How to bind a key such that it toggles between one particular application and the current application?

How to bind a key such that it toggles between one particular application and the current application?

I want to bind a key such that a particular application (in my case Pidgin or Calculator) gets the focus, then when I am done and press key again, the focus returns to the previously focused application, as though I never changed focus.
For example:
Before:
Chromium —Alt+Tab→ Files —Alt+Tab→ Chromium
Use of key binding:
Chromium —(bound key)→ Pidgin (answer to chat) or Calc (calculate something) —(bound key)→ Chromium
After using above:
Chromium —Alt+Tab→ Files (not Pidgin) —Alt+Tab→ Chromium
Other alternatives are welcome.