Phosphaethynolate Dimerization and Carbonyl Migration in Cyclopentadienyliron Carbonyl Systems: A Theoretical Study

Successive decarbonylation and cyclodimerization of the mono- and binuclear FeCp(PCO)(CO)(2), Fe2Cp2(PCO)(2)(CO)(4), and Fe2Cp2(CO)(2)(P-2) complexes have been investigated using density functional theory calculations at the M06L/DZP level. For the mononuclear complexes, the lowest energy FeCp(PCO)(CO)(m) (m = 2, 1) structures always prefer the eta-P(CO) bent configuration relative to other models. However, the lowest energy structure for the species of stoichiometry FeCp(PCO) is actually FeCp(P)(CO) with a formal Fe P triple bond. The binuclear complexes are much more complicated. The Fe2Cp2(PCO)(2)(CO)(4) structure, highly entropy controlled, has a P2C2 ring originating from cyclodimerization of two P=C=O groups from two FeCp(PCO)(CO)(2) monomers. The lowest energy Fe2Cp2(PCO)(2)(CO)(3) structure has a mu-P(CO) group donating two lone-pair electrons to each iron atom in a nonbonded Fe...Fe unit. The most favorable structure for the species of stoichiometry Fe2Cp2(PCO)(2)(CO)(2) has an end-to-end diphosphene P2 bridge connecting two FeCp(CO) fragments in a trans arrangement. The lowest energy Fe2Cp2(PCO)(2)(CO) structure has P(P) and mu-C(O) groups bridging an Fe Fe single bond. The lowest energy structures for the species of stoichiometriesFe(2)Cp(2)(P-2)(CO)(2) and Fe2Cp2(P-2)(CO) have a central P2Fe2 tetrahedron as well as two or one CO group(s) bridging Fe Fe or Fe=Fe bonds. The lowest energy carbonyl-free Fe2Cp2P2 structure has a rhombic Fe2P2 core with no direct P-P bond.