. . "divalent metal cation; X is ???OMa, ???O???(CmH2mO)n???R1, in which R1 is a hydrogen atom, an aliphatic hydrocarbon radical containing from 1 to 20 carbon atoms, a cycloaliphatic hydrocarbon radical containing 5 to 8 carbon atoms or an optionally hydroxyl, carboxyl, or sulphonic substituted aryl radical containing 6 to 14 carbon atoms, m is 2 to 4, n is 0 to 100, ???NHR2 and/or ???N(R2)2, in which" . . .