Barriers transition metal dimers

Olefin insertion has been observed for complexes of nearly every transition metal and most lanthinides and actinides as well, the notable exception being group 17 metals. As discussed in section V.A, low insertion barriers have been computed for most classes of catalytically active complexes as well. One might speculate that any metal alkyl complex with the potential for a vacant coordination site would make polymer. This expectation is not experimentally realized because chain termination pathways compete with propagation. These termination pathways can be activated by a number of factors. Polymerization catalysts can be converted to oligomerization or dimerization catalysts simply by changes in the solvent or counteranion. The commonly proposed termination pathways are discussed below. 

Barriers to rotation around the Cca —N bonds have been determined experimentally for diaminocarbenes (3) and (4) and their protonated and lithiated counterparts the possible involvement of lithium or a proton in the dimerization of these acyclic diaminocarbenes was also reported. A computational study of the dimerization of diaminocarbenes has been performed via rate constant calculations using general transition-state theory calculations. Such a dimerization has been shown to be a rapid equilibrium between the carbenes and the tetra-A-alkyl-substituted enetetramines (5), by characterization of metathesis products when two different tetramines were mixed. The thermodynamic parameters of this Wanzlick equilibrium have been determined for the A-ethyl-substituted compound the enthalpy of dissociation has been evaluated at 13.7kcalmol and the entropy at 30.4calmor K . Complex-ation of diaminocarbenes by alkali metals has been clearly established by a shift of the C NMR signal from the carbene carbon of more than 5 ppm. ... 

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