Electron-rich piano-stool iron σ-acetylides.: Theoretical and phenomenological investigation of electronic substituent effects in iron(II) acetylides

The electronic structures of the electron-rich Fe(II) acetylide complexes (eta(2)-dppe)(eta(5)-C5Me5)Fe(C=CC6H4-X) (1a-j: dppe = 1,2-bis(diphenylphosphino)ethane; X = NO2, CN, CF3, Br, F, H, Me, Bu-t, OMe, NH2) have been investigated using density functional theory calculations. The crystal structures of the bromo- (1d), methyl- (1g), methoxy- (1i), and amino-substituted complexes (1j) are reported to complement the available structural data. The structural data, the first ionization potentials, and the relevant electronic and vibronic transitions calculated for the model complexes are compared to the experimental data for the corresponding Fe(II) complexes among 1a-j. The influence of the X substituent on the electronic properties is then more specifically investigated by means of linear free-energy relationships (LFERs). Correlations between the experimental data gathered for 1a-j and electronic substituent parameters (ESP's) provide additional information on the way the remote para substituent influences the electronic properties of these Fe(II) acetylide complexes. This dual theoretical/phenomenological approach gives a consistent picture of the bonding between the Fe(II) center and the functional phenylacetylide fragment.