We are focusing on transition between bonding states of hydrogen in hydrides, as a phenomenon specific to hydrides.An example was found in YMn2 hydrides, that is,the hydrides showed the continuous transition from the metal hydride YMn2Hx to the complex hydride YMn2H6 with [MnH6]5-complex anion, even under rather mild conditions (5 MPa, 423 K).