The present invention provides a labeled nucleoside having formula: ##STR00001## where X represents an aliphatic diamine, and the labeling moiety comprises a label and a spacer, where the spacer is coupled at one end to a platinum atom and at the other end to the label, which spacer comprises a chain having at least four carbon atoms and at least one oxygen atom in the chain. The invention further provides a method for labeling a nucleoside, kits for labeling a nucleoside, and kits for producing a labeled nucleic acid.