An aqueous composition comprising (a) metal ions comprising tin ions and silver ions and (b) at least one complexing agent of formula C1, C2 or C3 R 1 -X 1 -S-X 21 -[D 1 -X 22 -] n S-X 3 -R 2 R 1 -X 1 -S-X 31 -D 2 -[X 32 -S-] n X 3 -R 2 R 3 -X 1 -S-X 41 -[D 3 -X 42 -] n S-X 3 -R 4 wherein X 1, X 3 are independently selected from a linear or branched C 1 -C 12 alkanediyl, which may be unsubstituted or substituted by OH; X 21, X 22 are independently selected from (a) X 1, which may be further substituted by -X 5 --COOR 12, -X 5 -SO 2 -O-R 12, a C 2 to C 6 polyoxyalkylene group of formula -(O-CH 2 -CHR 11) z -OH, or a combination thereof, and (b) -X 1 -NH-CO-X 6 -CO-NH-X 1 -; X 31, X 32 are independently selected from a chemical bond and X 1; X 41, X 42 are independently selected from X 1; X 5 is a linear or branched C 1 to C 10 alkyl; X 6 is selected from X 1 and a divalent 5 or 6 membered aromatic group; R 1, R 2 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 C 1 -C 6 -alkyl group, which may be substituted by -COOR 12 or -SO 2 -O-R 12, and which aromatic N-heterocyclic group may optionally further comprise, under the proviso that X 21 is substituted by at least one OH, one S atom; R 3, R 4 are independently selected from a monovalent 5 or 6 membered aliphatic N-heterocyclic group comprising one N atom and one O atom; D 1 is independently selected from S, O and NR 10; D 2 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; D 3 is independently selected from S and NR 10; n is an integer of from 0 to 5; z is an integer from 1 to 50; R 10 is selected from H and a linear or branched C 1 -C 12 alkyl; and R 11 is selected from H and a linear or branched C 1 to C 6 alkyl; and R 12 is selected from R 10 and a cation. X 1, X 3 are independently selected from a linear or branched C 1 -C 12 alkanediyl, which may be unsubstituted or substituted by OH; X 21, X 22 are independently selected from (a) X 1, which may be further substituted by -X 5 --COOR 12, -X 5 -SO 2 -O-R 12, a C 2 to C 6 polyoxyalkylene group of formula -(O-CH 2 -CHR 11) z -OH, or a combination thereof, and (b) -X 1 -NH-CO-X 6 -CO-NH-X 1 -; X 31, X 32 are independently selected from a chemical bond and X 1; X 41, X 42 are independently selected from X 1; X 5 is a linear or branched C 1 to C 10 alkyl; X 6 is selected from X 1 and a divalent 5 or 6 membered aromatic group; R 1, R 2 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 C 1 -C 6 -alkyl group, which may be substituted by -COOR 12 or -SO 2 -O-R 12, and which aromatic N-heterocyclic group may optionally further comprise, under the proviso that X 21 is substituted by at least one OH, one S atom; R 3, R 4 are independently selected from a monovalent 5 or 6 membered aliphatic N-heterocyclic group comprising one N atom and one O atom; D 1 is independently selected from S, O and NR 10; D 2 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; D 3 is independently selected from S and NR 10; n is an integer of from 0 to 5; z is an integer from 1 to 50; R 10 is selected from H and a linear or branched C 1 -C 12 alkyl; and R 11 is selected from H and a linear or branched C 1 to C 6 alkyl; and R 12 is selected from R 10 and a cation.