5,10,15,20-Tetraaryl-21-vacataporphyrin (1), an annulene-porphyrin hybrid containing a butadiene fragment in the macrocycle perimeter, gives paramagnetic iron(II) complexes 2. The porphyrin 1 is devoid of one donor atom of the coordination core; hence, metal ion is bound in the macrocyclic cavity by only three pyrrolic nitrogen atoms. The coordination sphere in 2-X (where X = Cl, Br, I) is completed by a halide anion. The butadiene fragment flexibility and constraints of coordination lead to two stereoisomers with the chain oriented inward (2-i-X) or outward (2-o-X) of the macrocyclic center. Axial halide subtraction (AgBF4 addition) leads to two new forms differing in the butadiene chain configuration. The H-1 NMR spectra of all complexes show characteristics typical for high-spin iron(II) complexes of porphyrinoids. The dependence of the relaxation times T-1 versus Fe-II center dot center dot center dot H distances (estimated by MM+ models) for three of the isomers is in accordance with the in, out, and/or zigzag geometries. The 2-o-X complex is more reactive than 2-i-X and reacts at room temperature with dioxygen to form the iron(II) 21-oxaporphyrin complex, conserving the iron(II) oxidation state. After the addition of imidazole or excess of methanol to a mixture of 2-o-X and 2-i-X, single five-coordinate complexes with out annulene configuration and two axial ligands are formed.