A monomer which luminesces in the visible region upon excitation is described of the formula: M.sup.n+ (L).sup.n- (CL).sub.x in which n+ is the valency of M, (L) represents one or more anionic ligands with a total valency of n such that at least one of the ligands possesses the formula: Ch-X--Y wherein Ch represents a chelating group which is a ligand fragment comprising the chelate binding sites and the part of the rest of the ligand with which the binding sites are conjugated, Y represents an olefinic group and X represents a spacer comprising a chain of at least 4 carbon and/or hetero atoms, x is 0, 1, or 2, CL represents a neutral co-ligand and M represents a metal atom of group 2, 12, 13, d-block or f-block with the proviso that if Y is part of a styrene or substituted styrene group then M is a f-block or d-block metal.