Migratory stationary points

While the migratory insertion/BHE of the [CpM(L)(H)(C2H4)]" complexes is characterized by the five stationary points depicted in Scheme 1.1, additional isomers may be relevant for the insertion into a metal-alkyl bond. For example, for M = Cr it is known that the initial insertion product of the ethylene-alkyl complex I is a y-agostic complex Ily, which subsequently isomerizes to p- and a-agostic species Iip and Ila [26], as shown in Scheme 1.2. 

A different picture is obtained for the migratory insertion into the M-H bond in late TMs according to Scheme 1.1. Erom inspection of the stationary points of the insertion in the [CpRh(PH3)(H)(C2H4)] complex in Figure 1.3, it follows that... 

Figure 1.3 (a-e) Stationary points for the migratory insertion process. B3LYP/SDD geome-... 

Pathways (a)-(b)-(c)-(d) and (a )—(b )—(c )—(d ) correspond to the original mechanism proposed by Cossee [268,276,277] and are still valid, apart from some minor modifications [1], for heterogeneous catalysts. For metallocene-based catalysts of classes II and partially V, this mechanism gives rise to successive additions at the same site (from a configurational point of view) and is known as the chain stationary insertion mechanism ( chain skipped insertion or site isomerisation without insertion mechanism) [143, 146, 345], The (a)-(b)-(c)-(a )—(b )—(c ) pathway corresponds to the chain migratory insertion mechanism found in the case of metallocene catalysts of classes I, III, IV and partially V [143, 146]. 

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