Utilizing a SHG inactive ligand, 2-thiobarbituric acid (H₃L), three metal complexes formulated as M(H₂L)₂(H₂O)₂·2DMF (M = Mg 1 and Ni 2) and Co(H₂L)₂(H₂O)₂ (3) were synthesized and structurally characterized by single-crystal X-ray diffractions. Complexes 1 and 2 exhibit a similar uninodal 3D acentric diamondoid framework. Complex 1 displays SHG response using 1064 nm radiation, whereas complex 2 is SHG inactive under the same condition. Complex 3 is also SHG inactive, which exhibits a centro-symmetrical chain-like structure built from Co(II) ions and double strands of H₂L⁻-bridge with the inversion center occupied by the metal(II) ion. In the three complexes, the negative charge of H₂L⁻ ligand is delocalized over the OC–CH–CO group and H₂L⁻ can be considered as a β-diketo derivative. H₂L⁻ shows a bis-monodentate coordination fashion and acts as a u₂-bridge in the three complexes, which is different from the classical chelating fashion of β-diketo ligand. In complexes 1 and 2, the two β-diketo oxygen atoms of H₂L⁻ are coordinated to two metal centers. Whereas in complex 3, H₂L⁻ ligand links two Co(II) centers via one oxygen and one sulphur atoms. The three complexes exhibit different UV-vis absorption, photoluminescence properties and thermal stabilities.