Reducing Character of Hydrides

Trend Orders:

$$\begin{aligned} \text{Group 15:} &\quad \text{NH}_3 < \text{PH}_3 < \text{AsH}_3 \\ \text{Group 17:} &\quad \text{HF} < \text{HCl} < \text{HBr} < \text{HI} \end{aligned}$$

Concept: Reducing capacity increases down the group; as the thermal stability of the hydride drops, the $$M\text{-H}$$ bond breaks more readily, making the release of hydrogen easier.

Notation:

• $$\text{NH}_3, \text{HF}$$ = Weakest reducing agents;
• $$\text{AsH}_3, \text{HI}$$ = Stronger reducing agents due to weaker bonds.