why 7^th 8^th and 9^th group of d and f block donot form any hydrides with hydrogen which is called hydrie gap

Hydrogen combine with many element forming hydride in which its oxidation state is -1. The 7th8thand 9thgroup of d and f block donot form any hydrides with hydrogen because these elememt have a very low affinity for hydrogen in their normal oxidation state..Example of such elements are Mn, Fe, Co, Ru, Re, Os etc.