The mechanism for how two metal centers can work synergistically to improve a hydrogenation reaction has been shown "in unprecedented detail and clarity" by Richard D. Adams, Burjor Captain, and Lei Zhu of the University of South Carolina, Columbia (J. Am. Chem. Soc. 2006, 128, 13672). In heterogeneous catalysis, two metals are commonly used together to improve reactivity and selectivity. But little is known about the possible mechanism of this "bimetallic cooperativity." Using a model hydrogenation system, the researchers first noted that an osmium complex, OsH(CO)4Sn(C6H5)3, doesn't react with phenylacetylene. But when the reaction is carried out in the presence of a platinum complex, Pt[P(tert-butyl)3]2, alkyne insertion into the Os-H bond proceeds smoothly. The structure of an intermediate (shown) unveiled how the platinum complex activates the osmium complex by forming Pt-H-Os and Pt-CO-Os bridges. In addition, the platinum atom has an empty d orbital, which provides an initial bonding site for the alkyne and sets the stage for it to insert into the Pt-H-Os bridge and form a new C-H bond.