Chromocene and Heterogeneous Catalysts

In Section 6.2, it was mentioned that one of the commercial catalysts for polyethylene (HDPE) is made by the reaction of chromocene with silica according 

Complex 6.37 has been shown to be an active catalyst for the manufacture of HDPE. Note that 6.37 is a 15-electron complex with chromium in the formal oxidation state of 3 +. The mechanism of polymerization involves generation of coordinative unsaturation through the dissociation of a THF molecule from 6.37. The evidence for an oxidation state of 3+ in the commercial catalyst comes from the fact that complex 6.38 is active for polymerization. However, complex 6.39, identical to 6.38 in every respect except the oxidation state of the metal ion, is inactive. Note that the oxidation state of chromium in 6.39 is 2+. 

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