Why is BiH3 the strongest reducing agent amongst all the hydrides of Group 15 elements ?
As we move down the group, the size of the element increases and, therefore, the length of the E—H bond increases and its strength decreases. In other words, as we move down the group, the E—H bond can break more easily to evolve H2 gas which acts as the reducing agent. Thus, Bi—H bond is the weakest amongst the hydrides of elements of group 15 and hence BiH3 is the stronger reducing agent.