K4Sn9 dissolves in ethylenediamine (en) to give equilibrium mixtures of the diamagnetic HSn93- ion along with KxSn9(4-x)- ion pairs, where x = 0, 1, 2, 3. The HSn93- cluster is formed from the deprotonation of the en solvent and is the conjugate acid of Sn-9(4-). DFT studies show that the structure is quite similar to the known isoelectronic RSn93- ions (e.g., R = i-Pr). The hydrogen atom of HSn93- (delta = 6.18 ppm) rapidly migrates among all nine Sn atoms in an intramolecular fashion; the Sn-9 core is also highly dynamic on the NMR time scale. The HSn93- cluster reacts with Ni(cod)(2) to give the Ni@HSn93- ion containing a hydridic hydrogen (delta = 28.3 ppm) that also scrambles across the Sn-9 cluster. The Sn-9(4-) ion competes effectively with 2,2,2-crypt for binding K+ in en solutions, and the pK(a) of HSn93- is similar to that of en (i.e., Sn-9(4-) is a very strong Bronsted base with a pK(b) comparable to that of the NH2CH2CH2NH- anion). Competition studies show that the HSn93- reversible arrow Sn-9(4-) + H+ equilibrium is fully reversible. The HSn93- anion is present in significant concentrations in en solutions containing 2,2,2-crypt, yet it has gone undetected for over 30 years.