Synthesis, Characterisation and Molecular Hyperpolarisabilities of Pseudo‐Octahedral Hydrido(nitrile)iron( II ) Complexes for Nonlinear Optics: X‐ray Structure of [Fe(H)(dppe) 2 (4‐NCC 6 H 4 NO 2 )][PF 6 ]·CH 2 Cl 2

A series of ionic pseudo‐octahedral trans ‐hydrido(nitrile)iron( II ) complexes with the general formula [Fe(H)(dppe) 2 (4‐NCR)][PF 6 ] [dppe = 1,2‐bis(diphenylphosphanyl)ethane; R = acceptor‐substituted conjugated ligand] have been synthesised by chloride abstraction from the starting compound trans ‐[FeHCl(dppe) 2 ] and fully characterised. First hyperpolarisabilities ( β ) 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 × 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)C 6 H 4 NO 2 }][PF 6 ] and [Fe(H)(dppe) 2 {4‐NCC 6 H 4 (CH)(CH)C 6 H 4 NO 2 }][PF 6 ]. These results clearly show the two‐photon resonance but also the shortcomings of the TLM when it comes to deriving reliable static β values. A structural study of the compound [Fe(H)(dppe) 2 (4‐NCC 6 H 4 NO 2 )][PF 6 ] by X‐ray diffraction shows that it crystallises in the centrosymmetric monoclinic space group P 2 1 / n , with four molecules in the unit cell and a pairwise antiparallel alignment of the dipoles. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)