Dodecahydro‐closo‐undecaborate [B11H12]–

DFT‐calculations of the geometries of the closo‐anion [B₁₁H₁₁]^2– in its ground state and in the transition state of its skeletal rearrangement and of the protonated species [B₁₁H₁₂]^– in its ground state were performed at the B3LYP/6‐31++G(d,p) level. The corresponding NMR shifts were computed on the basis of the optimized geometry by the GIAO method at the same level. Calculated and observed NMR data are in good agreement and thus prove the structure of [B₁₁H₁₂]^–, previously deduced from 2 D‐NMR spectra. The addition of water, ethanol, and pyridine to [B₁₁H₁₂]^– at low temperature gave the nido‐species [B₁₁H₁₃(OH)]^–, [B₁₁H₁₃(OEt)]^–, and [B₁₁H₁₂(py)]^–, respectively. The structures of these anions were investigated by NMR methods and the last two of them by crystal structure analyses of appropriate salts. The course of the addition reactions can be rationalized on the basis of the structurally characterized reaction components.