The icosahedral dicarboranes and their decapitated anion, 1-R ' -1,2-C2B10H10 (closo) and [7-R ' -7,8-C2B9H10](-) (nido), exert a distict influence at the alpha position of substituents attached to the cage carbon atom; The closo fragment is electron-withdrawing while the nido anion is electron-releasing. These effects are studied by P-31 NMR, phosphorus oxidation, and phosphorus protonation in [7-PR2-8-R ' -7,8-C2B9H10](-) species. The P-31 NMR chemical shift dependence is related to the R alkyl or aryl nature of [7-PR2-8-R ' -7,8-C2B9H10](-). No direct relationship to the nature of the R substituent on the nido-carboranylmonphosphine toward oxidation has been found. The basicity of the nido-alkylcarboranylmonophosphines is the highest while the lowest corresponds to the nido-arylcarboranylmonophosphines. Interpretation can be carried out qualitatively by considering the electronic properties of the cluster and the nature of the R groups. The influence of R ' is less relevant. Confirmation of the molecular structure of the oxidated and protonated nido-carboranylmonophosphine compounds was obtained by X-ray diffraction analysis of [NBu4][7-P(O)Ph-2-8-Ph-7,8-C2B9H10] and [7-PH(Pr-1)(2)-8-Me-7,8-C2B9H10].