In the chemical formula M2CO3, "M" represents a placeholder for a metal cation. The specific identity of "M" depends on the context or the compound you are referring to. "M" can represent various metals, and without additional information, it cannot be determined which metal cation it refers to.
For example, in sodium carbonate (Na2CO3), "M" represents the sodium (Na) cation. In potassium carbonate (K2CO3), "M" represents the potassium (K) cation. Similarly, in other carbonates, "M" would represent the specific metal cation present in that compound. The identity of "M" varies depending on the compound being discussed, and it could be any metal cation that can form a carbonate compound.