Intramolecular H-Atom Exchange

Metal bound H-atoms undergo a number of unique intermolecular and intramolecular exchange processes. In the words of Greg Kubas Transition metal complexes containing rf -H2 ligands and hydride ligands are unquestionably the most dynamic ligand systems known [4]. 

In general, the rapid exchange of metal-bound H-atoms benefits from the nature of the Is valence orbital of hydrogen. The spherical shape of the Is orbital allows H-atom exchange to occur by associative mechanisms (with low barriers) in which H-H bond formation coincides with metal-hydrogen bond breaking. 

For complexes that have only one filled orbital that is capable of M H2 back-donation or in which the filled d orbitals have drastically different energies, the barrier to rotation can be quite large. The d complexes, [CpTa(CO)- 

In the transition state for rj -H2 rotation there can be complete loss of M - H2 back-donation. 

Interactions between the bound tf -H2 ligand and the co-ligands can also affect the barrier to H2 rotation by making some orientations of the / -H2 ligand significantly more stable others. In general, these interactions may be electrostatic or may result from orbital overlap between the metal-bound hgand 

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