Stoichiometric vs. Catalytic Mechanisms

Whether a reaction requires stoichiometric or catalytic quantities of a transition metal has a strong bearing on how one draws a mechanism for the reaction. The mechanism of a reaction that requires stoichiometric quantities of the metal can be written in a linear fashion like a polar or pericyclic mechanism. However, the mechanisms of metal-catalyzed reactions are usually drawn in a circular fashion, showing how the original metal species is regenerated at the end of each catalytic cycle. Whether a mechanism is catalytic or stoichiometric in metal is usually clear from the reaction conditions. 

Because it is not the case that every line represents exactly two electrons in a drawing of an organometallic or inorganic compound, it follows that the curved-arrow convention for showing the movement of electrons cannot be applied unambiguously when reaction mechanisms inolving transition metals are drawn. For this reason, in mechanisms involving transition metals, the name of each individual mechanistic step (insertion, transmetallation, oxidative addition, etc.) is indicated in place of curved arrows. You may use curved arrows to show electron movement in some steps if you wish, but it is more important for you to name every step. 

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