Hydride and Dihydrogen Ligands

Simplified one-electron MO diagram showing the bonding modes in a metal-dihydrogen bond. 

Direct reaction with H2 (g), including oxidative addition  

The MO of H2 donates a pair of electrons to the metal to form a three-centered, two-electron bond, while a filled d-orbital on the metal r-backbonds into the empty a MO on the dihydrogen li d. As a result of the. -backbonding, the H—H bond is weakened, lypical H—H bond lengths for coordinated H2 are 82-90 pm, compared with a bond length of only 74.1 pm in 

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RUTHENIUM AND OSMIUM COMPLEXES CONTAINING HYDRIDE AND DIHYDROGEN LIGANDS... 

Metal hydrides were among the earliest fluxional compounds to be studied. The more recent discovery of dihydrogen complexes and stereochemical nonrigidity in complexes containing both hydride and dihydrogen ligands has provided one of the areas of most intense interest in DNMR in recent years. ... 

In the case of hydride-dUiydrogen complexes, one can observe stereochemical nonrigidity wherein the effects of scrambling of the nuclei between the hydride and dihydrogen ligands can be observed (equation 29). The first example of this was reported by Crabtree for [(benzoquinolyl)(PPh3)2lr(H2)(H)]+. In most cases it appears that the mechanism involves the formation of a very low... 

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