Why does phosphine reduce HgCl2 to Hg but LiAlH4 / LiH reduces it to Hg2Cl2?


> 
> Why does phosphine reduce $\ce{HgCl\_2}$ to $\ce{Hg}$ but $\ce{LiAlH4}$
>  or $\ce{LiH}$ reduces it to $\ce{Hg2Cl2}$ ?
> 
> 
> 


It seems phosphine has more reducing tendency.But why? 


Moreover can it be predicted whether a reducing agent will be reduce $\ce{HgCl\_2}$ to $\ce{Hg}$ or $\ce{Hg2Cl2}$ ?


