Enneacarbonyldiiron reacts with 3,3,4,4-tetramethyl-1,2-diazetine (L) or its mono-N-oxide (L′) to form complexes (L)Fe(CO)4 (3), (L)Fe2(CO)7 (4), (L)Fe2(CO)6 (5), or (L′)Fe(CO)4 (6), and small amounts of deoxygenated 4 and 5, respectively. Formation and stability of the complexes is compared with those of ligands with smaller and larger ring sizes and discussed in terms of the electronic properties of the ligand lone-pair Orbitals. The molecular structure of 4 is established by X-ray analysis.