The potentially hexaden!ste ligand N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (C,,H,,N,, tpen) reacts in ethanolic solution with M(CI04),.6H20 (stoichiometry 1:l; h i = hln(II), Fe(II), Co(II), Ni(II), Cu(11)) yielding the corresponding [hl(tpen)](CIO,), complexes. The new complex [Cr(tpen)](PF& was also prepared from a dimethyl sulfoxide solution of tpen and CrC1,.6H,O. The crystal structure of the violet, [Ni(tpen)](ClO,), .2/3H,O complex has been determined by X-ray crystallography. Crystal data: monoclinic, space group C2/c; a=41.205(5), b=9.492(2), c=23.930(3)A, /T= 108.02(3)", V=8,900(6)A3, Z = 12, I),,,= 1.55gcm-'; final R=0.067 from 3379 observed reflections. The structure consists of two distinct distorted octahedral Ni(I1) cations, perchlorate anions and waters of crystallization.Electrochemical behaviour (cyclic voltammetry) and spectroelectrochemis!ry in acetonitrile shows a reversible or qzrasi-reversible one electron transfer process. The nickeI(I1) and copper(I1) complexes can be also reduced reversibly to the corresponding (formal) nickel(1) and copper(1) complexes. Stabilization of different oxidation states is discussed in terms of the flexibility and a-acceptor capability of the ligand tpen.