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 (η 5 :η 5 -C 5 H 4 RMeSiSiMeRC 5 H 4 )Fe 2 (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-η K -C 5 H 4 Fe(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.