Synthesis and formation of ironcarbonyl complexes of 3',3'-dimethyl-2,3,5,6-tetramethylidenespiro[bicyclo[2.2.1]heptane-7,2'-oxirane]. Single-crystal and molecular structure of syn,exo2,3-anti,endo5,6-trans-μ[(1R,2R,3S,4S,5S,6R)-Ç2,3,C-η:Ç5,6,C-η―(3',3'-dimethyl-2,3,5,6-tetramethylidenespiro[bicyclo