A molecular-orbital study of tricarbonyl(1–5-η-cycloheptadienylium)iron and tricarbonyl(1–5-η-cyclo-octadienylium)iron

Abstract

The self-consistent charge and configuration (SCCC) molecular-orbital method has been used to examine the electronic structures of the cyclodienyl complexes [Fe(C₈H₉)(CO)₃]⁺ and [Fe(C₈H₁₁)(CO)₃]⁺. The geometrical conformations adopted by the rings and the barriers to Fe(CO)₃ rotation have been calculated and compared with those of [Fe(C₅H₅)(CO)₃]⁺ and [Fe(C₆H₉)(CO)₃]⁺. It was found that distortions of the dienyl it molecular orbitals introduced by the saturated part of the ring are critical to the structure adopted. Both the complexes prefer geometries in which the rings are non-planar: in a non-symmetrical conformation in the case of [Fe(C₇H₉)(CO)₃]⁺, in which the hydrogen atoms of the C₂H₄ moiety are almost staggered with respect to one another and the Fe(CO)₃ group rotated by Ca. 10° out of the ‘mirror plane’ of the complex; and a ‘boat’ geometry in the case of [Fe(C₈H₁₁)(CO)₃]⁺, once again displaying a staggered arrangement of the methylene hydrogen atoms. These predictions are in agreement with known structural data. Interconversion processes between conformational isomers are examined.