The arene osmium metalla-prism [(p-cymene)(6)Os-6(donq)(3)(tpt)(2)](6+) ([2](6+)) (donq 5,8-dioxydo-1,4-naphthoquinonato, tpt 2,4,6-tri(pyridin-4-yl)-1,3,5-triazine) has been prepared from the dinuclear arene osmium precursor [(p-cymene)(2)Os-2(donq)Cl-2] (1) in the presence of the tridentate panel tpt and silver triflate. The assembly of the arene osmium metalla-prism has also been achieved in the presence of one equivalent of planar guest molecules, Pt(acac)(2) (acac acetylacetonato) and 1-(4,6-dichloro1,3,5-triazin-2-yl) pyrene (pyrene-R), to give the corresponding guest-encapsulated systems, [Pt(acac)(2)subset of(2)](6+) and [pyrene-R subset of 2](6+). All complexes were isolated as triflate salts and characterised by infrared, UVevisible, NMR and by elemental analysis. The empty metalla-prism has also been characterised by mass spectrometry. The cytotoxicities of the dinuclear and hexanuclear osmium complexes have been established using ovarian A2780 and A2780cisR cancer cell lines and compared with the cytotoxicities of their ruthenium analogues. (C) 2011 Elsevier B. V. All rights reserved.