Alternative force field models for ansa-zirconocene complexes : vibrational and structural studies on Me 2 Si-bridged and tert-butyl-substituted representatives

A force field model for ansa-zirconocene complexes is constructed, which represents each C atom of the C 5-ring ligands as being bonded directly to the metal center, rather than by way of the C5-ring centroids. This h 5 -model, which describes coordination geometries by mutual repulsions of the C5-ring and halide ligands, has no adjustable equilibrium or force-constant parameters for any of the angles at the Zr center. Nevertheless, vibrational frequencies and structural parameters are reproduced for a series of simple and Me2Si-bridged zirconocene dihalide complexes with an accuracy comparable to that of the more complex centroid model. Strong steric distortions, revealed by crystal structure determinations in a series of tert-butylsubstituted ansa-zirconocene dibromide and diiodide complexes, are likewise reproduced with remarkable accuracy by the h 5 -force field model.