When R is an electron-withdrawing group, the desired reaction B will be disfavored due to the relative lack of electron density of the carbon atom coordinating to Pd, and as a consequence reaction C will occur in solvents other than NMP. The formation of [B12H12] 2-which we have observed here under several reaction conditions had been seen before during BHC. The source of the hydrogen atom remained unresolved at that time, but hydrodehalogenation of carboranes has been found before under similar conditions. 11 Under Pd catalysis, water has been shown to act as hydride donor. As we cannot exclude water (even in dry solvents it can form through boroxine condensation of the boronic acid), water from solvent or reagents might be responsible for this reaction. We speculate that the hydride replaces the iodide, and the hydroxyl cation reacts with the iodide to hypoiodite, which then undergoes further reactions due to its high reactivity. For practical purposes, and future applications of this reaction with other substrates, the proper choice of ligand, solvent, base, and heating might minimize hydrodehalogenation. The hypoiodic acid postulated to be formed in reaction C is known to be very reactive, and it could be responsible for the great number of largely unidentified side products which we have found when reaction B does not proceed, but the starting [B12H11I] 2- disappears. For organic halides, reaction B, and possibly also reaction D, seem to be considerably faster that reaction C, unless a solvent or additive is used that can provides the hydride for hydrodehalogenation.