We report a high-yield heterogeneous solid/liquid phase synthetic method to a series of nido-6-metalladecaboranes. The hydridoirida- and hydridorhoda-decaboranes, [6,6,6-H(PPh3)2-nido-6-MB9H13] [M = Ir (1), Rh (2)] are isolatable in 98% yields from the reaction of the square-planar M(I) complexes, [MCl(PPh3)3] (M = Rh, Ir), with K[B9H14]. The same synthetic procedure, but using [MCl(CO)H(PPh3)3] (M = Ru, Os) as metal starting reagents produces the CO-ligated clusters, [6,6,6-(CO)(PPh3)2-nido-6-MB9H13] [M = Ru (3), Os (4)], in yields of 83% and 95%, respectively. These highly convenient syntheses permit the investigation of the reaction chemistry of the new nido-6-metalladecaboranes. Thus, the CO-ligated compounds, 3 and 4, react with the square-planar platinum(II) complex, [PtCl2(PMe2Ph)2], in the presence of potassium triethylborohydride, to give the bimetallic clusters, [1,1,1-(CO)H(PPh3)-isocloso-1-RuB9H8-μ-(1,2)-{Pt(PMe2Ph)2}] (5) and [7,7-(PMe2Ph)2-9,9,9-(CO)(PPh3)2-nido-7,9-PtOsB9H11] (6), and the monometallic nido-5-osamadecaborane, [5,5,5-(PPh3)2(CO)-nido-5-OsB9H13] (7). This reactivity illustrates the potential of polyhedral boron-based clusters as molecular scaffolds ("B-frames") for the construction of multimetallic species. Single-crystal X-ray diffraction analyses have revealed the molecular structures of 3, 5, 6 and 7; the compounds are also studied by multielement NMR spectroscopy, mass spectrometry, IR spectroscopy, and in some cases computationally. Futhermore, the rotation of the {M(X)(PR3)2} moiety (X = H, CO), as PH3-ligated models, is studied by means of DFT-calculated relaxed potential energy surface scans, giving some insight into the lability of the metal-to-borane fragment interaction and of the exo-polyhedral ligands.