Synthesis of new donor/acceptor η5-cyclopentadienyl and η5-indenyliron(II) complexes with p-benzonitrile derivatives. Crystal structures of [Fe(η5-C5H5)(CO)(P(OC6H5)3)(p-NCC6H4NO2)][BF4]·CH2Cl2 and [Fe(η5-C9H7)(CO)(P(OC6H5)3)(p-NCC6H4NO2)][BF4]

New complexes of the type [Fe(η5-Cp or Ind)(L)(L′)(p-NCR)][BF4] (L, L′=CO, P(OC6H5)3, P(C6H5)3; R=C6H4N(CH3)2, C6H4NO2, (E)-C(H)C(H)C6H4NO2, (E)-C(H)C(H)C6H4N(CH3)2) have been synthesised and characterised. Spectroscopic data were analysed in order to evaluate the tuning of the electronic density at the metal centre and the extension of the π-delocalisation on the molecule, due to the presence of coligands with different acceptor/donor abilities. The structures of two complexes [Fe(η5-C5H5)(CO)(P(OC6H5)3)(p-NCC6H4NO2)][BF4] and [Fe(η5-C9H7)(CO)(P(OC6H5)3)(p-NCC6H4NO2)][BF4] were determined by X-ray crystallographic analysis. The compounds crystallised in the centrosymmetric space groups P1̄ and P21/n, respectively. Bond distances within the nitrile ligand are discussed in order to evaluate the nature of iron–nitrile bonding in these complexes.