A COMPARISON OF THE ENERGY BARRIERS TO BRIDGE REVERSAL IN TRISULPHUR-BRIDGED AND TRISELENIUM-BRIDGED [3]METALLOCENOPHANES

The [3]metallocenophanes with trichalcogen chains as bridging groups, namely [M(C5H4E)2E'] (M = Ru, Os, E = E' = S; M = Ru; E = E' = Se), have been shown by NMR spectroscopy to be fluxional by a bridge reversal process. From two-dimensional NMR exchange experiments (2D-EXSY), accurate energy data for the bridge reversal fluxion have been obtained and compared with previous data for analogous [3]ferrocenophanes. The DELTA-G double-ending-dagger(298 K) values for the process lie in the range 67-93 kJ mol-1. Magnitudes increase in the order Fe < Ru < Os, reflecting the increasing strength of metal-ring bonding, and in the order Se3 < S3 due to the different torsional energies of the chalcogen-chalcogen bridge bonds.