Redox-Switchable Second-Order Molecular Polarizabilities with Electron-Rich Iron σ-Aryl Acetylides

μ·β products were experimentally derived by EFISH for several Fe(II) and Fe(III) σ-aryl acetylides of formula [(η²-dppe)(η⁵-C<INF>5</INF>Me<INF>5</INF>)Fe(C&tbd1;C−Ar)]ⁿ⁺, n(PF<INF>6</INF>) ^- (dppe = 1,2-bis(diphenylphosphino)ethane; n = 0, 1), where Ar = p-Py, m-Py, o-Py (<BO>1a</BO>−<BO>c</BO>/<BO>1a</BO>⁺); Ar = p-C<INF>5</INF>H<INF>4</INF>-N(CH<INF>3</INF>)⁺, o-C<INF>5</INF>H<INF>4</INF>-N(CH<INF>3</INF>)⁺ (<BO>2a</BO>,<BO>b</BO>⁺); Ar = p-C<INF>6</INF>H<INF>4</INF>X with X = NO<INF>2</INF>, CN, CF<INF>3</INF>, H, OMe, NH<INF>2</INF> (<BO>3a</BO>−<BO>f</BO>/<BO>3a</BO>−<BO>f</BO> ⁺). For comparison purposes, EFISH data were also gathered on the Ru(II) analogues (<BO>4a</BO>,<BO>b</BO>) of <BO>3a</BO>,<BO>b</BO>. Significant figures for μ·β were determined, with the Fe(II) complexes <BO>2a</BO>⁺ and <BO>3a</BO>,<BO>b</BO> possessing the most electron-deficient aryl rings opposed to the metal center, indicating good second-order molecular polarizabilities. For most representatives among <BO>3a</BO>−<BO>f</BO>/<BO>3a</BO>−<BO>f</BO>⁺ and <BO>4a</BO>,<BO>b</BO>, β and β<INF>0</INF> values could be derived using DFT-computed dipole moments. The substituent influence on β and β<INF>0</INF> is especially marked in the neutral complexes and follows the electron-accepting properties of X:  NO<INF>2</INF> > CN > H > OMe ≈ NH<INF>2</INF>. The comparison between <BO>4a</BO>,<BO>b</BO> and <BO>3a</BO>,<BO>b</BO> indicates that the Fe(II) center is more efficient than Ru(II) as an organometallic donor group when opposed to strong acceptors. In addition, we also show with (<BO>3a</BO>−<BO>f</BO>/<BO>3a</BO>−<BO>f</BO>⁺) that the NLO response can be fine-tuned by variation of the X substituent in each redox state. Thus, given the facile and reversible oxidation to the corresponding Fe(III) parents, Fe(II) acetylides might conveniently be used for switching purposes in molecular devices for NLO.