Question

Iron (III) chloride reacts with triphenylphosphine to form trigonal bipyramidal trans-FeCl3(PPh3)2 but with tricyclohexyl phosphine to form pseudooctahderal FeCl3(PCy3). Offer an explanationfor these structural difference

Answer 1

The distortions from the octahedral geometry usually forces the orbitals to either stabilised or lowering of energy or the MO’s to destabilized or the increasing in energy.The explanation arises from the purely crystal field treatment of the splitting of the d-orbitals in the particular geometry and the effect of the distortions.

The chloride and the phosphine ligands offer no contribution of the pi-bonding reign so we mainly deal with the sigma bonding between the ligands and the metal.The orbital interactions make the metal orbitals (dxy and dx2-y2) and dz2 are raised in energy, whereas the dxz and dyz are unchanged. The dxy and the dx2-y2 set of orbitals are raised only by the the interaction with the chloride ions whereas dz2 orbital is expected to be raised by both the by bot the chloride ions and the phosphine ligands. Now the phosphine ligands here are differing by the means of their sigma donation and the better sigma donor is PCy3 which raises the energy of the dz2 orbital to a higher extent with respect to the PPh3 ligand.

Thus the Pcy3 ligand owing to its high sigma donation destablises the dz2 orbital of the metal to a such extent with respect to the PPh3 that the PCy3 is found to assume a more or less octahedral arrangement or pseudo octahedral geometry .