Unexpected direct iron-fluorine bonds in trifluorophosphane iron complexes: an alternative to bridging trifluorophosphane and difluorophosphido groups

The iron trifluorophosphane complexes Fe(PF(3))(n)] (n=4, 5), Fe(2)(PF(3))(n)] (n=8, 9), H(2)Fe(PF(3))(4)], and Fe(2)(PF(2))(2)(PF(3))(6)] have been studied by density functional theory. The lowest energy structures of Fe(PF(3))(4)] and Fe(PF(3))(5)] are a triplet tetrahedron and a singlet trigonal bipyramid, respectively. Both cis and trans octahedral structures were found for H(2)Fe(PF(3))(4)] with the cis isomer lying lower in energy by approximately 10 kcal mol(-1). The lowest energy structure for Fe(2)(PF(3))(8)] has two Fe(PF(3))(4)] units linked only by an iron-iron bond of length 2.505 A consistent with the formal Fe=Fe double bond required to give both iron atoms the favored 18-electron configuration. In the lowest energy structure for Fe(2)(PF(3))(9)] one of the iron atoms has inserted into a P-F bond of one of the PF(3) ligands to give a structure (F(3)P)(4)Fe<--PF(2)Fe(F)(PF(3))(4)] with a bridging PF(2) group and a direct Fe-F bond. A bridging PF(3) group is found in a considerably higher energy Fe(2)(PF(3))(9)] structure at approximately 30 kcal mol(-1) above the global minimum. However, this bridging PF(3) group keeps the two iron atoms too far apart (approximately 4 A) for the direct iron-iron bond required to give the iron atoms the favored 18-electron configuration. The preferred structure for Fe(2)(PF(2))(2)(PF(3))(6)] has a bridging PF(2) group, as expected. However, this bridging PF(2) group bonds to one of the iron atoms through an P-Fe covalent bond and to the other iron through an F-->Fe dative bond, leaving an uncomplexed phosphorus lone pair.