Analogies between binuclear phospholyl and cyclopentadienylmanganese carbonyl complexes: seven-electron donor bridging phospholyl rings

Abstract

Phosphacymantrene, (η⁵-C₄H₄P)Mn(CO)₃, is a known stable compound and a potential precursor to binuclear (C₄H₄P)₂Mn₂(CO)_n derivatives analogous to isovalent (C₅H₅)₂Mn₂(CO)_n derivatives. In this connection the structures and energetics of binuclear derivatives have been investigated by density functional theory. The lowest energy (C₄H₄P)₂Mn₂(CO)_n (n = 5, 4) structures have seven-electron donor η⁵,η¹-C₄H₄P ligands bridging the two metal atoms, which are too far apart for a direct Mn–Mn bond. However, the lowest energy (C₄H₄P)₂Mn₂(CO)₃ structure is predicted to be a triply carbonyl bridged structure with a formal MnMn triple bond of length 2.17 Å analogous to (η⁵-C₅H₅)₂Mn₂(μ-CO)₃, which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy (C₄H₄P)₂Mn₂(CO)₂ structures have one terminal η⁵-C₄H₄P ring and one bridging seven-electron donor η⁵,η¹-C₄H₄P ring. However, only slightly higher energy (C₄H₄P)₂Mn₂(CO)₂ structures are found with two η⁵-C₄H₄P rings and thus analogous to previously found (η⁵-C₅H₅)₂Mn₂(CO)₂ structures. Such a triplet (η⁵-C₄H₄P)₂Mn₂(CO)₂ structure is predicted to have a formal MnMn triple bond of length 2.20 Å. The two analogous singlet (η⁵-C₄H₄P)₂Mn₂(CO)₂ structures have shorter MnMn bonds of length 2.07 Å, which are interpreted as the formal quadruple bonds required to give each Mn atom the favored 18-electron configuration.