A new series of linear N4O2 amine phenols (H(2)badd, H(2)Brbadd, and H(2)Clbadd) based on N,N’-bis(3-aminopropyl)-ethylenediamine (tnentn) were prepared and characterized by spectroscopic techniques. Monocationic hexadentate metal complexes with the tnentn-based amine phenols were obtained from the reactions of Ga3+ and In3+ with the Linear amine phenol in the presence of a weak base (acetate). The molecular structure of [Ga(Brbadd)]ClO4 has been determined by X-ray crystallography; crystals of [Ga(Brbadd)][ClO4] (C22H30Br2ClGaN4O6) are orthorhombic : a = 12.462(1) Angstrom, b = 21.835(2) Angstrom, c = 9.961(2) Angstrom, Z = 4, space group P2(1)2(1)2(1). The structure was solved by the Patterson method and was refined by full-matrix least-squares procedures to R = 0.031 (R(w) = 0.029) for 1924 reflections with I greater than or equal to 3 sigma(I). The Ga3+ ion is coordinated in a distorted octahedral geometry by an N4O2 donor atom set. The four nitrogen atoms of the tetraamine backbone form the equatorial plane of the octahedron, and the two phenolate oxygen atoms are coordinated trans to each other. Water-soluble 1,10-bis-(2-hydroxy-5-sulfonylbenzyl)-1,4,7,10-tetraazadecane (H(6)Sbad(2+)), 1,12-bis(2-hydroxybenzyl-5-sulfonylbenzyl)-1,5,8,12-tetraazadodecane (H(6)Sbadd(2+)), and N,N’-bis-(2-hydroxy-5-sulfonylbenzyl)-N,N’-bis(2-methylpyridyl)-ethylenediamine (H(6)Sbbpen(2+)) were also prepared and characterized; potentiometric titrations of these three Ligands, in the absence and presence of Ga3+ and In3+, were performed to determine deprotonation constants of the ligands and the thermodynamic stabilities of Ga and In amine phenol complexes. The formation constants of the Ga3+ and In3+ complexes with Sbad(4-) (Ga3+, log beta = 28.33(8); In3+, log beta = 24.54(2)), Sbadd(4-) (Ga3+, log beta = 28.27(5); In3+, log beta = 24.56 (5)), and Sbbpen(4-) (Ga3+, log beta = 35.33(8); In3+, log beta = 34.85(5)) were obtained. All six of the Ga and In hexadentate amine phenol complexes were found to be very stable in aqueous solution. With the exception of [In(Sbadd)](-), all the complexes were calculated to be thermodynamically stable with respect to demetalation by transferrin at physiological pH. The linear amine phenols showed a selectivity for Ga3+ over In3+, while the Sbbpen(4-) derivative was indiscriminate in binding the Ga3+ and In3+ ions.