Unsaturated Binuclear Cyclopentadienylmanganese Carbonyl Derivatives Related to Cymantrene

Abstract

The cyclopentadienylmanganese carbonyls Cp2Mn2(CO)n(Cp = η5-C5H5; n = 5, 4, 3, 2) have been studied by density functional theory using the B3LYP and BP86 functionals. The pentacarbonyl Cp2Mn2(CO)5 is predicted to have a monobridged Cp2Mn2(CO)4(µ-CO) structure with an Mn−Mn distance of 2.804 Å (BP86), suggesting the single bond required for the 18-electron configuration. Its predicted infrared ν(CO) frequencies agree very well with the transient ν(CO) frequencies assigned to Cp2Mn2(CO)4(µ-CO) in the laser photolysis of CpMn(CO)3 in hydrocarbon media. The tetracarbonyl Cp2Mn2(CO)4 has an Mn═Mn distance of 2.509 Å (BP86), indicative of an Mn═Mn double bond, but is thermodynamically unstable with respect to disproportionation into Cp2Mn2(CO)5 + Cp2Mn2(CO)3. The tricarbonyl Cp2Mn2(CO)3 has three bridging CO groups with an Mn≡Mn distance of 2.167 Å (BP86), suggesting an Mn≡Mn triple bond similar to the experimentally known structure of the stable (η5-Me5C5)2Mn2(CO)3. A triplet structure for Cp2Mn2(CO)2 with an Mn≡Mn triple bond (2.202 Å, BP86) is energetically preferred over a singlet structure with an Mn—Mn quadruple bond (2.067 Å, BP86).