A series of ionic η⁵-monocyclopentadienyl–metal compounds possessing p-substituted benzonitrile ligands has been studied by hyper-Rayleigh scattering at the fundamental wavelength of 1.064 µm. Upon systematic variation of the metal ion, the first hyperpolarizability β was found to increase along the sequence Co, Ni, Ru, Fe, with about a three-fold increase from Ru to Fe. This yields very high values for the iron complexes, e.g., β=410×10^–30 esu for [Fe(η⁵-C₅H₅)(dppe)( p-NCC₆H₄NO₂)]⁺ [PF₆ ]^– dissolved in methanol. The high β values are attributed to π back-donation resulting in an extension of the conjugated π-system from the Fe^II organometallic fragment, acting as a good donor group, via the nitrile to the acceptor group NO₂ . Complexes with single phenyl rings as conjugated chains perform better than their biphenyl analogues, which is explained in terms of non-planarity of the coordinated biphenyl ligands in solution. By comparing complexes with electron donor and acceptor substituted ligands it is demonstrated that the organometallic moiety can be used as an extremely effective donor group but not as a good acceptor group.