Synthesis, characterization, structural analysis, DFT investigations, molecular docking, in vitro cytotoxicity and antioxidant activity of Os(II)-cymene complexes of the type [(η6-p-cymene)OsBr(CH3CN)n(PPh3)m]+BF4−

• A novel series of osmium-cymene complexes was prepared by the standard Schlenk techniques. • Structures of novel complexes were confirmed by spectroscopic data, microanalysis, mass spectroscopy and X-ray crystallography data. • Synthesized complexes were screened against two cancer cell lines (MCF-7 and A549) and a normal cell line (HEK293). • Molecular docking data were found to be in agreement with the in vitro cytotoxicity results. Abstract This study investigates the synthesis of complexes 1, 2 and 3 and characterisation using the various methods such as vibrational (FTIR and Raman), NMR ( 1 H, 13 C, 19 F, and 31 P) spectroscopy, thermal analysis, elemental analysis, mass spectrometry, conductivity and the crystal structures of 2 and 3 are also reported. Introducing an aryl phosphine moiety increases the thermodynamic stability and the thermal stability of complexes 2 and 3 . In complex 2 , π-π interactions have been demonstrated to play a crucial role in stabilising the crystal packing. Hirshfeld surface analysis provides insight into the non-covalent interactions of complexes 2 and 3 . Molecular docking studies provide insight into the interactions of the complexes with protein receptors associated with the cancer cell lines screened in the in vitro cytotoxicity studies. Osmium(II)-cymene complexes exhibit an interesting trend in in vitro cytotoxicity against cancer cell lines, MCF-7 and A549, as well as the healthy cell line HEK293. Complex 3 demonstrated relatively high inhibitory activity against the MCF-7 and A549 cells, with IC 50 values of 2.9 ± 0.18 µM and 2.4 ± 0.19 µM, respectively. This was followed by complex 2 , which had IC 50 values of 2.7 ± 0.22 µM for MCF-7 cells and 3.4 ± 0.23 µM for A549 cells. In contrast, complexes 2 and 3 were less toxic to normal HEK293 cells, exhibiting IC 50 values of 8.7 ± 0.06 µM and 10.80 ± 0.25 µM, respectively. Furthermore, molecular docking was conducted to validate the findings from the in vitro anticancer activities. Results show that PPh 3 significantly enhances the in vitro cytotoxicity and antioxidant activities of complexes 2 and 3 , suggesting that metal-PPh 3 complexes are promising scaffolds for the rational design of multifunctional therapeutic agents with potential applications for future drug development strategies.