Sterically promoted zirconium-phosphorus pi-bonding: structural investigations of [Cp2Zr(Cl){P(H)Dmp}] and [Cp2Zr{P(H)Dmp}(2)] (Dmp=2,6-Mes(2)C(6)H(3))

The new terminal zirconocene-organophosphanido complexes [Cp2Zr(Cl){P(H)Dmp}] (1) and [Cp2Zr{P(H)Dmp}(2)] (2) (Dmp = 2,6-Mes(2)C(6)H(3)) 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) Angstrom provide evidence for moderate Zr-P pi-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) Angstrom, in conjunction with the phosphorus geometries, indicate that one phosphanido ligand is engaged in substantial pi-bonding to the zirconium center while minimal, if any, of such interactions are present for the other phosphanido ligand. The solid state structures and P-31 NMR spectra for 1 and 2 are very different than previously reported [Cp(2)zr(Cl){P(H)Mes*}] and [Cp2Zr{P(H)Mes*}(2)] (Mes* = 2,4,6-(Bu3C6H2)-Bu-t) 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 ir. interactions with the metal centers are afforded if such steric interactions do not prevent achieving proper alignment of the PR, group for optimal M-P pi-bonding. (C) 2000 Elsevier Science S.A. All rights reserved.