Three new complexes of the [Re6(μ3-Se)8]2+core-containing clusters with the water-soluble trialkylphosphine ligand 1,3,5-triaza-7-phosphaadamantane (PTA), [Re6(μ3-Se)8(PEt3)5(PTA)](SbF6)2(P5PTA, 1), [Re6(μ3-Se)8(PEt3)5(PTAH)][Re6(μ3-Se)8(PEt3)5(PTA)](SbF6)5(2), and [Re6(μ3-Se)8(PEt3)5(PTAH)]2(HBr)(SbF6)2Br4(3) have been prepared and structurally characterized. Unexpected protonation of the cluster-bound PTA ligand was observed when coordination of 1with HgI2was attempted, affording compound 2featuring a protonated PTA ligand, PTAH. Deliberate acidification of 1with HBr produced compound 3, and the protonation was investigated by using 31P NMR spectroscopy. Crystallographic studies revealed distinct structural distortions of the ligand as a result of protonation. [ABSTRACT FROM AUTHOR]