Binuclear phospholyl iron carbonyls: The limited role of the phosphorus atom in metal complexation

Abstract The structures and thermodynamics of the binuclear phospholyl iron carbonyls (C 4 H 4 P) 2 Fe 2 (CO) n ( n  = 6, 5, 4, 3, 2) have been investigated using density functional theory. The low-energy (C 4 H 4 P) 2 Fe 2 (CO) n ( n  = 4, 3, 2) structures are found to have direct iron–iron bonds and terminal five-electron donor pentahapto η 5 -C 4 H 4 P rings with the phosphorus lone pairs not involved in the bonding to the iron atoms. They are thus analogous to the corresponding cyclopentadienyliron carbonyl derivatives. However, they differ from the binuclear phospholyl manganese carbonyls (C 4 H 4 P) 2 Mn 2 (CO) n ( n  = 5, 4) for which structures with bridging seven-electron donor η 5 , η 1 -C 4 H 4 P phospholyl rings are the lowest energy structures by substantial margins. Partially bonded phospholyl rings, as well as direct Fe–Fe bonds, are found in the carbonyl-rich (C 4 H 4 P) 2 Fe 2 (CO) n ( n  = 6, 5) species. The pentacarbonyl (C 4 H 4 P) 2 Fe 2 (CO) 5 does not appear to be a viable species since it is disfavored relative to CO loss and to disproportionation into (C 4 H 4 P) 2 Fe 2 (CO) 6  + (C 4 H 4 P) 2 Fe 2 (CO) 4 .