Why is bis-indenylchromium a dimer? A DFT investigation

Abstract Unsubstituted bis-indenylchromium has been shown to be a dimer, Cr 2 (Ind) 4 , whereas the monomeric sandwich-type structure, Cr(Ind) 2 , has been only observed for substituted relatives. DFT calculations indicate that dimerization allows the building of a quadruple Cr-Cr bond whereas it can still participate in five formal 2-electron metal-ligand bonds. Despite of this apparently favorable situation with respect to dimer stability, the energetic balance in favor of the dimer is computed not to be very large. Calculations on a series of related Cr, Mn, Fe and Co relatives indicate that Cr 2 (Ind) 4 appears unique in terms of its stability relatively to its monomer. However, dimeric species such as Cr 2 (Cp) 2 (Ind) 2 , Cr 2 (Cp) 4 or Mn 2 (Ind) 4 appear to be not that much unstable relatively to their monomer for being observed under some specific circumstances.