MECHANISTIC STUDIES OF THE 1,4-CIS POLYMERIZATION OF BUTADIENE ACCORDING TO THE PI-ALLYL INSERTION MECHANISM .1. DENSITY-FUNCTIONAL STUDY OF THE C-C BOND FORMATION REACTION IN CATIONIC (ETA(3)-ALLYL)(ETA(2)- ETA(4)-BUTADIENE)NICKEL(II) COMPLEXES [NI(C3H5)(C4H6)](+) AND [NI(C3H5)(C4H6)(C2H4)](+)/

The 1,4-cis polymerization of butadiene according to the pi-allyl inse rtion mechanism has been studied theoretically by density functional t heory (DFT) for the ligand free cationic butenylnickel(II) complexes [ Ni(C3H5)(C4H6)](+), I and [Ni(C3H5)(C4H6)(C2H4)](+), II, DFT energy pr ofiles have been determined for the insertion of s-cis-butadiene into the (eta(3)-butenyl)nickel(II) bond in the supine and prone orientatio ns of the reacting ligands. The primary goal of this study aims to sho w that the insertion of cis-butadiene into the nickel(II)-allyl bond c an occur within the pi-coordination of the reacting parts which is cha racterized by an insertion barrier that should make the process feasib le. Due to the lack of coordinative saturation of nickel(II) in the si mpler model I, the insertion was calculated to be endothermic, and no clear difference between the supine/prone arrangements was apparent. T he influence of the next double bond of the growing polymer chain for an adequate description of the geometrical aspects, as well as reliabl e energetics of the insertion, was demonstrated by II. The insertion w as calculated to be exothermic by 11.6 kcal/mol for supine and 17.3 kc al/mol for prone, while the activation barrier was estimated to be 26. 4 kcal/mol for supine and 3.9 kcal/mol for prone. Preference is given to the prone orientation in kinetic and in thermodynamic control.