The complex [7,10-beta-H-7-CO-7,7-(PPh3)(2)-isonido-7,8,9-ReC2B7H9]has been synthesized by treatment of the complex salt [NHMe3][3,3-Cl-2-3,3-(CO)(2)-closo-3,1,2-ReC2B9H11] with PPh3 in refluxing THF (tetrahydrofuran) and isolated as intensely colored orange-red microcrystals. Spectroscopic NMR and IR data have suggested that the product has a highly asymmetric structure with two inequivalent PPh3 ligands and a single CO ligand. Measurement of B-11 NMR spectra in particular have indicated seven distinct boron vertexes, although the resulting cage degradation by removal of two BH vertexes was confirmed only following X-ray crystallographic analysis, which revealed the pentadecahedral isonido-7,8,9-ReC2B7 architecture. The B-11 NMR resonances span an enormous chemical shift range (Delta delta = 113), and this appears to be a direct consequence of the deshielding of the boron vertex directly opposite the quadrilateral ReCCB aperture. The new complex has been shown by electrochemical measurements to undergo a reversible one-electron oxidation. Digitally simulated cyclic voltammograms support a proposed square scheme (E-1/2 = 0.58, 0.69 V vs ferrocene) involving a reversible isonido-closo transition of the metallacarborane cage. Most unusually for a metallacarborane complex, ambient temperature solutions in CH2Cl2 and DMF have been shown to be intensely turquoise-blue fluorescent (lambda(em) = 442 nm, Phi = 0.012). Fluorescence spectroscopy measurements in MeTHF (2-methyltetrahydrofuran) glass at 77 K have indicated that the likely cause of such a broad emission is dual fluorescence (lambda(em) = 404, 505 nm), with both emissions displaying vibronic structure. Following excited- state lifetime decay analysis, the emissive behavior has been accredited to metal- perturbed (IL)-I-1 states, with the lower energy emission arising from a slight geometric distortion of the initially excited complex.