1,3-Diphosphacyclobutadiene as a ligand in binuclear manganese carbonyl derivatives: Role of the ring phosphorus atoms

Abstract

The binuclear 1,3-diphosphacyclobutadiene manganese carbonyls (Me2C2P2)2Mn2(CO)n (n=6, 5, 4, 3) have been investigated by density functional theory. The lowest energy (Me2C2P2)2Mn2(CO)n (n=6, 5) structures are found to have two mononuclear (Me2C2P2)2Mn2(CO)m fragments linked by P→M dative bonds. The lowest energy isomers with terminal η4-Me2C2P2 rings and Mn–Mn bonds lie ∼15 and ∼22kcal/mol above these global minima for (Me2C2P2)2Mn2(CO)6 and (Me2C2P2)Mn2(CO)5, respectively. For the (Me2C2P2)2Mn2(CO)4 system, singlet structures with terminal η4-Me2C2P2 rings and formal MnMn triple bonds are of comparable energies to triplet structures consisting of (Me2C2P2)2Mn2(CO)m fragments linked by two P→M dative bonds. All of the low-energy (Me2C2P2)2Mn2(CO)3 structures have terminal η4-Me2C2P2 rings and three bridging CO groups with short Mn–Mn distances suggesting formal multiple bonds.