Bond angles and Lone Pairs

I have a pretty good understanding of VSEPR theory and how to predict bond angles etc.

However, one thing bugs me.

Supposing I have NH3.... I know that there is only one lone pair and that bond angle is 107.3 degrees.

Now, what if I were asked to predict the bond angle of PH3 ?

I know it is around 104 degrees.

My question is why this decrease in bond angle for these two similar compounds containg elements inn the same group ?

Does have to do with the electronegativity ?

Yes.

N is smaller and more electronegative. This increases both the LP-BP repulsion and the BP-BP repulsion. But the increase in the latter dominates --> larger HNH bond angle than HPH

One other question.

What is the reaction mechanism when Ammonia reacts with Ag ions ?

I know a ligand is formed but what part does Ammonia play ?