A Systematic Entry to Butterfly-Type [M4(CO)12(μ4-XY)] Clusters

The photochemical generation of M(CO)3 fragments from simple carbonyl complexes and their addition to X = Y‐bridged trinuclear clusters provides a general route to tetranuclear clusters having butterfly structures. The detailed structure depends on the kind of unsaturated ligands XY and their electron count (see picture). Such a cluster expansion should change the reactivity of the ligands XY, thereby allowing preparatively useful transformations.