Synthesis, characterisation and molecular hyperpolarisabilities of pseudo-octahedral hydrido(nitrile)iron(II) complexes for nonlinear optics: X-ray structure of <tex>[Fe(H)(dppe)_{2}(4-NCC_{6}H_{4}NO_{2})][PF_{6}]\cdot CH_{2}Cl_{2}$</tex>

A series of ionic pseudo-octahedral trans-hydrido(nitrile)iron(II) complexes with the general formula [Fe(H)(dppe)(2)(4-NCR)][PF6] [dppe = 1,2-bis(diphenylphosphanyl) ethane; R = acceptor-substituted conjugated ligand] have been synthesised by chloride abstraction from the starting compound trans-[FeHCI(dppe)(2)] and fully characterised. First hyperpolarisabilities (beta) have been determined by hyper-Rayleigh scattering (HRS) at the fundamental wavelength of 1072 nm and the high near-resonant values obtained (up to 1130 x 10(-30) esu) are interpreted in terms of the two-level model (TLM) and are correlated with IR and NMR spectroscopic data. Wavelength-dependent HRS has been performed in the 1072-1580 nm range for two of the compounds, namely [Fe(H)(dppe)(2)(4-NC(CH)(CH)C6H4NO2}][PF6] and [Fe(H)(dppe)(2){4-NCC6H4(CH)(CH)C6H4NO2}][PF6]. These results clearly show the two-photon resonance but also the shortcomings of the TLM when it comes to deriving reliable static beta values. A structural study of the compound [Fe(H)(dppe)(2)(4-NCC6H4NO2)][PF6] by X-ray diffraction shows that it crystallises in the centrosymmetric monoclinic space group P2(1)/n, with four molecules in the unit cell and a pair-wise antiparallel alignment of the dipoles.