* R4 cards are not officially supported by Nintendo: R4 cards are third-party devices that allow you to play ROMs (copied game files) on your DS. They are not designed for official online connectivity or trading.
* Trading requires official connectivity: Pokémon trading in the DS games requires an official Nintendo Wi-Fi connection or a physical link cable. R4 cards lack this functionality.
* Different game generations: LeafGreen is from Generation III, while Diamond is from Generation IV. Even if you could connect them, the games are not compatible for trading.
What you can do:
* Trade within the same generation: If you have two DS games from the same generation (e.g., both Generation IV games like Diamond and Pearl), you can trade between them using a Wi-Fi connection (if available) or a physical link cable.
* Use an emulator: You can use a DS emulator on your computer, allowing you to play ROMs of both LeafGreen and Diamond. Some emulators offer features like network connectivity and trading, but this is still unofficial and may require technical setup.
* Use online trading services: There are unofficial online trading services where you can trade Pokémon between different generations. However, be cautious with these services, as they may pose security risks.
Important Note: Trading Pokémon between different game generations, even through emulators or unofficial services, can affect the legality of the Pokémon (e.g., it might not be recognized as legitimate in official online play).