[Big-Daddy]

From what I can observe, it seems the Ag-X bonds in silver halides get stronger down the group. For example, Ag-Cl is the only bond which can be broken to produce Ag (s) by sunlight (which is why AgCl precipitate "darkens" in sunlight - you get some silver Ag (s) in it!). Ag-Cl is weak enough that NH3 molecules can break it up to form [Ag(NH₃)₂]+ instead. This takes much more NH3 for Ag-Br and does not really happen for Ag-I.

But isn't this the opposite to the expected trend? For instance H-I is the weakest bond, whereas H-F is the strongest; it gets weaker down the group, not stronger! Same for just X₂. So why is it different for Ag-X?

[Borek]

For example, Ag-Cl is the only bond which can be broken to produce Ag (s) by sunlight

AgBr and AgI are light sensitive as well. Google for enthalpies of formation for AgCl, AgBr and AgI.

[Big-Daddy]

For example, Ag-Cl is the only bond which can be broken to produce Ag (s) by sunlight

AgBr and AgI are light sensitive as well. Google for enthalpies of formation for AgCl, AgBr and AgI.

Ah I see. So these two halide precipitates will also darken in sunlight? Indeed, more easily than AgCl? (as AgI has an enthalpy of formation half as exothermic as that of AgCl.)

Still does not explain the ammonia problem though ...

[UG]

One explanation I can see is a soft-soft Lewis acid-base type interaction where you have 'soft' acids like Ag⁺ preferring 'soft' bases like I^- so it is harder to displace with NH₃ (a hard base)