Stereochemistry of the Thermal Rearrangement through Intramolecular Metathesis of Si−Si and Fe−Fe Bonds: First Evidence for a Nonconcerted Mechanism

meso and rac isomers of (η5:η5-C5H4RMeSiSiMeRC5H4)Fe2(CO)2(μ-CO)2 (R = Ph, n-Bu) were synthesized and successfully separated for the first time. They were found to undergo the title rearrangement reaction in a nonstereospecific fashion, to give the products [RMeSi-η5-C5H4Fe(CO)2]2 as mixtures of meso and rac isomers in ratios of about 1:1. This is direct evidence to rule out the concerted free radical mechanism that is currently used to explain this reaction. A new mechanism involving activation of the Si−Si bond by a coordinatively unsaturated iron center is suggested.