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 s terically demanding ligand Dmp have been prepared and structurally characte rized. A flattened pyramidal geometry for the phosphorus atom of 1 and a sh ortened 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 p hosphorus atoms. The corresponding Zr-P bond lengths of 2.726(2) and 2.519( 2) Angstrom, in conjunction with the phosphorus geometries, indicate that o ne phosphanido ligand is engaged in substantial pi-bonding to the zirconium center while minimal, if any, of such interactions are present for the oth er 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 rel ated sterically demanding group Mes*. Comparisons to other structurally cha racterized zirconocene(IV) and hafnocene(IV) complexes having terminal phos phanido ligands suggest that upon increasing the steric congestion at the p hosphorus atoms greater ir. interactions with the metal centers are afforde d 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. Al l rights reserved.