Compounds derived from η⁵-monocyclopentadienylmetal (metal - Fe^ll, Ru^ll, Ni^ll and Co^lll) and bis-η⁵-cyclopentadienylmolybdenum^lV / tungsten^lV moieties containing para-substituted benzonitriles p-NCC₆H₄R (R = donor or acceptor group) as chromophores were identified as possessing significant values of Second Harmonic Generation (SHG) efficiency. Best results were found in compounds with p-NCC₆H₄NO₂ coordinated nitrile, for [MCp((+)-DIOP)(p-NCC₆H₄NO₂)][X] (M = Fe^ll, X = PF₆^- and M = Ru^ll, X = CF₃SO₃^-) which powder efficiencies are 38 and 10 times that of urea standard, respectively. NMR data together with SHG powder efficiencies were analyzed for compounds [Ru(η⁵-C₅H₅)((+)-DIOP)(p-NCC₆H₄NO₂)][X] (X = PF₆^-, NO₃^-, BF₄^-, CIO₄^-, CF₃SO₃^-, p-CH₃C₆H₄SO₃^-, Cl^-) in order to get some insight on the role played by the counterion and to appreciate the ability of NMR data to probe the relationship between molecular structures and SHG values.