Unsaturated binuclear cyclopentadienylmanganese carbonyl derivatives related to cymantrene

The cyclopentadienylmanganese carbonyls Cp₂Mn ₂(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 Cp₂Mn ₂(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 v(CO) frequencies agree very well with the transient v(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 + Cp ₂Mn₂(CO)₃. The tricarbonyl Cp ₂Mn₂(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).