The invention relates to bi-functionalised metallocenes of general formula (I) where Me=a transition metal, preferably chosen from Fe, Ru and Os, Y and Z, when identical are selected from --(CH.sub.2).sub.n--O--, (CH.sub.2)--O--[(CH.sub.2).sub.2--O].sub.P-- and --(CH.sub.2).sub.q--CONH--(CH.sub.2).sub.r--O--, or Y=--(CH.sub.2).sub.S--NH-- and Z=--(CH.sub.2).sub.t--COO--, .sub.n=a whole number from 3 to 6 inclusive, p=a whole number from 1 to 4 inclusive, q=a whole number from 0 to 2 inclusive, r=a whole number from 0 to 2 inclusive, s=a whole number from 2 to 5 inclusive, t=a whole number from 3 to 6 inclusive, R and R'=H atoms or are protective groups used in oligonucleotide and peptide synthesis, where at least one of R or R' is protective group used in oligonucleotide and peptide synthesis and R and R' are as defined below: (i) when Z and Y are selected from (CH.sub.2).sub.n--O--, --(CH.sub.2)--O--[(CH.sub.2).sub.2--O].sub.p-- and --(CH.sub.2).sub.q--CONH--(CH.sub.2).sub.r--O--, then R and R' are protective groups used in oligonucleotide synthesis and R is a group which can leave a free OH group after deprotection, preferably a photolabile group such as monomethroxythoxytrityl, dimethoxytrityl, t-butyldimethylsilyl, acetyl or trifluroacetyl, and R' is a phosphorylated group which can react with a free OH, preferably a phosphodiester, phosphoramidite or H-phosphonate and (ii) when Y=--(CH.sub.2).sub.s--NH-- and Z=--(CH.sub.2).sub.t--COO--, then R is a protective group used in the synthesis of peptides and is an amino-protecting group, preferably 9-fluorenyloxycarbonyl, t-butoxycarbonyl or benzyloxycarbonyl and R'=H. The above is applied in marking.