Investigation of the reaction product of 1,4,7,10-tetraphenyl-cyclododecahexa-1,2,3,7,8,9-ene with dodecacarbonyl-triiron by X-ray diffraction and MS: comparative structural study of the hexacarbonyl-μ2-(1η2-1,2-ene-2η3-3,4,5-allyl-1σ1-5)diiron(FeFe) system