Diiron sigma-pi alkenyl complexes: Crystals structures of cis-[Fe-2(Co)(4)(mu-RC CH2)(mu-dppm)], (R=Ph, CH2, OH), trans-[Fe-2(CO)(4)(mu-HC=CHPh)(mu-PPh(2))(mu-dppm)] and the alpha,beta-unsaturated acyl complex trans-[Fe-2(CO)(4){mu O=C-C(Ph)=CH2}(mu-PCy(2))(mu-dppm)]

Insertion of phenylethyne into cis-[Fe-2(CO)(4)(mu-H)(mu-CO)(mu-PPh(2))(mu-dppm)] 1a affords isomers cis-[Fe-2(CO)(4)(mu-PhC=CH2)(mu-PPh(2))(mu-dppm)] 2a and trans-[Fe-2(CO)(4)(mu-HC=CHPh)(mu-PPh(2))(mu-dppm)] 2b which differ in both the regioselectivity of the alkyne insertion (alpha and beta) and the relative orientations of the phosphido and diphosphine ligands (cis and trans) respectively. In contrast, reaction with trans-[Fe-2(CO)(4)(mu-H)(mu-CO)(mu-PCy(2))(mu-dppm)] 1b appears to give only the alpha-substituted product trans-[Fe-2(CO)(4)(mu-PhC=CH2)(mu-PCy(2))(mu-dppm)] 3a, however, thermolysis of the crude reaction mixture leads to the low yield isolation of the alpha, beta-unsaturated acyl complex trans-[Fe-2(CO)(4){mu-O=C-C(Ph)=CH2}(mu-PCy(2))(mu-dppm)] 4, formally resulting from CO insertion into the unidentified beta-substituted isomer. Reaction of 1a with propargyl alcohol affords chromatographically separable isomers cis-[Fe-2(CO)(4){mu-C(CH2OH)=CH2}(mu-PPh(2))(mu-dppm)] 5a and trans-[Fe-2(CO)(4){mu-HC=CH(CH2OH)}(mu-PPh(2))(mu-dppm)] 5b, while with 1b an inseparable mixture of trans-[Fe-2(CO)(4){mu-C(CH2OH)=CH2}(mu-PCy(2))(mu-dppm)] 6a and trans-[Fe-2(CO)(4)(mu-HC=CH(CH2OH))(mu-PCy(2))(mu-dppm)] 6b results. Complexes 2a, 2b, 4 and 5a have been characterised by single crystal X-ray diffraction analyses. All mu-alkenyl complexes undergo 'windshield-wiper' fluxionality on the NMR timescale, with free energies of activation varying dramatically, an effect which can be correlated with the mode of binding of the alkenyl ligand in the solid state.