MOLECULAR AND ELECTRONIC-STRUCTURES OF BIS[1,4-BIS(TRIMETHYL-SILYL)CYCLOOCTATETRAENE] SANDWICH COMPLEXES OF TITANIUM AND ZIRCONIUM

The reaction between 2 equivalents of [Li2{C8H6(SiMe3)2}] and [MCl4(thf)2] (M = Ti or Zr) in tetrahydrofuran (thf) afforded the sandwich complexes [M{C8H6(SiMe3)2}2]. X-Ray studies showed that the zirconium compound adopts a structure in which one ring is bound in an eta8 fashion whilst the other adopts a novel eta3 co-ordination mode. This unusual bonding situation has been studied using extended-Huckel molecular-orbital calculations and photoelectron spectroscopy. In contrast, the titanium compound adopts a solid-state structure in which one ring is eta8 bound and the other is less readily classified, being intermediate between the eta3 structure found for Zr and the eta4 structure found in [Ti(C8H8)2]. Variable-temperature solution NMR studies on [M{C8H6(SiMe3)2}2] showed that the two rings remain indistinguishable even at 193 K for M = Zr, whereas for M = Ti the two rings only become equivalent on the NMR time-scale above 328 K.