Compositions comprising a tripeptide having the sequence XC1C2; wherein X is any amino acid such that XC1C2 is capable of binding a metal in a square planar orientation or square pyramidal orientation or both; and wherein C1 and C2 are the same or different; and wherein C1 and C2 individually are chosen from a cysteine and a cysteine-like nonnatural amino acid, as well as metal-XC1C2 complexes and methods for forming such complexes.