Chapter 1 Introduction 1 General Introduction

The in-plane channel occurs in a stepwise fashion whereby two intermediate -complexes are involved in addition to the incipient resting state -complex, 16a. The first intermediate observed, 17, can be characterized as a -complex in which the ethylene moiety is oriented perpendicular to the plane of the Ni-diimine ring. Rotation of the coordinated ethylene group in 17, such that both carbon atoms lie in the diimine ring plane, yields the second intermediate 18. Here, the alkyl group, the Ni-diimine fragment and the ethene moiety are all coplanar. The optimized QM/MM structures of both these intermediates, 17 and 18,^◊ are shown in Figure 3.7. The bulky substituents act to destabilize the in-plane -complexes, 17 and 18, compared to their pure QM counterparts. Structures 17 and 18, respectively, lie 8.4 and 16.6 kcal/mol higher in energy than the resting state structure 16a, whereas the equivalent pure QM complexes lie 6.0 and 11.2 kcal/mol higher than the pure QM resting state. The most noticeable geometric consequence of the bulky ligands occurs in 18 where the C_-C(olefin) distance decreases from 2.65 Å in the pure QM complex to 2.44 Å in the QM/MM structure. Furthermore, the Ni-C_-C_ angle increases from 133° to 140° as a result of the bulky aryl ligands. The transition state for the in-plane insertion, TS[18-20], which is shown in Figure 3.8 lies 17.3 kcal/mol higher in energy than the resting state structure 16a. This is only slightly higher than the 16.3 kcal/mol barrier observed with the pure QM system. Interestingly, the C(olefin)-C_ distance in TS[18-20] is actually 0.16 Å larger than in its pure QM counterpart. We suggest that an earlier transition state is formed due to the bulky groups. Since the in-plane insertion channel is less favorable than the out-of-plane channel when the bulky ligands are included, the transition states linking the -complex 16a to 17 and 17 to 18 were not located. We, however, do not expect these transformation barriers to be large based on our pure QM study⁵⁹ where these barriers were found to be less than 2 kcal/mol. The kinetic product of the in-plane insertion is a -agostic Ni-pentyl cation, 20, which lies 1.0 kcal/mol above the resting state complex, 16a. Species 20 which is pictured in Figure 3.8 has two -agostic hydrogens as evidenced by the short 1.84 and 1.99 Å Ni-H_ distances.