ZR-91 CHEMICAL-SHIFTS AND LINE WIDTHS AS INDICATORS OF COORDINATION GEOMETRY DISTORTIONS IN ZIRCONOCENE COMPLEXES

Zr-91 NMR chemical shifts and line widths (Delta nu(1/2)) are reported for a number of ring-bridged and ring-substituted zirconocene dichlor ide, dibromide, and dimethyl complexes. Ab initio computations at the SCF level employing basis sets of moderate size suggest that the magni tude of the electric field gradient (EFG) at the Zr atom dominates Del ta nu(1/2) when the substituents X at Zr are varied (X = Br, Cl, Me). Substituents at the cyclopentadiene (Cp) rings affect the computed EFG s much less; in these cases, the line widths Delta nu(1/2) are governe d by the molecular correlation times tau(c), which were obtained for s everal zirconocene dichlorides from T-1(C-13) measurements. Experiment al trends in delta(Zr-91) of zirconocenes are well reproduced computat ionally with the IGLO (individual gauge for localized orbitals) or GIA O (gauge including atomic orbitals) SCF methods employing large basis sets. Model calculations suggest that Delta(Zr-91), as well as the EFG , are quite sensitive to the inclination and twist angles of the Cp ri ngs and, to a lesser extent, to the CpZrCp' angle. A substantial deshi elding, Delta(Zr-91) ca. 700 ppm, is predicted for (C5H5)(2)ZrMe(+), p resumably the active olefin-polymerizing catalyst.