PI-BONDING OF THE DIHYDROGEN LIGAND PROBED BY MOSSBAUER-SPECTROSCOPY

The isomer shift and quadrupole coupling constant of the iron nucleus in the complexes trans-[Fe(H2)H(dppe)2]BF4 and trans-[Fe(H2)H(depe)2]B Ph4 are measured and interpreted to suggest that the dihydrogen ligand is a weak sigma-donor and strong pi-acceptor compared to CO, N2 and a series of primarily sigma-donor ligands. The strong pi-acceptor prope rty of H-2 may explain why the point for this ligand deviates from the curve defined by a plot of the H-1 NMR chemical shift of the terminal hydride versus the isomer shift of the iron for the series of complex es trans-[Fe(L or X-)H(diphos)2]+.