The Nature of the Indenyl Effect

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Abstract
The η5‐to‐η3 coordination shift of cyclopentadienyl (Cp=C5H5−) and indenyl (Ind=C9H7−) ligands in molybdenocene complexes, [(η5‐Cp′)(η5‐Cp)Mo(CO)2]2+ (Cp′=Cp or Ind), driven by a two‐electron reduction of those species, was studied and compared by means of molecular orbital calculations (B3LYP HF/DFT hybrid functional, DZP basis sets). The results obtained, in terms of optimized geometries, relative energies, and bond analysis parameters, compare well with the experimental data, and verify the well‐known indenyl effect, that is, a significantly more facile η5‐to‐η3 rearrangement for the indenyl ligand when compared to cyclopentadienyl. However, the study of the folding of free Cp and Ind, combined with the (η5/3‐Cp′)−M bond analysis, shows that the observed difference is not the result of an intrinsic characteristic of the indenyl ligand, such as the traditionally accepted aromaticity gain in the benzene ring formed in η3‐Ind complexes. Instead, it is directly related to the Cp′−M bond strength. While the difference in the energy required to fold the two free ligands is negligible (≤1 kcal mol−1 for folding angles up to 20°), the (η5‐Cp)−M bond is stronger than that of (η5‐Ind)−M; however, the opposite situation is found for the η3 coordination mode. The net result, for Cp′=Ind, is a destabilization of the η5 complexes and a stabilization of the η3 intermediates or transition states yielding smaller activation energies and faster reaction rates for processes in which that is the rate‐determining step.

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