Can you trade Pokemon between Pokemon Diamond and Pokemon Leaf Green?

Answer 1

Sorry that this is so long.

You can trade Pokemon from LeafGreen to Diamond but not the other way around. You can do this by going to the Pal Park which is found on route 221 after you get the National Dex. Before you talk to the Pal Park Ranger you have to have at least 6 Pokemon in your Pokemon storage box on the PC from the game which you want to trade from. The Ranger will then ask you to choose which Pokemon you want to trade over and after you have selected them he gives you (i think) 6 Park Balls. You go out into the Park which contains different areas, for example: in the Ocean area you would find a Pokemon which fits in with that area like a water Pokemon such as Sharpedo or Gyarados. Generally just walk or surf through the different areas to capture the 6 Pokemon, don't worry about not being able to capture a Pokemon as the Park Balls have a 100% capture rate, you cannot run and the Pokemon will more-or-less automatically appear to you (in my experience).

You have a time limit in the Park though so try to be quick. If you capture all 6 Pokemon you will be congratulated by the Pal Park Ranger and be given a special berry. Hopefully now you will have your traded Pokemon :)