We describe the synthesis of the ruthenacyclic carbamoyl complexes [Ru(2-NHC(O)C5H3NMe)(CO)(2)(o,o-Me-2-C6H3S)(L)] (L = H2O or MeCN), which have a labile water or acetonitrile ligand at their sixth coordination sites. Steric bulk around the ruthenium center is essential in preventing isomerization and dimerization, and embedding within papain can be achieved via coordination of its sole free cysteine residue. The observed chemistry parallels that of the natural [Fe]-hydrogenase.