The reaction between [IrCl(CO)(PMe(3))(2)] and the Cs[arachno-6-SB(9)H(12)] salt in CH(2)Cl(2) yields pale-yellow 11-vertex [8,8,8-(CO)(PMe(3))(2)-nido-8,7-IrSB(9)H(10)] (4). Reaction of this CO-ligated iridathiaborane with Me(3)N=O affords pale-yellow 11-vertex [1,1,1-(H)(PMe(3))(2)-isonido-1,2-IrSB(9)H(9)] (6), which is also formed from the thermal decarbonylation of 4. Compound 4 has a conventional cluster structure based on classical 11-vertex nido geometry, with the iridium center and the sulfur atom in the adjacent 8- and 7-positions on the pentagonal open face. Compound 6 exhibits an 11-vertex isonido structure based on an octadodecahedron with the {Ir(H)(PMe(3))(2)} occupying the apical position of connectivity six, but with one long non-bonding Ir-B distance generating the quadrilateral isonido open-face. Compound 6 reverts to 4 upon reaction with CO, and the Lewis acid character of 6 is further demonstrated in the reaction with EtNC to give [8,8,8-(EtNC)(PMe(3))(2)-nido-8,7-IrSB(9)H(10)] (7). The three new compounds 4, 6, and 7 have been characterized by single-crystal X-ray diffraction analyses and by NMR spectroscopy. Each of the nido iridathiaboranes 4 and 7 exhibits two different {Ir(L)(PMe(3))(2)}-to-{SB(9)H(10)} conformers in solution and in the solid state. Density functional theory (DFT) calculations reveal that the iridium atom inverts the nido-isonido-closo energy profile previously found for the rhodathiaborane congener [8,8-(PPh(3))(2)-nido-8,7-RhSB(9)H(10)] (3), demonstrating how the structure of these 11-vertex clusters can be controlled and fine-tuned by the tailoring of the metal center.