Can Pokemon Pearl and Pokemon sapphire trade?

No.

But you can migrate Pokemon from sapphire to your pearl or diamond game.

To do this, put your sapphire, emerald, or ruby cartridge into your ds' GBA slot (while it's off).

When you get to the main menu of Pokemon diamond or pearl, there will be an option to Migrate from sapphire, emerald, or ruby. Once you have clicked it, you will be able to choose 6 Pokemon from your PC to migrate to your game. When you're finished, it will go back to the main menu of Pokemon pearl or diamond. Go to Pal park, located south of sandgem town. You will need surf to get there. Once there, talk to the guy blocking the entrance to the door behind him, talk to him and he will give you park balls which will be used to 'catch' the Pokemon you migrated. Note you can only migrate once every 24 hours. Happy migrating!