Sterically promoted zirconium–phosphorus π-bonding: structural investigations of [Cp2Zr(Cl){P(H)Dmp}] and [Cp2Zr{P(H)Dmp}2] (Dmp=2,6-Mes2C6H3)

The new terminal zirconocene–organophosphanido complexes [Cp2Zr(Cl){P(H)Dmp}] (1) and [Cp2Zr{P(H)Dmp}2] (2) (Dmp=2,6-Mes2C6H3) bearing the sterically demanding ligand Dmp have been prepared and structurally characterized. A flattened pyramidal geometry for the phosphorus atom of 1 and a shortened ZrP bond length of 2.638(1) Å provide evidence for moderate ZrP π-bonding. Compound 2, however, displays both pyramidal and planar phosphorus atoms. The corresponding ZrP bond lengths of 2.726(2) and 2.519(2) Å, in conjunction with the phosphorus geometries, indicate that one phosphanido ligand is engaged in substantial π-bonding to the zirconium center while minimal, if any, of such interactions are present for the other phosphanido ligand. The solid state structures and 31P NMR spectra for 1 and 2 are very different than previously reported [Cp2Zr(Cl){P(H)Mes*}] and [Cp2Zr{P(H)Mes*}2] (Mes*=2,4,6-tBu3C6H2) which carry the related sterically demanding group Mes*. Comparisons to other structurally characterized zirconocene(IV) and hafnocene(IV) complexes having terminal phosphanido ligands suggest that upon increasing the steric congestion at the phosphorus atoms greater π interactions with the metal centers are afforded if such steric interactions do not prevent achieving proper alignment of the PR2 group for optimal MP π-bonding.