The present invention relates to the use of a compound of formula (Ia) as a reagent for linking a compound of formula R 1 -H which comprises a first functional moiety of formula F 1 to a second functional moiety of formula F 2 wherein X, X', Y, R 1 , R 2 , R 3 , R 3 ', F 1 and F 2 are as defined herein. The present invention also provides related processes and products. The present invention is useful for creating functional conjugate compounds, and specifically conjugates in which at least one of the constituent molecules carries a thiol group.