[CrimpJiggler]

I'm reading about transitional metal catalysts and 50% of the examples I come across involve rhenium.  I notice that a lot of the examples with this metal involve hydride ligands. The main example of a "coordinatively saturated complex" that people seem to use is rhenium nonahydride or other rhenium hydride complexes. Whats so special about rhenium when it comes to catalysts?

[Yakimikku]

I'm not too familiar about Rhenium and I don't know if there is anything particularly special about it in terms of catalysis. However, in more of a generalization--the variety of accessible oxidation states of transition metals aids in their ability to catalyze a variety of reactions (for instance, think oxidative addition/reductive elimination steps in many catalytic cycles). In terms of coordinatively saturated vs. unsaturated, unsaturation can allow easier access for species to add to a metal center and can be another important factor in catalysis. As for hydrides, they are useful for particular types of reactions, such as hydrogenation or hydroformylation. I hope this helps your exploration of transition metal catalysis!

[CrimpJiggler]

I don't know what the hell I was thinking when I started this thread. I was getting rhenium mixed up with rhodium and even then, rhodium doesn't pop up in the examples any more frequently than other transitional metals. Thanks for the reply though, I learned a few things from it. I've read about palladium catalysts for hydrogenations of double bonds but I never really thought about how they worked. It all makes sense now. The sigma bond of H2 adds to palladium then palladium, being an electron rich metal, does some serious back bonding into H2s sigma antibonding orbital, causing the H-H bond to break, yielding 2 new H+ ligands. The H-Pd-H complex is still very coordinatively unsaturated so it can accept alkenes as ligands and undergo some kind of catalytic cycle to reduce the double bond.

[Yakimikku]

You're welcome! Glad to help out!

Just one thing to stress about in your comment: in the oxidative addition of H₂ to a metal, you gain two hydride ligands (H^-), not H⁺. The H's go from a 0 oxidation state to -1, so 2 electrons are extracted from the metal, ie if you started with Pd⁰:

Pd⁰ + H-H    H-Pd²⁺-H