We report quantum-chemical computations of Proton Affinities (PA) of icosahedral amino boranes, carboranes and Co-containing metallacarboranes with a relative error of ~ 2% - when experimental data available - by means of the B3LYP and BP86 functionals. Use of larger basis sets for simple systems such as NH3, CH3NH2, and borazine (B3H6N3) reduces the error to ~ 0.5 % indicating the validity of these functionals for these computations and prediction of PA for unavailable experimental data on amino-derived (car) boranes and metalla(car)boranes. The computed PA show that, from an electronic structure point of view, when substituting an exo H atom by an NH2 group in B12H12(2-), CB11H12(-), (ortho, meta, para)-C2B10H12, and the metallacarborane [3-Co(1,2-C2B9H11)2](-)= COSAN the most similar system to be compared with is the anion NH2-BH3(-) – computed PA(B3LYP/ cc-pVTZ) = 1505 kJ·mol-1 – rather than methylamine CH3NH2 or borazine, the two latter with experimental PA of 900 and 803 kJ·mol-1 respectively. The largest PA for a given isomer correspond, following this order, to: 1-NH2-B12H11(2-)(-)BH3NH2 , 12-NH2-CB11H11(-), cisoid 8-NH2-COSAN, transoid 9-NH2-COSAN, 9-NH2-1,2-C2B10H11, 9-NH2-1,7-C2B10H11, and 2-NH2-1,12-C2B10H11. The rule for larger PA applies for isomers with the NH2 groups farthest aways from (non-metal) carborane C(cage) atoms. Pyramidalization energy computation shows an enhanced facility for planarization of the amino group in cisoid 8-NH2-COSAN as compared to cisoid 1-NH2-COSAN.