Reaction of nickelocene with diazoalkanes

Nickelocene catalyzes the reaction of diazomethane to produce polymethylene; even in the presence of carbene acceptors polymethylene is the only product formed. Nickelocene catalyzes the reaction of ethyl diazoacetate, giving diethyl maleate and diethyl fumarate. In the presence of cyclohexene, ethyl norcaranecarboxylate is also formed. Nickelocene is recovered unchanged from these reactions. The catalytic effects of nickelocene seem to be unique to it; for example, neither ferrocene nor dicyclopentadienylchromium have similar properties. A mechanism for these reactions is proposed.