n ab initio study of a promising nido-carboranylporphyrin for the boron neutron capture therapy of tumors, and of its closo precursor is reported. Base-induced deboronation of neutral ZnDCP, believed to exist as a mixture of 3 stereoisomers, produces the tetraanionic ZnDCP4- as a complex mixture of isomers. 1H NMR data and ab initio calculations support these findings. The position of the axial pyridine ligand in ZnDCP4- and the orientation of the endo hydrogen atoms on the open faces of the nido-carborane cages significantly influence the total energy of the ZnDCP4- structures. It is suggested that the "cocktail" of isomers possibly enhances the biological activity of tetra(nido-carboranyl)porphyrins, such as ZnDCP4-.