The first ab initio investigation of a realistic Ziegler-Natta system is presented that includes dynamical electron correlation to determine more reliable geometries and energetics during the ethylene insertion process. The systems Cp(2)TiCH(3)+-C2H4 and Cl2TiCH3+-C2H4 were investigated with the Moller-Plesset second-order perturbation method and the local density functional method. The quality of the MP2 energetics has been checked through single point coupled cluster singles doubles (triples) calculations for the C1(2)TiCH(3)(+)-C2H4 system. The results strongly support the mechanism proposed by Brookhart and Green. Agostic interactions were found to be of crucial importance. Two basic concepts of the Cossee-Arlman mechanism could not be confirmed : neither a precoordinated monomer nor a transition state was found to be a critical point on the potential energy surface for Cp(2)TiCH(3)(+)-C2H4. The educt Cp(2)TiCH(3)(+) has carbenoid character. The ethylene insertion is then rationalized by reference to a 2 + 2 addition which is symmetry allowed through the availability of d orbitals.