Steric limit of cis-[(C(5)R(5))Fe(CO)(2)](2) complexes in solution

The symmetrical dimers [(C5H5)Fe(CO)(2)](2) (1), [(C(5)Me(5))Fe(CO)(2)](2) (2), and [(C(5)Me(4)H)Fe(CO)(2)](2) (3) have been used to synthesize the unsymmetrical dimers (C(5)Me(4)H)(C5H5)Fe-2(CO)(4) (4), (C(5)Me(5))(C5H5)Fe-2(CO)(4) (5), and (C(5)Me(5))(C(5)Me(4)H)Fe-2(CO)(4) (6). All of the new compounds show cis trans isomerization to varying extents, depending on the degree of substitution. The crystal structures of 3 and 4 have been determined. The limit of the observable cis-bridged isomer is found in 6 which is 14% cis in CH3CN solution.