Nonlocal Density Functional (DF) calculations have been carried out on the insertion of ethylene into the metal-CH3 bond of the Kaminsky type metallocenes : Cp2ZrCH|s(+,3) (2a), (SiH2Cp2)ZrCH|s(+,3) (3a), Cp2ScCH3 (4a), and the constrained geometry catalyst (CGC) (SiH2(Cp)NH)ZrCH|s(+,3) (5a). The objective has been to study the energy profile for the insertion involving (2a), and how it is modified by introducing the silane bridge in (3a), replacing one Cp with NH in (5) or switching to the neutral system (4a). The DF calculations reveal that the insertion into the bis-Cp systems proceeds with a modest barrier of 3 kJ/mol, 4 kJ/mol and 14 kJ/mol for (2), (3), and (4) , espectively. This barrier is marginally influenced by going from the charged species Cp2ZrCH|s(+,3) (2a) to the neutral system Cp2ScCH3 (4a) and is unchanged in going from the unbridged system Cp2ZrCH|s(+,3) (2a) to the silane bridged system (3a). The systems (2a), (3a), and (4a) all form a p-complex with a very shallow minimum at the beginning of the insertion process. The insertion process for the zirconium CGC (5a) exhibits a clear activation barrier and a pronounced minimum for the p-complex. The deeper potential well of the p-complex in (5a) compared to (2a), (3a) and (4a) can be related to a reduction in the steric interaction between ethylene and the ligands on the metal center as one Cp-ring is replaced by a NH ligand. The higher barrier in the case of (5a) is a consequence of the stable p-complex which has to be abandoned in order to proceed to the insertion product. Calculations have also been carried out on two chain terminating steps. The first step involves b-hydride elimination. It is concluded that this process is unfavourable for early transition metal centres with an endothermicity of 176 kJ/mol. The second step is concerned with the activation of an ethylene C-H bond by the metallocene to form an alkane and a vinyl zirconocene. It is concluded that this process is viable as a chain terminating step.