ACN is a better solvent than methanol for both [NMe(4)] [7-(2'-pyridyl)-nido-7,8-C(2)B(9)H(11)] and its protonated anion. The investigated laboratory preparations of the salt and of its protonated anion are electrophoretically pure solids stable for 2 months at 4 degrees C. At a longer storage, the solid salt is more stable than the solid protonated anion. In the 40:60 v/v water-methanol solvent, decomposition products of the salt anion are detectable after one-week storage of the salt solution at 4 degrees C. The protonated anion does not decompose for almost 1 year in water-organic solutions at 4 degrees C. The exchange of the proton between the protonated anion and the solution is reversible and fast at room temperature. The pH dependence of the mobility of the [7-(2(-pyridyl)-nido-7,8-C(2)B(9)H(11)](-) anion reveals that the basicity of the nitrogen atom in the pyridine ring is not significantly affected by the bonding of the pyridyl group to the nido-7,8-C(2)B(9)H(11) cluster in position 7 and that the proton from the solution is accepted by the nitrogen atom in the 2-pyridyl ring. The UV-spectra of the salt and of its protonated anion indicate that the accepted proton is probably slightly shifted to the open face of the nido-7,8-C(2)B(9)H(11) cluster. The [1](-) is chiral.