Oxalate (IUPAC: ethanedioate) is the dianion with the formula C2O42−, also written (COO)22−. Either name is often used for derivatives, such as salts of oxalic acid, for example sodium oxalate Na2C2O4, or dimethyl oxalate ((CH3)2C2O4). Oxalate also forms coordination compounds where it is sometimes abbreviated as ox. Many metal ions form insoluble precipitates with oxalate, a prominent example being calcium oxalate, the primary constituent of the most common kind of kidney stones.