Alternative force field models for ansa-zirconocene complexes - vibrational and structural studies on Me2Si-bridged and tert-butyl-substituted representatives

A force field model for ansa-zirconocene complexes is constructed, which re presents each C atom of the C-5-ring ligands as being bonded directly to th e metal center, rather than by way of the C-5-ring centroids. This eta(5)-m odel, which describes coordination geometries by mutual repulsions of the C -5-ring and halide ligands, has no adjustable equilibrium or force-constant parameters for any of the angles at the Zr center. Nevertheless, vibration al frequencies and structural parameters are reproduced for a series of sim ple and Me2Si-bridged zirconocene dihalide complexes with an accuracy compa rable to that of the more complex centroid model. Strong steric distortions , revealed by crystal structure determinations in a series of tert-butyl-su bstituted ansa-zirconocene dibromide and diiodide complexes, are likewise r eproduced with remarkable accuracy by the eta(5)-force field model. (C) 199 9 Elsevier Science B.V. All rights reserved.