Effects of an interannular bridge on spectral and electronic properties of bis(cyclopentadienyl)- and bis(indenyl)zirconium(IV) complexes

Interconnection of the indenyl ligands of bis(indenyl)zirconium dichloride complexes by a 1,1'-positioned Me 2 Si or (CH 2 ) 2 bridge causes pronounced bathochromic shifts of the longest wavelength UV/vis absorption bands by ca. 50 nm, while much smaller shifts are caused by a Me 2 Si bridge in otherwise analogous bis(cyclopentadienyl) complexes. Extended Huckel MO calculations show that overlap between the empty 4d orbitals of the ZrCl 2 2 + fragment and the highest filled π orbitals of the indenyl anions is decreased when a Me 2 Si bridge forces the indenyl ligands into a lateral orientation. Consequences of this orbital mismatch, e.g. on electron densities at the Zr center, are discussed.