From Bulbapedia, the community-driven Pokémon encyclopedia.
Metallica Island (Japanese: メタリカ島 Metallica Island) is an anime-exclusive location in the Kanto region. It is located between Cinnabar Island and the Seafoam Islands. It is the home to the Battle Palace.
The island does contain its own Pokémon Center, but most of the island is a large forest, where Palace Maven Spenser trains. It appears in Cutting the Ties that Bind and Ka Boom with a View!.
Pokémon seen on Metallica Island