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