B-11 NMR data on substitution-induced electron-density redistribution in monosubstituted closo-boranes

High-resolution NMR spectroscopy data are used to quantitatively estimate t he degree of electron delocalization over atomic centers of a monosubstitut ed molecular system. The results of systematic studies of the high-symmetry closo-boranes containing electron-deficient chemical bonds are analyzed. T he trends in the substitution-induced electron-density redistribution are e xamined with invoking the free-electron molecular-orbital model and the tra jectory approximation for the roots of secular LCAO MO equation. The electr on-density delocalization in the icosahedral and octahedral structures is d iscussed. It is pointed out that the degree of delocalization significantly increases upon passing from icosahedron to octahedron. The delocalization concept is shown to play an important role in the cluster systems, causing their spherical aromaticity.