Carbonyl migration from phosphorus to the metal in binuclear phosphaketenyl metal carbonyl complexes to give bridging diphosphido complexes

Abstract

Alkali metal salts of the 2-phosphaethynolate anion PCO⁻ synthesized from reactions of CO with NaPH₂ or K₃P₇ have recently become available in quantities for the synthesis of transition metal complexes of the potentially ambidentate PCO ligand (Angew. Chem., Int. Ed., 2013, 38, 10064). This is exemplified by the recently reported rhenium carbonyl complex (triphos)Re(CO)₂(PCO) (triphos = MeP(CH₂PPh₂)₃). Density functional theory studies on the related manganese carbonyl complexes Mn(CO)_n(PCO) (n = 5, 4, 3) and Mn₂(CO)_n(PCO)₂ (n = 8, 7, 6, 5) are now reported. For the binuclear systems the low-energy Mn₂(CO)₈(PCO)₂ structures are singlet spin state structures having two bridging P-bonded phosphaketenyl μ-PCO ligands without a direct Mn–Mn bond. Carbonyl loss from Mn₂(CO)₈(μ-PCO)₂ is predicted to lead to migration of CO groups from phosphorus to manganese resulting in Mn₂(CO)_n+2(μ-P₂) structures with bridging diphosphido groups as the lowest energy Mn₂(CO)_n(PCO)₂ isomers (n = 7, 6, 5). Isomeric Mn₂(CO)₆(PCO)₂ structures with dihapto bridging η²-μ-PCO ligands at ∼30 kcal mol⁻¹ above the global minimum are also found representing intermediates in the migration of CO groups from phosphorus to manganese. For the mononuclear systems the P-bonded Mn(CO)_n(PCO) (n = 5, 4) phosphaketenyl structures are found to lie 20 to 28 kcal mol⁻¹ in energy below the isomeric O-bonded Mn(CO)_n(PCO) phosphaethynoxy isomers consistent with previously reported results by Grützmacher and coworkers on R₃E(PCO)–R₃E(OCP) systems (R = iPr, Ph; E = Si, Sn, Ge, Pb). The lowest energy structure for the tricarbonyl Mn(CO)₃(PCO) is a singlet structure with an unusual trihapto η³-PCO ligand. However, higher energy isomeric Mn(CO)₃(PCO) structures with P-bonded phosphaketenyl or O-bonded phosphaethynoxy ligands and tetrahedral Mn coordination are also found.