An aqueous composition comprising (a) metal ions comprising tin ions and silver ions and (b) at least one complexing agent of formula (C1) R1—X1—S—X21[D1-X22—]n—S—X3—R2, (C2) R1—X1—S—X31-D2-[X32—S—]nX3—R2, (C3) R3—X1—S—X41-[D3-X42—]nS—X3—R4 wherein X1, X3 are independently selected from a linear or branched C1-C12 alkanediyl, which may be unsubstituted or substituted by OH; X21, X22 are independently selected from X1, which may be further substituted by —X5—COOR12, —X5—SO2—O—R12, a C2 to C6 polyoxyalkylene group of formula —(O—CH2—CHR11)z—OH, or a combination thereof, and —X1—NH—CO—X6—CO—NH—X1—; X31, X32 are independently selected from a chemical bond and X1; X41, X42 are independently selected from X1; X5 is a linear or branched Ci to C10 alkyl; X6 is selected from X1 and a divalent 5 or 6 membered aromatic group; R1, R2 are independently selected from a monovalent 5 or 6 membered aromatic N-heterocyclic group comprising one N atom or two N atoms which are separated by at least one C atom, and its derivatives received by N-alkylation with a C1-C6-alkyl group, which may be substituted by —COOR12 or —SO2—O—R12, and which aromatic N-heterocyclic group may optionally further comprise, under the proviso that X21 is substituted by at least one OH, one S atom; R3, R4 are independently selected from a monovalent 5 or 6 membered aliphatic N-heterocyclic group comprising one N atom and one O atom; D1 is independently selected from S, O and NR10−; D2 is (a) a divalent 5 or 6 membered aliphatic heterocyclic ring system comprising 1 or 2 S atoms, or (b) a 5 or 6 membered aromatic heterocyclic ring system comprising at least two N atoms and optionally one or two S atoms; D3 is independently selected from S and NR10−; n is an integer of from 0 to 5; z is an integer from 1 to 50; R10 is selected from H and a linear or branched C1-C12 alkyl; R11 is selected from H and a linear or branched C1 to C6 alkyl; and R12 is selected from R10 and a cation.