Ligands that are capable of forming metal complexes are incorporated directly into peptides at nonbiologically active locations. The metal complex serves as a bifunctional agent and as a spacer molecule. In one aspect of the invention, the ligands are prepared by replacing a nonbiologically active peptide spacer sequence with either Cys-Gly-Gly-Glu(η-)CO- (SEQ ID NO:1) or Cys-Gly-Gly-Lys(ε-)NH-CO(CH2)2-CO- (SEQ ID NO:2). In these examples, unnatural peptide bonds are used to attach the ligand to the terminal end of the peptide. Peptides incorporating such ligands are also disclosed. Other spacer ligands which may be incorporated into peptides include the following natural peptide sequences: -Cys-Gly-His-, -Asp-Gly-Cys-, -Glu-Gly-Cys-, -Gly-Asp-Cys-, and -Gly-Glu-Cys-. Unnatural tripeptides which function as spacer ligands include: -Cys-Gly-(imidazolyl glycyl)-, -isoCys-Gly-(imidazolyl glycyl)-, and -isoCys-Gly-His-. When the above peptide sequences are present in a nonbiologically active peptide spacer, they are able to form metal complexes with desired metal ions, and the resulting complexes serve as bifunctional agents and as spacer molecules in the peptide.