Synthesis and structure of bimetallic allyl, alkoxysilyl complexes [Fe{Si(OMe)3}(CO)3(.mu.-dppm)M(.eta.3-2-RC3H4)] M = Pd, Pt; R = H, Me) and of [Fe{.mu.-Si(OMe)2(OMe)}(CO)3(.mu.-dppm)Pd(SnPh3)], a Sn-Pd-Fe-Si chain complex with a .mu.2-.eta.2-SiO bridge