The addition of the anion of 1,3,5-trimethyl-2-thiobarbituric acid 1 to π-bonded unsaturated hydrocarbons (olefin, cyclohexadienyl, cycloheptadienyl, cycloheptatrienyl) in cationic complexes of rhenium, iron, ruthenium and chromium provides a method for the introduction of organometallic fragments into the barbituric acid moiety. Substitution of the C-5-hydrogen atom gives the complexes 4-9. The dianions of 1,3-dimethylbarbituric acid 2 and 1,3-dimethyl-2-thiobarbituric acid 3 yield the bimetallic complexes 10-15. The structures of 10, 11 and 13 were determined by X-ray diffraction. Due to the protection of the N-atom there was no self-assembly via hydrogen bonds. The complexes may be useful as covalent markers for barbiturate drugs in carbonyl metalloimmunoassays.