Organometallic complexes for second-order non-linear optics: synthesis andmolecular quadratic hyperpolarizabilities of eta(5)-monocyclopentadienyliron(II) nitrile derivatives with different phosphines. X-ray crystal structure of [FeCp(DPPE)(p-NCC6H4NO2)][PF6]center dot CH2Cl2

New complexes of the type [FeCp(P_P)(p-NCR)][PF6] (P_P=(S)-PROLOPHOs, DPPE and (R)-PROPHOS ((S)-PROLOPHOS= (S)-N-(diphenylphosphino)-2-diphenylphosphi nooxymethylpyrrolidine; DPPE = 1,2-bis(diphenylphosphino)ethane; (R)-PROPHO S = (R)-(+)-1,2-bis(diphenylphosphino)propane); R = C6H4N(CH3)(2), C6H4NO2, (E)-C(H)=C(H)C6H4NO2, C6H4C6H4NO2) have been synthesized and characterized . Spectroscopic data were evaluated in order to correlate the tuning of the electronic density at the metal center with the first molecular hyperpolar izabilities (beta) determined by hyper-Rayleigh scattering at 1064 nm. Expe rimental beta values increase with chain length and with the enhancement of the donor ability of the bidentate phosphine. [FeCp(DPPE)((E)-p-NC-C(H)=C( H)C6H4NO2)][PF6] is found to have the highest beta value (570 x 10(-30) esu ) in the series. Molecular orbital calculations were carried out to underst and the geometric and electronic factors involved and also to compute the s tatic hyperpolarizabilities, beta (0). These results are consistent with th e trend observed in the beta (0) values derived from experiment on the basi s of the two-level model. The complex [FeCp(DPPE)(p-NCC6H4NO2)][PF6] was cr ystallographically characterized. The compound crystallizes in the centrosy mmetric space group Pbcn. Bond distances within the nitrile ligand were dis cussed in order to evaluate the extension of pi -backdonation suggested by spectroscopic data. (C) 2001 Elsevier Science B.V. All rights reserved.