The synthesis of the new Fe(II) complex [CpFe(CO)2(N4C–C5H4N)] (2) is described. 1H- and 13C-NMR spectroscopy data of (2) indicate the presence of interannular conjugation in the pyridyl-tetrazolate ligand, implying coplanarity between the two rings. Addition of electrophiles to 2 resulted in the formation of cationic complexes such as [CpFe(CO)2(4-MeN4C–C5H4N)][O3SCF3] (3), [CpFe(CO)2(4-HN4C–C5H4N)][O3SCF3] (4) and of the doubly protonated complex [CpFe(CO)2(4-HN4C–C5H4N–H)][O3SCF3]2 (5). In all cases, the out-of-plane rotation of the pyridyl ring occurred as a consequence of the quaternization of the N-4 of tetrazole ring. X-ray structure of complex 5 indicates a torsion angle of 20.9(2)° between the aromatic rings. Protonation reactions were found to be reversible and complexes 4–5 were easily converted into the starting complex 2 by addition of a base.