate TPEN, whilst [Cu(II)(TPEN)(PF6)(2) contains penta-coordinated Cu(II), with one pendant picolyl arm of TPEN. A survey of structures of metal-TPEN complexes reveals the versatility of the TPEN ligand. Complexes of V(V), Cr(III), Rh(III), Re(VII), Fe(II), as well as Cu(II) are known which contain hypodentate TPEN with 3-5 of its nitrogens coordinated to the metal. Solution NMR spectroscopic studies also suggest that there is dynamic conformational flexibility in the structures of the [Zn(TPEN)](2+) and [Cd(TPEN)](2+) complexes in aqueous solution. (c) 2005 Elsevier Ltd. All rights reserved.