The title compound, [FeCl2(C12H26N4)]PF6, is the first mononuclear Fe3+ complex of an ethylene cross-bridged tetraaza-macrocycle to be structurally characterized. Comparison with the mononuclear Fe2+ complex of the same ligand shows that the smaller Fe3+ ion is more fully encapsulated by the cavity of the bicyclic ligand. Comparison with the mu-oxo dinuclear complex of an unsubstituted ligand of the same size demonstrates that the methyl groups of 4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane prevent dimerization upon oxidation of the metal centre. Nax -Fe3+ -Nax bond angles (ax is axial), and thus the degree of encapsulation by the ligand, are quite different between the mononuclear and dinuclear mu-oxo species, which is probably the consequence of steric considerations.