Diene–metal π bonding. Some unexpected effects of Group 5 donor ligands on carbon-13 nuclear magnetic resonance parameters and X-ray crystal structures

Abstract

A number of η⁴-cyclohexadiene–Fe(CO)₂L and η⁴-2-methoxycyclohexadiene–Fe(CO)₂L complexes have been prepared and their ¹³C n.m.r. spectra recorded. Methoxy-substituent effects suggest a depopulation rather than the expected enhanced population of the diene I.u.m.o. (lowest unoccupied molecular orbital) as the π-acceptor strength of L decreases. X-Ray crystal-structure determinations for L = PPh₃ were in agreement, and an explanation is proposed. Complexes [Fe(CO)₂(PPh₃)(C₆H₈)](7) and [Fe(CO)₂(PPh₃)(C₆H₇OMe)](8) both crystallise in the monoclinic space group P2₁/c with Z= 4. The cell parameters are a= 10.289(4), b= 24.651(10), c= 9.282(4)Å, and β= 109.37(3)° for (7) and a= 14.444(7), b= 18.052(10), c= 9.229(4)Å, and β= 100.57(4)° for (8). The structures were solved by a combination of Patterson and Fourier-difference techniques, and refined by blocked-cascade least squares to R= 0.053 for 3 325 unique observed reflections (7) and to R= 0.062 for 2 952 reflections (8).