A precise chemistry of ternary complex formation between osmium reagents, 2,2'-bipyridine (Bpy) and DNA strand has been characterized. Enzymatically digested products of ternary complexes formed for a thymidine in an oligonucleotide has given two diastereoisomers corresponding the 5S,6R- and 5R,6S-isomers of T-Os-Bpy in 70:30 ratio.