Properties of Metal Carbonyl Hydrides

The majority of metal carbonyl hydrides show dynamic properties, that is, stereochemical lability. The positions of atoms change with a noticeable rate (stereoisomerization). These properties are most often investigated by NMR. The nuclear magnetic resonance method may be used only if the stereoisomerization occurs fast enough to influence the line shape and simultaneously allow the interaction between nuclear spins. 

The activation energy for stereoisomerization of metal carbonyl hydrides varies considerably. Complexes possessing the coordination numbers 6 and 4 are generally inert, while compounds which have coordination numbers 5 or higher than 6 are usually stereochemically labile. Because of crucial importance in catalytic processes, the isotopic H-D exchange between hydrido complexes and hydroxyl solvents, deuterium, and olefins is of particular interest. Often, exchange reactions with solvents such as D2O or EtOD are catalyzed by acids and bases. 

The exchange of coordinated hydrido ligands with deuterium readily takes place if oxidative addition is possible  

The exchange reaction between deuterium of [MDL ] and the ortho hydrogen in triarylphosphines also occurs through oxidative addition. The exchange between [MHL ] and C2D4 takes place when the ligand migration reaction is possible  

Therefore, the hydrido complexes are catalysts or are formed as intermediates in many reactions catalyzed by transition metal complexes, as in the isomerization of unsaturated hydrocarbons  

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