We know metals of group 7,8,9 cannot form hydrides. This is because these metals accept the only negative electron of H atom and the metal ion becomes negatively charged and a ligand is formed in this way.
If the opposite would have occurred, like if the metal lost an electron and became positively charged then the compound formed with hydrogen would be known as a hydride.
This trait seen is known as hydride gap.
So, only group 6 elements form hydrides.
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