The present invention provides an aqueous composition comprising tin ions and at least one compound of formula I wherein X1 is selected from a linear or branched C1-C12 alkanediyl, which may optionally be interrupted by O or S or a C5 to C12 aromatic moiety, R11 is a copolymer of ethylene oxide and a further C3 to C6 alkylene oxide, wherein the content of ethylene oxide is from 5 to 30% by weight, R12 is selected from H, R11, R40, R13, R14, are (a) independently selected from H, R11, R40, or (b) may together form a divalent group X13; X13 is selected from a linear or branched C1-C12 alkanediyl, which may optionally be interrupted by O, S or NR43; R40 is H or a linear or branched C1-C20 alkyl, R43 is selected from H, R11 and a linear or branched C1-C20 alkyl.