Ligand Displacement from TpMn(CO)2L Complexes: A Large Rate Enhancement in Comparison to the CpMn(CO)2L Analogues

The mechanism and energetics of ligand (L) displacement from TpMn(CO)2L (L = cyclooctene, HSiEt3) complexes was studied using time-resolved infrared spectroscopy and DFT. While the dissociative substitution mechanism remains unchanged, the results demonstrate a remarkable increase in the ligand displacement rates in comparison to those for the Cp analogues. When Tp is the ancillary ligand, the cyclooctene and HSiEt3 complexes react almost 1011 and 108 times faster, respectively. The observed rate enhancement is primarily a result of significantly weaker binding of molecules to the Mn center when Tp is the supporting ligand. The experimental findings are supported by DFT calculations, which confirm that, in addition to the larger cyclooctene and HSiEt3 ligands, small molecules such as CO, N2, and H2 also have significantly lower binding enthalpies to the TpMn(CO)2 fragment in comparison to the Cp analogue.