Electronic Interactions in the Electroactive Iron σ‐Ethynyl‐2‐Phenyltetramethyldisilanes [Cp*(dppe)Fe–C≡C–{Si(CH₃)₂}₂–1,4‐C₆H₄X](PF₆)n (n = 0, 1)

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Treatment of the triflate complex [Cp*(dppe)FeOTf] { 5 ; Cp* = η 5 ‐C 5 (CH 3 ) 5 , dppe = 1,2‐bis(diphenylphosphanyl)ethane, OTf = CF 3 SO 3 } with an excess of HC≡C–[Si(CH 3 ) 2 ] 2 –1,4‐C 6 H 4 X ( 6a – e ) in a mixture of THF and triethylamine (NEt 3 ) provides the new complexes { 1a : X = H; 1b : X = CH 3 ; 1c : X = N(CH 3 ) 2 ; 1d : X = CF 3 ; 1e : X = Br}, which were isolated in 64 to 81 % yields as orange powders. The cyclic voltammetry, and multinuclear NMR, IR, and UV/Vis spectroscopic data obtained for 1a – e , as well as the X‐ray crystal structures determined for 1a , 1c , and 1e , reveal the importance of the σ–π conjugation (hyperconjugation) between the Si–Si σ‐bond and the vicinal C≡C π‐orbitals in the description of the electronic structure of the ground state of these complexes. When compounds 1a – e are reacted with [(C 5 H 5 ) 2 Fe](PF 6 ) (1 equiv.) at 20 °C, they provide [ 1a – e ](PF 6 ), which partially decompose in solution during the reaction process. They were isolated as mixtures with the mononuclear [Cp*(dppe)Fe=C=CH 2 ](PF 6 ) ( 7 ) and binuclear [Cp*(dppe)Fe=C=CH–CH=C=Fe(dppe)Cp*](PF 6 ) 2 ( 8 ) vinylidenes. Reduction of these mixtures with cobaltocene affords the parent neutral compounds 1a – e in analytically pure forms. The iron(III) complexes were characterized by IR and Mössbauer spectroscopy, which clearly showed that the iron(III) nucleus sensed the electronic effect of the X substituents.