Quantum-Mechanical Exchange Coupling and Hindered Rotational Phenomena

Quantum-Mechanical Exchange Coupling and Hindered Rotational Phenomena 

A soft vibration exists in the polyhydride complex that combines in-plane and out-of-plane MH2 bending to facilitate the approach of the two H that 

A chemical path initiated by the vibration allows exchange via approach of the H within a distance that varies according to the complex but is shorter than the sum of the van der Waals radii. In some cases an H2 ligand is clearly involved but in others (hr and Os systems) it is not so clear, although short dHH have been calculated. 

A rotational tunneling process through this barrier exists similar to that observed by INS for H2 complexes and can be viewed as a transition between coordinated ortho- and para-hydrogen. 

A unique case of QEC involving only a H2 ligand is known. The complexes [CpiM(H2)(L)]+ (M = Nb, ) show blocked rotation oftheH2 ligand in the NMR 

Several metallocene complexes such as (C5H4SiMe3)2NbH3, [CpIrH3(PR3)] ,  

Calculations suggest that this species exchanges a pair of H atoms through a tunneling path involving a transient H2 complex, and more recent calculations show that an complex 19 is a transition state in the H-exchange process with a 

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