The incorporation of iodine atoms onto the boron vertices of the o-carborane framework causes, according to spectroscopic data, a uniform increase in the acidic character of the Cc-H (Cc = cluster carbon) vertices, whereas the incorporation of methyl groups onto the boron vertices of the o-carborane framework reduces their acidity. Methyl groups when attached to boron are electron-withdrawing in boron clusters, whereas iodine atoms bonded to boron act as electron donors. This has been proven on B-methyl and B-iodinated o-carboranes with NMR spectroscopy measurements and DFT calculations of natural bond orbital (NBO) charges, which show a cumulative buildup of positive cluster-only total charge (CTC) on B-methyl o-carboranes and a cumulative buildup of negative cluster-only total charge for B-iodinated o-carboranes.