wherein M(II) is a bivalent metal, Xa is a substituted or non-substituted residue that forms a monocyclic or polycyclic aromatic ring together with carbon atoms that are adjacent thereto at its both ends, Ya is a substituted or non-substituted residue that forms a nitrogen-containing aromatic heterocycle together with a nitrogen atom and carbon atoms that are adjacent thereto at its both ends, and the substituents when present in these residues are independently selected from the group consisting of halogen atoms, substituted and non-substituted alkyl groups, substituted and non-substituted alkoxy groups, substituted and non-substituted alkylthio groups, substituted and non-substituted aryl groups, substituted and non-substituted aryloxy groups, substituted and non-substituted arylthio groups, nitro group, and substituted and non-substituted amino groups.