Metal–metal bonding in co-ordination complexes. Part IX. Crystal structure of trans-di-µ-carbonyl-dicarbonyldi-π-cyclopentadienyldi-iron (Fe–Fe), a redetermination

The crystal structure of the trans-isomer of di-µ-carbonyl-dicarbonyl-di-π-cyclopentadienyldi-iron has been redetermined by three-dimensional methods. The space group is P21/c with a= 7·046(1), b= 12·453(2), c= 7·990(1)A, β= 108·44(1)°, and Z= 2. The 904 reflections measured by counter diffractometry were used in the refinement by block-diagonal least-squares methods. With individual anisotropic thermal parameters and no allowance for hydrogen atom contributions, R= 0·044, WR= 0·056.Bond lengths and angles are: Fe–Fe 2·534(2); Fe–C(bridge) 1·910, 1·918(5); Fe–C(terminal) 1·748(6); Fe–C(ring) 2·082–2·121(8); C–O(bridge) 1·188(6); C–O(terminal) 1·157(7); and C–C 1·346–1·405(11)A; and Fe–C(bridge)–Fe 82·9(2); C(bridge)–Fe–C(bridge) 97·1(4); C(bridge)–Fe–C(terminal) 93·7(2); Fe–C(bridge)–O 138·4–139·1(3); and Fe–C(terminal)–O, 178·4(8)°.The identity of the Fe–Fe distance in both cis- and trans-isomers makes it unlikely that the former is stabilised preferentially by a π- or δ-contribution to the metal–metal bond as has been suggested.