The term "platinum group" is used to refer to a set of six chemical elements: platinum, palladium, rhodium, ruthenium, iridium, and osmium. Antonyms for this group of elements would be metals that are not part of this specific set. These could include copper, iron, gold, silver, zinc, and many others. While these metals may have different properties and uses, they are not considered part of the platinum group due to their unique chemical characteristics. Therefore, it is important to use the term "non-platinum group" if referring to metals that are not part of this specific set.