The X-ray diffraction structures of the olefin complexes [CpFe(CO)2(H2CCHDo)]PF6 (Do = OEt and NMe2) have been determined to further evaluate the previous report that the distance from Fe to the olefin carbon substituted by Do
(referred to as Cβ) is long or even nonbonding. These Fe–C distances are determined here to be long [2.402(10)
Å for Do = OEt] or nonbonding [2.823(11)
Å for Do = NMe2]. DFT optimization of the geometries of these, together with CpFe(CO)2 − n(PH3)n(H2CCHDo])+ for n = 1 and 2, show (a) agreement with experiment for n = 0, (b) a progression of Fe–Cβ distances to shorter values with increasing n for Do = OEt, (c) persistence of the Fe–Cβ distance at a nonbonding value for all n when Do = NMe2 and (d) the shortest Fe–Cβ distances for the weakest π-donor substituent, Do = F. These results are rationalized in terms of increased localization of nucleophilicity on the olefin Cα as the π-donor ability of Do strengthens. Therefore, not all olefins will show η2-binding.
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