π-Donor olefin substituents alter olefin binding to CpFe(CO)2+

Abstract

The X-ray diffraction structures of the olefin complexes [CpFe(CO)₂(H₂CCHD_o)]PF₆ (D_o = OEt and NMe₂) have been determined to further evaluate the previous report that the distance from Fe to the olefin carbon substituted by D_o (referred to as C_β) is long or even nonbonding. These Fe–C distances are determined here to be long [2.402(10) Å for D_o = OEt] or nonbonding [2.823(11) Å for D_o = NMe₂]. DFT optimization of the geometries of these, together with CpFe(CO)_2 − n(PH₃)_n(H₂CCHD_o])⁺ for n = 1 and 2, show (a) agreement with experiment for n = 0, (b) a progression of Fe–C_β distances to shorter values with increasing n for D_o = OEt, (c) persistence of the Fe–C_β distance at a nonbonding value for all n when D_o = NMe₂ and (d) the shortest Fe–C_β distances for the weakest π-donor substituent, D_o = F. These results are rationalized in terms of increased localization of nucleophilicity on the olefin C_α as the π-donor ability of D_o strengthens. Therefore, not all olefins will show η²-binding.