Reaction of [Pt3(CNR)6] with arachno-4-R′-4-CB8H13 in THF (tetrahydrofuran) gives the platinamonocarborane species [9,9-(CNR)2-6-R′-arachno-9,6-PtCB8H11] (R = But, R′ = H (1); R = Xyl (C6H3Me2-2,6), R′ = H (2); R = But, R′ = Ph (3)). The related compound [9,9-(PMe2Ph)2-arachno-9,6-PtCB8H12] (5) can be synthesized by treating arachno-4-CB8H14 with [PtMe2(PMe2Ph)2] in CH2Cl2. Compounds 1–3 and 5 are all formed in high yields (>80%), enabling reactivity studies to be carried out. A single isocyanide ligand in 1 can be easily exchanged for a phosphine, as is the case with [9-CNBut-9-PPh3-arachno-9,6-PtCB8H12] (6), or, by using bidentate dppe (Ph2PCH2CH2PPh2), both isocyanide groups are replaced, giving [9,9-dppe-arachno-9,6-PtCB8H12] (7). Treatment with [NEt4][CN] allows the isocyanides in 1 to be replaced by cyanide ligands, giving the dianionic complex [NEt4]2[9,9-(CN)2-arachno-9,6-PtCB8H12] (8) or the monoanionic species [NEt4][9-CNBut-9-CN-arachno-9,6-PtCB8H12] (9) depending on the stoichiometry of the reaction. Reaction of 1 or 3 with I2 yields the platinum(IV) complexes [9,9-(CNBut)2-9,9-I2-6-R′-arachno-9,6-PtCB8H11] (R′ = H (10), Ph (11)), respectively, by an oxidative addition reaction. Using the same reagent, compound 7 forms solvent-dependent, boron-substituted products: [9,9-dppe-4,8-I2-arachno-9,6-PtCB8H10] (12) is obtained in CH2Cl2, whereas [9,9-dppe-4-{O(CH2)4I}-8-I-arachno-9,6-PtCB8H10] (13) results when THF is used as solvent.