Are polyatomic ions only involving non-metals?

Not all polyatomic ions consist of non-metals. It's not true that only non-metals share electrons. Transition metals that are electron rich, eg Fe(II) can donate electrons to pi-acceptor ligands such as CO. Electron poor metal can also accept electrons from donor ligands. Examples of polyatomic ions containing metals are MnO4-, [Cr2O7]2-