Computational study of structures and properties of metallaboranes: cobalt bis(dicarbollide).

A density functional study at the BP86/AE1 level is presented for the cobalt bis(dicarbollide) ion [3-Co-(1,2-C2B9H11)2]- (1) and selected isomers and rotamers thereof. Rotation of the two dicarbollide moieties with respect to each other is facile, as judged by the small energetic separation of the three rotamers located (within 11 kJ mol(-1)) and by the low barriers for their interconversion (at most 41 kJ mol(-1)). Among the isomers differing in carbon atom positions that contain two equivalent dicarbollide ligands, the 1,7 ("carbon apart") form [2-Co-(1,7-C2B9H11)2]- is the most stable, 121 kJ mol(-1) below 1. The electronic structure of 1 is characterized in terms of molecular orbitals, population analysis, and excitation energies from time-dependent density functional theory, relevant to UV/Vis spectroscopy. Experimental 11B NMR chemical shifts of 1 are reproduced to better than 5 ppm at the GIAO-B3LYP/II' level, and the computed delta(11B) values are only little affected by rotational averaging or the presence of a polarizable continuum. Larger such effects are found for the as-yet unknown 59Co chemical shift, for which a value in the range between -1800 and -2400 ppm is predicted. Even though the accuracy achieved for the theoretical delta(11B) values is somewhat lower than that for heteroboranes at conventional ab initio levels, the level of density functional employed can afford qualitatively reliable chemical shifts, which can be useful in assignments and structural refinements of heteroboranes containing transition metal.