The tetracysteinyl peptide cyclo[Lys(1,12)](Gln-Cys-Gly-Val-Cys-Gly-Lys-Cys-Ile-Ala-Cys-Lys)(subset of L(Cys . SH)(4)) was synthesized by solid-phase methods using an Fmoc/t-Bu/allyl strategy on a PAL-PEG-PS support. The formation of the 1:1 complexes with M = Fe2+, Co2+, and Ni2+ was observed by spectrophotomeric monitoring of reactions in aqueous solution at pH 7.5. Size exclusion chromatography indicated that the peptide is a monomer and the complexes are dimers [M-2(subset of L(Cys . S)(4))(2)] in aqueous buffer at pH 7.5. Cobalt and nickel K-edge X-ray absorption data and EXAFS analysis of [Co-2(subset of L(Cy . S)(4))(2)] and [Ni-2(subset of L(Cys . S)(4))(2)] as lyophilized solids rue reported. Derived bond distances are Co-S = 2.30 Angstrom and Ni-S = 2.21 Angstrom. From the collective results provided by absorption spectra, K-edges, EXAFS, and bond length comparisons with known structures, it is shown that [Fe-2(subset of L(Cys . S)(4))(2)] and [Co-2(subset of L(Cys . S)(4))(2)] possess distorted tetrahedral structures and [Ni-2(subset of L(Cys . S)(4))(2)] has distorted square planar stereochemistry. The Co(TT) chromophore is particularly distinctive of the assigned structure, displaying three components of the parent tetrahedral ligand field transition (4)A(2) --> T-4(1)(P) (610, 685, 740 nm). The observed structures conform to the intrinsic stereochemical preferences of the metal ions. Structures for the binuclear complexes are suggested. These are the first characterized metal complexes of a cysteinyl cyclopeptide and among the few well-documented complexes of synthetic cyclopeptides. This study is a desirable first step in the design of cyclic peptides for the binding of mononuclear and polynuclear metal centers.